1 Introduction

Religious diversity is often perceived—in public and academic debates alike—as a major challenge to social integration in contemporary Western Europe. Historically, this perception is premised on ideas of religious unity that are deeply engrained in European social thought, from politico-theological concepts of the social bond (religio vinculum societatis) to Durkheimian notions of the sacred, and to sociological theories of normative integration. Indeed, urban pockets of religious heterogeneity notwithstanding, most European societies were relatively homogeneous from the sixteenth up until the twentieth century. Starting with the Spanish reconquista, the emergence of territorial nation-states in early modern Europe went hand in hand with religious cleansing, confessional wars, and the expulsion of dissenters.Footnote 1 The Jewish population—despite legal emancipation since the French Revolution—suffered particularly from repeated persecutions and pogroms throughout the continent, to be almost entirely extinguished by Nazi Germany during the Holocaust. Confessional milieus were even reinforced after World War II, when both Catholic and Protestant churches postured as moral authorities in the anti-totalitarian reconstruction of Western European civilization. However, since the 1960s, European societies have experienced an unprecedented religious diversification. To start with, rates of Christian church membership and participation have declined drastically, as decades of survey research on secularization amply document. At the same time, the rise of new religious movements and popular spiritualities has entailed a more pluralistic religious economy. The most far-reaching transformation of religious demographics, however, has resulted from sustained migration from the Global South, driven by post-war labor recruitment, family resettlement, and refuge. As a consequence, historically entrenched remnants of the confessional age and secular settlements have been put to a severe test, giving rise to intense public controversies over whether and how religious diversity undermines national solidarity and threatens social cohesion.

Most of these public controversies have focused on the presence of Islam. As by far the largest non-Christian group in Western Europe, Muslims hail from different origin countries with vastly heterogenous traditions of Islam. Accurate numbers of the Muslim population are notoriously hard to obtain. In Germany, the last official census with reliable data on religious affiliation was conducted in 1987, and in France, where such census dates back even to 1872, it has long been legally prohibited to collect official data on religious (and ethnic or racial) affiliation.Footnote 2 Census limitations such as these have forced demographers to rely on rough estimates of the migrant population from Islamic countries, which, by design, miss out undocumented migrants, Muslims from non-Islamic countries, and native converts, and, at the same time, run the risk of inflating the number of Muslims by equating national origin with religious affiliation while ignoring highly variable degrees of religiosity. It is generally accepted, though, that in terms of total numbers as well as population shares the largest Muslim populations in contemporary Western Europe reside in Germany (5.3 million or 6.4%, mainly from Turkey, the Balkans, and, more recently, from Syria); France (about 4.1 million or 6.0% of the population, mainly from former colonies in the Maghreb and Sub-Saharan Africa); Great Britain (about 3.4 million or 5.2%, mainly from postcolonial India, Pakistan, and Bangladesh), and the Netherlands (0.9 million or 5.2%).Footnote 3 Public perceptions of Muslims and their increased visibility in Western Europe have been strongly interwoven with colonial legacies as well as with contemporary geopolitics, above all with ongoing conflict in the Near East. Starting with the Rushdie affair in Great Britain (1989) and the foulard affair in France (1990), gaining momentum with the terrorist attacks of 9/11 (2001), and further intensifying during the so-called refugee crisis (2015/2016), public debates have focused on Islam’s putative threat to security, its presumed incompatibility with liberal values, and its overall challenge to social integration.

Responding to these public debates, social scientists have studied migration-driven religious diversity and the presence of Islam from various disciplinary angles and with increasingly specialized research agendas. Sociologists of religion have focused mainly on the intensity, forms, and functions of religiosity among Muslim migrants and their children in secularizing Christian-heritage societies (reviewed in Voas and Fleischmann 2012). Migration scholars have (re-)discovered the religious factor in inter-generational processes of immigrant integration as well as in group-specific patterns of discrimination (reviewed in Kogan et al. 2020; Drouhot and Nee 2019; Statham and Tillie 2016). Comparative politics have shifted from controversies over national, post-national, and multicultural citizenship to studying the politics of secularism and religious diversity (reviewed in Koopmans 2013; see also Bramadat and Koenig 2009). Yet, the undeniable advances of these strands of research notwithstanding, their increasing specialization runs the risk of losing sight of the larger picture of how religious diversification has transformed Western European societies—culturally, socio-structurally, and institutionally.

The present article is therefore aimed at advancing academic debates on religious diversity and social integration in Western Europe by synthesizing strands of research that previous reviews have tended to compartmentalize. Drawing on analytical tools from cultural sociology, it analyses mutually reinforcing micro-level mechanisms of symbolic, social, and institutional boundary-making among native majorities and ethno-religious minorities, which have fueled slow-moving macro-level processes of religious boundary reconfiguration. It does so by systematically reviewing social scientific research since about 2010. Earlier studies from the 1990s and 2000s, whether based on ethnographies, surveys, and experiments or on policy analysis, are not only well covered by existing reviews (Voas and Fleischmann 2012; Koopmans 2013; Drouhot and Nee 2019) but were also largely cross-sectional in design. More recent research has, despite persisting limitations, started to employ longitudinal designs that allow slow-moving processes of religious boundary reconfigurations to be addressed up-front. The review’s geographic scope is restricted to Western Europe, because dynamics of religious change in post-socialist Eastern Europe have followed rather different logics. The review privileges cross-national studies that cover Western Europe as a whole, but it also includes comparative case studies from the aforementioned four countries with the largest Muslim migrant populations (Germany, Great Britain, France, Netherlands). In doing so, this review is aimed at shedding analytical light on broader religious boundary reconfigurations that can be observed, with variable intensity, across Western European societies—despite their undeniable differences in historical background, migration patterns, and demographic profiles.

The article starts out by discussing the boundary paradigm in cultural sociology and its implications for analyzing religious diversity and social integration (Sect. 2). The subsequent section reviews the literature on anti-Muslim prejudice and on Muslim migrants’ religious acculturation, demonstrating that their continuous interplay has helped sustain religiously coded symbolic boundaries (Sect. 3). The next section discusses research on Muslim migrants’ socio-economic integration, highlighting how ethno-religious discrimination and religiously based (self-)segregation can, at least in some domains, jointly reproduce social boundaries (Sect. 4). The last section focuses on the governance of religious diversity, demonstrating how Muslims’ claims for public recognition and state policies toward Islam have successively shifted institutionalized religious boundaries (Sect. 5). By way of conclusion, the paper sketches scenarios for the interplay of symbolic, social, and institutional processes of religious boundary reconfiguration, thus reassessing the challenges posed by religious diversity for social integration in Western Europe (Sect. 6).

2 Theorizing Religious Boundary Dynamics

The social science literature on religious diversity in Western Europe has long been framed in the conceptual vocabulary of immigrant integration. Immigrant integration here refers interchangeably to the process by which individual migrants and their offspring become part of the receiving society’s cultural, social, and political systems, and to the receiving society’s degrees of solidarity and cohesion that result from these processes. The literature has repeatedly been criticized for embracing a linear or one-way concept of integration that puts the entire explanatory burden on migrants’ behavior, reproduces public discourses of progressive nationalism, and ultimately legitimates state-power in liberal democracies.Footnote 4 Upon scrutiny, however, much of the more recent literature conceives of immigrant integration as a two-way process involving the outsiders and the established alike, to recall Norbert Elias’ terminology. For instance, neo-assimilationists conceive of immigrant integration (or “assimilation,” as North American terminology has it) as the declining salience of categorical markers of difference, to be explained as resulting both from unintended consequences of immigrants’ purposive action and from shifting cultural, social, and political contexts (Alba and Nee 2003, p. 38). In other words, immigrant integration is increasingly seen as an inherently relational process through which mainstream society is ultimately remade.

A theoretical framework that foregrounds the relational character of immigrant integration even more explicitly is the boundary paradigm inspired by anthropologist Frederick Barth (1969) and sociologist Pierre Bourdieu (1985). Avoiding the essentializing “groupism” that does characterize some strands of immigrant integration research, this approach argues that ethnic groups, including migrants and natives, are analytically posterior to boundary-making, that is, to practices of cognitive classification and social distancing (Brubaker 2009; Lamont and Molnár 2002; Wimmer 2008 and 2013). It should be conceded that neo-assimilationist theory does pay attention to these practices, distinguishing between the brightening, blurring, and shifting of boundaries (Alba and Nee 2003, p. 59–63; Zolberg and Woon 1999; see also Phalet et al. 2013). However, the boundary paradigm can be taken one step further by unpacking the slow-moving historical dynamics through which boundary configurations are formed and transformed (Loveman and Muniz 2007, p. 918). More precisely, macro-level processes of boundary (trans)formation can be explained as resulting from mutually reinforcing micro-level mechanisms that are sustained by variable boundary-making strategies of dominant and subordinate actors.Footnote 5

To theorize the historical (trans-)formation of boundary configurations, it is helpful to analytically dissect symbolic, social, and institutional boundary dynamics. Symbolic boundaries typically consist of shared cultural schemas that categorically distinguish in-group from out-group; social boundaries translate these categorical distinctions into unequal access to socio-economic resources (Lamont and Molnár 2002), whereas institutional boundaries anchor them in the legal-administrative apparatus of the state (Fox and Guglielmo 2012). How these analytical dimensions of boundaries are conceptually related is open to debate (see Edgell et al. 2020).Footnote 6 But it seems obvious that the degree of their empirical alignment is crucial to determine whether boundaries are bright or blurry, stable or in flux. Consider the example of Jews in pre-revolutionary Europe, where cultural repertoires of antisemitism aligned with socio-spatial segregation and legal exclusion to produce a bright and stable boundary configuration. A key question in the study of contemporary religious diversity is therefore to assess the extent to which cultural, social, and institutional boundaries align.

An additional point merits attention to fully leverage the analytical potential of the boundary approach to the study of religious diversity in Western Europe. Whereas orthodox Barthian constructivism has tended to downplay the “cultural stuff” of group boundaries, recent theoretical contributions have highlighted the distinctive character of religious as opposed to racial, ethnic, or linguistic boundary dynamics (see notably Brubaker 2015). Indeed, inherent properties of the religious field—such as divinely sanctioned norms, organizational forms, modes of authority—shape actors’ strategies of boundary-making. These properties vary across macro-contexts; for instance, the North American literature on immigrant integration regards religious beliefs, practices, and identities as a “bridge” to the societal mainstream, whereas the European literature tends to perceive them rather as a “barrier” (see Zolberg and Woon 1999; Foner and Alba 2008; Connor 2014; Kivisto 2014; Drouhot and Nee 2019). Capturing these contextual variations requires identifying not only generic mechanisms of immigrant integration but distinctively religious micro-level mechanisms of cultural, social, and institutional boundary-making. Certainly, the demographic composition of immigrant populations matters for explaining greater contentions over immigrants’ religion in Western Europe, but so do popular attitudes toward religious outsiders and their behavioral consequences, as well as deep-seated institutional arrangements between Christianity and the State (Alba and Foner 2015, pp. 130–138).

In sum, the boundary paradigm promises to shed light on those cultural, socio-structural, and institutional processes that affect the extent to which religion operates as salient marker of difference in Western European nation-states. It thereby allows assessment of whether religious diversity is indeed posing a challenge to social integration—conceived at the collective, not at the individual, level.

3 Prejudice, Acculturation, and Symbolic Boundaries

That Muslims face relatively bright symbolic boundaries in Western Europe may sound like a truism. Across many countries, social science research shows that religious differences have increased in salience for public claims-making about immigration and integration since the 2000s (Berkhout and Ruedin 2017). Media discourses, drawing heavily on cultural repertoires that represent Muslims as Oriental, premodern, or uncivilized Others, articulate what has been variably labeled as “Islamophobia,” “anti-Muslimism,” or “anti-Muslim racism” (e.g., Dolezal et al. 2011; Korteweg and Yurdakul 2021; for a critical analysis see Bleich 2011). Emphasizing Islam’s differences from Christianity just as much as its collision with putatively European values such as secularism, gender equality, and freedom of sexual orientation, these discourses provide fertile ground for the rhetorical combination of “identitarian Christianism” with the defense of liberalism that—notably in the Netherlands, but also elsewhere in Western Europe—characterizes right-wing populism (Brubaker 2017; Cremer 2023).

To fully capture processes of symbolic boundary reconfiguration, however, it is necessary to analyse their underlying micro-level mechanisms. Two distinctive fields of research speak to this question, by studying the formation of anti-Muslim prejudices and hostilities among the native majority (Sect. 3.1), and by scrutinizing religious acculturation and intergenerational value transmission among Muslim minorities (Sect. 3.2). Read conjointly, they identify mutually reinforcing mechanisms of cultural polarization that sustain the salience of religiously coded symbolic boundaries in Western Europe (Sect. 3.3).

3.1 Anti-Muslim Attitudes in Western Europe

Building on well-established research findings on nationalism and xenophobia, several studies analyse religious aspects of nationalist exclusion and anti-Muslim attitudes above and beyond generic anti-immigrant prejudice. At the outset, it should be acknowledged that, country variations notwithstanding (Trittler 2017), Christianity has lost considerable relevance in definitions of national belonging across Western Europe. A fuzzy-set analysis of cross-national data from the European Social Survey (ESS, wave 2003), for instance, reveals that in most Western European societies religious belonging pales in importance for deciding who should be allowed to immigrate when compared with language, culture, education, and occupation (Bail 2008). That said, substantial parts of the Western European population do hold anti-Muslim attitudes. An analysis of the 1999–2000 wave of the European Values Study (EVS) finds that respondents dislike Muslims more strongly than immigrants in general, although levels of prejudice are lower in Western than in Eastern Europe (Strabac and Listhaug 2008, p. 278). Another survey, conducted 10 years later across six Western European countries, finds that a quarter of respondents distance themselves from Muslims and even that more than half oppose the Islamic headscarf (Helbling 2014).

What accounts for these attitudes? Research suggests that individual-level correlates of anti-Muslim prejudice (age, education, occupational status, right-wing orientation, etc.) are similar to those factors that predict generic anti-immigrant sentiment (e.g., Strabac and Listhaug 2008; Savelkoul et al. 2011). Likewise, contextual factors that correlate with anti-Muslim hostility (unemployment rates, Muslim group size, etc.) align with general socio-psychological theories of group-conflict and ethnic threat, although the empirical evidence is less conclusive than for individual-level factors (e.g., Strabac and Listhaug 2008, p. 280).Footnote 7 Upon scrutiny, however, there does seem to be a distinctively religious aspect to anti-Muslim attitudes. Many people perceive Muslims as being more strictly religious and hence to hold conservative gender and sexuality attitudes that are increasingly rejected by Western Europeans (for the Netherlands see Sniderman and Hagendoorn 2007, p. 22). A survey experiment that was recently (2015–2017) fielded in Great Britain, France, and Germany (as well as in Austria and Switzerland) demonstrates that it is not Muslims per se, but extremely devout Muslims who elicit negative reactions among respondents, including among secular left-leaning liberals (Helbling and Traunmüller 2020; Helbling et al. 2022). In fact, majority members who are nonreligious and supportive of civic (not ethnic) nationalism seem particularly prone to anti-Muslim attitudes, at least in strongly secularized countries in Northwestern Europe (for EVS data see Ribberink et al. 2017; Simonsen and Bonikowski 2019).Footnote 8

To sum up, research on popular attitudes suggests that Muslim immigrants in Western Europe might face a symbolic boundary that conjoins the Christianity/Islam divide with perceptions of cultural incompatibility between secular liberalism and religious fundamentalism. That said, recent longitudinal research suggests that the prevalence of anti-Muslim attitudes has slightly declined in Western (but not in Eastern) Europe over the past decade (for a pseudo-panel analysis of EVS data 1990–2017 see Bell et al. 2021).

3.2 Religious Acculturation Among Muslim Migrants in Western Europe

A quite different line of research, firmly grounded in migration studies and the sociology of religion, focuses on the religiosity of Muslim (and other non-Christian) immigrants and their children. Quantitative research, drawing on increasingly powerful national or cross-national surveys among the immigrant population, studies whether religious identities, beliefs, and practices decline over time and across generations—as acculturation theorists might expect in highly secularized contexts (for a review see Voas and Fleischmann 2012). Some variations by receiving country and origin group notwithstanding, a rather consistent pattern emerges from this research.

First, the migratory event as experienced by newcomers seems to disrupt routine religious practices such as worship attendance among Muslim (as well as Christian) arrivals, reflecting pragmatic expediencies in the early phase of adaptation (Diehl and Koenig 2013; van Tubergen 2013; see also Fleischmann and Khoudja in this special issue). Second, first-generation migrants seem to become less religious with increasing length of stay, notably if they are highly educated, if they are employed, and if the majority population is highly secularized (van Tubergen and Sindradottir 2011). The key question, third, concerns the religiosity of the second generation that has been exposed to Western European education systems, while confronting an often hostile environment. Scattered studies with survey data from the 1990s and early 2000s were rather inconclusive; some found Muslim religiosity to decline among the second generation alongside increased socio-structural integration (for the Netherlands see Maliepaard et al. 2010, p. 463; Phalet et al. 2009) whereas others suggested a reproduction of religious identities (for Great Britain see Bisin et al. 2008; for Germany see Diehl and Koenig 2009). Cross-national survey results from the 2000s and 2010s, however, converge on demonstrating greater intergenerational stability of religiosity among Muslims (and other religious minority groups) compared with mainstream Christians (for ESS data, 2002–2018, see Molteni and van Tubergen 2022). Data on adolescent–parent dyads taken from the Children of Immigrants Longitudinal Survey (CILS4EU, wave 2010/2011) in Germany, Great Britain, and the Netherlands (and Sweden) suggest a remarkable intergenerational stability in religious salience (“how important is religion to you”?) among Muslim migrant families (Jacob and Kalter 2013). That Muslim families are particularly successful in transmitting religious orientations across generations is also confirmed by analyses of the French Trajectories and Origins (TeO) survey (2008/2009), although they suggest that decreasing religious homogamy may eventually result in religious decline in the third generation (Simon and Tiberj 2013; Soehl 2017a). The most sophisticated study on the topic, also drawing on the TeO survey, concludes that it is a confluence of mechanisms—parental religious socialization and transnational cultural ties among them—that sustains the striking religious stability among Muslims across socio-economic strata (Drouhot 2021). Longitudinal studies with CILS4EU data also confirm that religious identities and practices are more stable over time among Muslim youth than among Christian youth (Simsek et al. 2019). Fourth, acculturation research shows that among Muslims—just as among religiously devout people in general—strong religiosity correlates with traditional value orientations, including above all conservative gender role attitudes and intolerance of homosexuality (for Germany see Diehl et al. 2009, Kretschmer 2018; for the Netherlands see Maliepaard and Alba 2016; for cross-national evidence see Soehl 2017b). However, more refined cross-national analyses suggest that among certain subgroups, notably among second generation female Muslims, religiosity might become increasingly decoupled from traditional value attitudes (Röder 2014; van Klingeren and Spierings 2020). A similar decoupling is indicated by research on patterns of self-identification. A meta-analysis of various survey-based studies finds that Muslim migrants prioritize their Muslim identity over that of their receiving society (Stockemer and Moreau 2021). However, identification patterns seem to be driven less by religiosity per se than by generic immigration-related factors (for France see Maxwell and Bleich 2014; for cross-national evidence see Leszczensky et al. 2020).

It is an often-noted limitation of quantitative research that the heterogeneity of Islamic religiosities can hardly be captured with standardized questionnaire items such as self-rated religiosity, worship attendance, prayer, fasting, and the like. Indeed, qualitative research draws a more multifaceted picture of the religious lives of Muslims in Western Europe, with second-generation women receiving particular attention in light of sustained public headscarf controversies. It suggests that strong Islamic beliefs are one among several avenues of personal-identity formation for second-generation Muslims who face cross-cutting pressures from their receiving society and ethnic community. Critical of essentialist notions of Islam, recent qualitative studies document the variability of individuals’ appropriations of Islam, as instantiated by headscarf-wearing “Neo-Muslimas,” who affirm their publicly stigmatized Islamic identity while strongly contesting traditional (and male) Islamic authorities (for France see Parvez 2017). Inevitably, these new modalities of being Muslim come along with claims for equal recognition and cultural belonging in public spheres across Europe (Göle 2015; van Es 2016) and thus intersect with institutional boundary dynamics (see Sect. 5.1). A similar picture emerges from ethnographic studies that focus on Islamic communities and organizations and their ways of re-negotiating religious symbols, rituals, and norms. Ethnographic research in France, for instance, suggests that Muslims’ religious identities, norms, and authority structures might become increasingly disentangled from origin cultures (Bowen 2010). Extended field-work among Turkish Islamic associations in Germany highlights the complex transition from the first generation that is transnationally oriented toward the Turkish political-religious field, to the second generation that formulates its Islamic diaspora identity in reaction to experiences with the receiving society (Schiffauer 2009); even radical Muslim organizations seem to have shifted from initial linkages within global Salafism toward civic engagement and legal pragmatism in response to domestic opportunity structures (Emmerich 2023). Comparative ethnographies of mosques in Berlin and London show how Muslims struggle to articulate new forms of piety in the face of stigmatizing public discourses (Becker 2021). Taken together, qualitative studies document complex modes of Islamic believing and belonging, thus complementing quantitative findings on the continued salience of religion as boundary marker among significant shares of the Muslim-origin population.

3.3 Cultural Polarization and Symbolic Boundary Reproduction

In identifying mechanisms of anti-Muslim prejudice among native majorities and of religious transmission among Muslim minorities respectively, the two pieces of published research provide insights into macro-level processes of symbolic boundary formation. Indeed, there is both theoretical rationale and empirical evidence to assume that the aforementioned mechanisms are mutually reinforcing. Proponents of the boundary paradigm would theoretically expect that blocked acculturation (and perceived discrimination) prompt “reactive ethnicity” and sustain cultural heterodoxy (e.g., Wimmer and Soehl 2014). Experiences of anti-Muslim hostility might thus result in “reactive religiosity” among Muslim migrants and their offspring (Diehl and Koenig 2009; Phalet et al. 2013; Güveli 2015). Indeed, a cross-national study of immigrant Muslim subsamples from the ESS (waves 2002, 2004, 2006) finds that Muslims in xenophobic contexts, measured by public attitudes toward immigration, tend to declare themselves as more religious and to attend worship more often (Connor 2010; for counter-evidence with EURISLAM data see Torrekens and Jacobs 2016). Qualitative research among Muslim women (e.g., van Es 2016) likewise suggests that individual religiosities are partly responsive to experiences of stigma and perceptions of discrimination. At the same time, the relatively strong intergenerational transmission of religiosity among Muslims, often selectively showcased by media discourse, may sustain negative prejudices among the native majority (on media effects see Boer and van Tubergen 2019).

To conclude, the social mechanisms underlying the reproduction of the religiosity of Muslim migrants and the appeal of anti-Islamic rhetoric in the public sphere are mutually reinforcing. This “dual dynamics of cultural polarization” (Drouhot and Nee 2019, p. 188) sustains the brightness of religiously coded symbolic boundaries across Western Europe. That said, first indications of declining anti-Muslim attitudes and increasing decoupling of religiosity from other value attitudes, alongside changing political rhetoric among centrist politicians (“Islam belongs to Europe”), suggest that religiously coded symbolic boundaries are somewhat in flux and might in fact become more blurry over time.

4 Discrimination, (Self‑)Segregation, and Social Boundaries

How do the symbolic boundaries just outlined relate to social boundaries? Put differently, do religious differences contribute to the reproduction of social inequalities? That majority prejudices do not directly translate into hostile behavior vis-à-vis minorities has long been established in social psychology. Likewise, migration scholars have emphasized that acculturation does not always lead directly to socio-economic integration as articulated in migrants’ labor market performance, educational achievement, and social networks. At the same time, there are various causal pathways—discrimination, racialization, stigmatization among them—through which cultural distinctions reinforce social-economic inequalities (Lamont et al. 2014; Brubaker 2015).

Against this background, a rich research literature scrutinizes social mechanisms of discrimination and (self-)segregation among new religious out-groups in Western Europe, with Muslims as the primary focus (for a review see Drouhot and Nee 2019). It is worth noting at the outset that these mechanisms focus on different aspects of religion. The first addresses religious belonging as a marker of categorical difference that prompts discriminatory behavior by natives and, thereby, potentially reduces Muslim migrants’ chances of socio-economic success (Sect. 4.1). The second, by contrast, zooms in on individual religious beliefs and practices that shape migrants’ motivations and resources, thereby potentially sustaining patterns of socio-economic (self-) segregation (Sect. 4.2). Again, there is evidence that micro-level mechanisms might be mutually reinforcing, thus sustaining macro-level processes of social boundary reproduction (Sect. 4.3).

4.1 Religious Penalties and Socio-Economic Discrimination

Informed by the definition of “integration” as declining salience of ethnic differences, migration scholars have long studied the disadvantages experienced by immigrants and their offspring in the receiving society’s stratification system, analyzing whether their minority status affects their labor market and educational positions—net of class origin (for a review see Heath et al. 2008). Along these lines, recent scholarship asks whether belonging to a religious outgroup, notably being Muslim in Western Europe, comes with additional socio-economic disadvantages above and beyond generic migrant or ethnic penalties.

As regards the labor market, several studies have consistently documented distinctive “religious penalties” among first- and second-generation Muslims (but see Koopmans 2016). Thus, cross-national analysis of pooled ESS data (2002–2012) reveal that Muslims are more likely to face unemployment than non-Muslims, even after controlling for migration status, class background, educational levels, and individual religiosity (Connor and Koenig 2015), although evidence is less straightforward for occupational attainment (Connor and Koenig 2013). Another analysis of ESS data (2004–2010) also finds persisting negative net effects of Muslim affiliation for women’s labor force participation and weekly working hours (Kanas and Müller 2021). One major limitation of survey-based labor market research is that ethno-racial and religious markers of categorical difference often overlap among the immigrant population in Western Europe (consider Turks in Germany or Maghrebi in France). The strongest survey-based evidence comes from Great Britain, where demographics of the South Asian immigrant population do allow ethnic penalties to be disentangled from religious penalties. Overall, it confirms the existence of a distinctive disadvantage of Muslims in employment (Heath and Martin 2013; Khattab and Modood 2015), notably among Muslim women, for whom there does not even seem to be social mobility across generations (Cheung 2014; Khattab and Hussein 2018). Another limitation of survey-based labor market research lies in its inability to determine whether such “religious penalties” are in fact the result of discriminatory behavior. Scholars have therefore turned to experimental methods to assess the prevalence of taste-based or statistical discrimination vis-à-vis Muslims. An agenda-setting study by Adida et al. (2016), drawing on correspondence tests designed to disentangle racial from religious discrimination, shows clearly that French employers are least likely to respond to job applications from Senegalese Muslims, when compared with those from Senegalese Christians or French natives. Cross-national correspondence tests as well as a meta-analysis of national field experiments on hiring confirm that anti-Muslim discrimination is pervasive across Western European countries (di Stasio et al. 2021; Thijssen et al. 2022). Country-based testing studies, furthermore, suggest that such discrimination is driven by perceived value incompatibility (for Germany see Koopmans et al. 2019) and notably affects devout Muslims, veiled women in particular (for France see Valfort 2020; for Germany and the Netherlands see Fernándes-Reino et al. 2023).

Compared with the labor market, research on religious discrimination in education is far less conclusive. On the one hand, stigmatization of visible Muslims, as evinced most obviously by headscarf prohibitions for public school attendees in France, but also by biased portrayals of Islam in public school textbooks, seems to impose extra burdens for Muslims’ educational attainment. On the other hand, echoing well-established findings, whereby ethnic inequalities in education are largely driven by class origin, the relatively low attainment of second-generation Muslims, as measured by grades, certificates, or school transitions, almost entirely disappears once their socio-economic background is controlled (for German CILS4EU data see Ohlendorf et al. 2017). But even if class origin ultimately overrides religious discrimination in education, persisting ethno-religious penalties on the labor market may in the long run perpetuate educational disadvantages for Muslims. It seems fair to conclude, then, that religious differences translate at least to some extent into socio-economic inequalities for Muslims in Western Europe.

4.2 Religiosity and Socio-Economic (Self‑)Segregation

But what about mechanisms of (self-)segregation that might explain Muslims’ socio-economic disadvantages? Put differently, do their religious beliefs and practices prevent particularly devout Muslims from actively participating in key socio-economic domains? On balance, the research literature finds only relatively weak, if any, effects of high religiosity on socio-economic outcomes once key background variables such as parental socio-economic status, language proficiency, etc., are controlled. As regards the labor market, high religiosity has weak to null effects on employment among first- and second-generation Muslims (with ESS data Connor and Koenig 2013) as well as among recent arrivals (for cross-national evidence see Koenig et al. 2016). Gender-sensitive studies suggest, however, that among female migrants (both Muslim and Christian) high self-declared religiosity does come with lower rates of labor market participation and fewer weekly working hours, partly reflecting conservative gender attitudes (with ESS data Kanas and Müller 2021; for the Netherlands see Khoudja and Fleischmann 2015).

As regards education, empirical evidence is also mixed; some studies find null effects of self-declared religiosity or worship attendance on school performance among Muslims (for Germany see Ohlendorf et al. 2017), whereas others report even some positive effects of praying and community involvement on math performance (for Germany see Carol and Schulz 2018). In any event, high religiosity seems not to hinder educational achievement among Muslims. The only domain where empirical evidence does, on balance, point to mechanisms of self-segregation is that of social networks. Studies drawing on EURISLAM data covering Britain, Germany, and the Netherlands find strong religiosity among Muslims to decrease the probability of developing weak inter-ethnic ties (Damstra and Tillie 2016). With regard to strong ties, the same data suggest that religiosity might increase preferences for intra-ethnic marriages that sustain processes of ethno-religious group formation (Carol 2016). Similarly, studies based on CILS4EU data reveal that in-group preferences sustain relatively rigid friendship boundaries between Muslim and non-Muslim youth, even in religiously diverse classrooms (Kretschmer and Leszczensky 2022; Simsek et al. 2022). It should be noted, though, that among recent arrivals from Islamic countries religiosity seems rather unrelated to majority contacts, suggesting once more a potential decoupling of religiosity from social tie formation (for Britain, Germany, and the Netherlands see Malipaard and Schacht 2018).

4.3 Social Boundary Reproduction, a “Discriminatory Equilibrium”?

It seems fair to conclude that religious differences continue to translate in complex ways into socio-economic inequalities, thus potentially forming new cleavages in Western Europe. As in the case of symbolic boundaries, micro-level mechanisms of anti-Muslim discrimination and religious self-segregation can be mutually reinforcing and thereby sustain macro-level processes of social boundary formation. The aforementioned study on France, combining correspondence tests with group experiments and survey data, goes a far way in demonstrating the existence of a “discriminatory equilibrium,” whereby minorities’ experiences of discrimination prompt reactive religious in-group orientation that, in turn, strengthens the prejudices of native majorities (Adida et al. 2016).

However, the stability of this discriminatory equilibrium is far from obvious. Lacking longitudinal data, it remains difficult to assess the direction of slowly changing levels of both discrimination and (self-)segregation. Moreover, Muslims’ individual religiosity seems relatively decoupled from their socio-economic achievement and, in any event, pales into significance compared with underlying class dynamics. Finally, the shifting demographic composition of immigration from Islamic countries—consider the increased skill-level of recent immigrants from Turkey—might unsettle the discriminatory equilibrium. In sum, social boundaries between Muslim minorities and native majorities in Western Europe seem slightly less bright than the symbolic boundaries outlined above.

5 Governance of Religious Diversity and Institutional Boundary Shifts

The boundary configuration that combines Islam’s differences from Christianity with its putative threat to European secular values is not only reproduced in public discourse and popular attitudes but also articulated in institutional arrangements. Reflecting historical paths of state formation and nation building, these arrangements display variable relationships between state structures and Christian churches, ranging from full-scale establishment (as in Great Britain) over regimes of selective cooperation (as in Germany) to strict separation or laïcité (as in France). At the same time, these arrangements are shaped by constitutional principles of state neutrality and religious freedom that have been reinforced by European human rights law since the post-war period (Koenig 2007; Harms 2022).

A thriving research literature at the intersection of citizenship studies and comparative secularism has described and explained the institutional regulation of migration-driven religious diversity and, more specifically, the policy responses to increased public visibility of Islam. The following section reviews more recent trends in this literature, starting with the political claims-making by Muslim minorities (Sect. 5.1), before discussing national policy responses to Muslim claims for public recognition (Sect. 5.2) and tracing transformations of institutionalized religious boundaries (Sect. 5.3).

5.1 Muslim Claims for Public Recognition

Confronting prejudice and discrimination, Muslim minorities have engaged in various struggles of public recognition across Western Europe. Extended cross-national qualitative research suggests that Muslim civic and political leaders draw upon various social movement strategies to carve out a place for Islam in European public spheres (Göle 2015), despite difficulties of overcoming ethnic cleavages and decentralized authority structures. This activism is echoed by ordinary Muslims’ civic and political participation, with religious faith functioning as a motivational source for active citizenship (for Germany see Peucker 2018). Indeed, survey data suggest that civic engagement might be particularly pronounced among the highly religious Muslims, notably among the second generation (with ESS data Just et al. 2014) and that strong religious community involvement correlates with Muslims’ political participation (for Great Britain see McAndrew and Voas 2014; Sobolewska et al. 2015). Indeed, Muslims are increasingly visible—as voters as well as candidates—in party politics across Western Europe, with variations depending on citizenship regimes and local electoral institutions (for cross-national evidence see Dancygier 2017).Footnote 9

To be sure, compared with preeminent issues such as residence permits, dual citizenship, and racist discrimination, distinctively religious claims constitute but a small portion of migrants’ generic claims-making, as a standard-setting study, based on the coding of major British, French, German, and Dutch newspapers, established for the 1990s (Koopmans et al. 2005, p. 89). But these publicly articulated religious claims have increasingly challenged existing institutional religious boundaries in Western Europe. Just as the form and content of migrants’ generic political demands do not simply reflect factual or perceived disadvantages, Muslims’ religious claims for recognition are considerably shaped by preexisting political opportunity structures and public discourses as embedded in the institutional relationships of Western European nation-states with religious organizations.

Ideal-typically, Muslims’ religious claims for public recognition take the form of exemption or parity claims (Statham et al. 2005).Footnote 10 Exemption claims, for instance, include demands for toleration of ritual slaughter practices (halal), which would otherwise be prohibited by animal protection laws, or requests to have children excused from co-educatory sports and swimming lessons in public schools. Making new religious identities publicly visible and contesting prevalent conceptions of cultural membership, these claims—also found among other religious minorities such as turbaned Sikhs contesting the obligation to wear motorcycle helmets—typically give rise to intense public controversy, as they challenge the authority of secular statehood. This especially pertains to states with an expansively defined public sphere such as France, where even the least demanding religious exemption claims are perceived as a threat to public order, as notably attested by the 2004 law banning students from wearing headscarves (or other ostensible religious signs) in public schools (Joppke 2009). By contrast, parity claims follow a different logic as they demand the equal inclusion into the more or less formalized (neo-)corporatist systems of co-operation between state and churches that still characterize most Western European states. Depending on preexisting institutional arrangements, they may center on religious education in public schools; on public funding for private confessional schools; on the recognition as corporation of public law; on participation in radio or television broadcasting councils; and the like.

Clearly, not all everyday responses to discrimination and stigma take the form of political activism (Najib and Hopkins 2019). Yet, political claims for religious rights do find substantial support among Muslims, notably those who are highly religious (for the Netherlands see Verkuyten and Yildiz 2009; with EURISLAM data Carol et al. 2015). They are articulated through various channels of political contestation, including also supra- and international arenas such as the European Union, and pressure states to shift institutionalized religious boundaries by redefining the contours of religious freedom and by extending cooperative relations to a broader range of religious actors.

5.2 State Policies Toward Islam

How have Western European nation-states responded to Muslim claims of recognition, and what explains these responses? Macro-comparative scholarship aptly describes differences and commonalities in the accommodation of religious diversity. An early study assessing the situation of the 1990s found institutional recognition of Muslims to be most favorable in the Netherlands, with its tradition of pillarization and a strong discourse of multiculturalism (Rath et al. 2001, p. 258). Another comparative study that measures the accommodation of Islam across three public policy domains—Muslim practices in public schools; funding for Islamic schools; regulation of mosque building—found Great Britain to be most and France least accommodative, with Germany figuring in between (Fetzer and Soper 2004; on British receptivity to religious demands see also Bowen 2016, p. 231). These qualitative comparisons have more recently been supported by quantitative research that has tried to find suitable measures for the accommodation of religious diversity, while doing justice to the diversity of immigrant populations and citizenship regimes across Western Europe. The most prominent effort has been the Indicators of Citizenship Rights for Immigrants (ICRI) dataset that captures not only naturalization laws, political representation rights, and affirmative action, but also includes two indicators for the accommodation of minority religions, notably Islam (Koopmans et al. 2012; Koopmans and Michalowski 2017). The first covers allowances for religious, notably Islamic, practices outside public institutions, as indicated by allowances for the ritual slaughtering of animals, allowance for public calls to prayer (azan), and provisions for Muslim burials. The second comprises state recognition and funding of Islamic schools, Islamic religious instruction in state schools, the right of female teachers to wear the hijab, and Islamic religious programs in public broadcasting. Placing various European countries on a continuum between more or less inclusive policies with these indicators, the Netherlands and Great Britain appear once more as relatively open for religious diversity, whereas Germany and France seem considerably less accommodating (see Michalowski and Burchardt 2015).Footnote 11

To account for the variable accommodation of religious diversity across Western Europe, scholars have advanced various explanations. Some have argued that church–state relationships and the general openness of the public sphere toward religion are the key causal factor, compared with the resource mobilization of Muslim minorities, broader political opportunity structures, or codes of national identity (Fetzer and Soper 2004, pp. 18, 147). However, ICRI data suggest that institutional policy legacies might pale in significance to electoral mechanisms, with pluralist policies depending on larger immigrant electorates, and smaller shares of right-wing parties (Koopmans et al. 2012). In fact, as in the broader citizenship literature, scholarship has turned rather skeptical toward stylized typologies of path-dependent institutional models for explaining the variable accommodation of religious diversity. For instance, France corresponds to the model of strict separation only partially, as the state grants certain privileges (e.g., subsidies for church buildings) to the Catholic Church and maintains regular consultations with religious communities on military and prison chaplaincies (e.g., Bowen 2010). The strongest argument against assumptions of stylized national models is, however, that they fail to do justice to those processes through which institutionalized religious boundaries have become transformed over the past decades.

5.3 Religious Politics and Institutional Boundary Shifts

Indeed, repeated interactive sequences of Muslim claims-making and public policy responses have resulted in considerable shifts of institutionalized religious boundaries in many Western European nation-states. Recent scholarship addresses such boundary shifts by tracing policy processes on various scales. Micro-level ethnographies on local renegotiations of religious diversity in urban spaces suggest that state and civil society actors creatively move beyond static national policy models (for France, see for example, Martínez-Ariño 2021). Meso-level comparisons of hospitals, prisons, and the military show that public institutions, when encountering their Muslim citizens and their claims, adapt their practical schemas of dealing with religious difference (for cross-national comparisons, see for example, Bowen et al. 2014). Finally, macro-comparative analysis traces how governments have abandoned foreign policy instruments of regulating the Islamic field (e.g., through embassies) and adopted proactive approaches to include Muslim umbrella organizations in (neo-)corporatist modes of church–state-relations (e.g. Laurence 2012), even if countries such as Turkey or Morocco, through their foreign policies, continue to influence Islam abroad (Bruce 2019).

The upshot of this scholarship is that institutionalized religious boundaries have shifted considerably within many Western European nation-states. To be sure, one still finds instances of public policies aimed at brightening boundaries vis-à-vis Islam. Thus, “civic integration” policies in Germany, France, and the Netherlands are implicitly aimed at Muslim immigrants and their putative value orientations, e.g., when governmental agencies include questions concerning attitudes to arranged marriage, patriarchy, homosexuality, or terrorism in naturalization procedures (e.g., Joppke 2007, p. 15). Another prominent instance of “liberal exclusion” is the spread of (largely symbolic) legislation against the niqab that caters to populist movements and sustains bright boundaries against what is deemed religious fundamentalism (Koussens and Roy 2013; Burchardt et al. 2019). Critical governmentality studies put particularly strong emphasis on the domesticating effects of liberal-secular politics of integration (for Germany see Tezcan 2012 and Amir-Moazami 2022). But still, the expansion of Muslims’ individual and collective religious rights seems rather striking. Despite different starting points, all Western European countries have moved considerably to the pluralist pole over the past two decades, as longitudinal ICRI data suggest (Michalowski and Burchardt 2015). Constitutional courts, the European Court of Justice, and the European Court of Human Rights stand out as major motors of transforming institutional arrangements of religious diversity, prompting scholars to interpret the integration of Islam as a predominantly law-driven process (Joppke and Torpey 2013). Since 2015, the European Commission has even established the position of a coordinator on combating anti-Muslim hatred, and similar state efforts of combatting anti-Muslim prejudice and discrimination have emerged at national levels.Footnote 12 Indeed, under both international, European and constitutional law, with their heavy emphasis on equality and nondiscrimination, liberal democracies can hardly deny parity claims to new religious actors, although exemption claims have given rise to more ambivalent judicial politics (de Galembert and Koenig 2014; Koenig 2015). It should be noted that the simultaneous decline of church membership across Western Europe has prompted a more general legitimation crisis of existing church–state relationshipsFootnote 13 and may induce novel institutional arrangements of religious diversity. In sum, moving away from historically entrenched majority privileges, while reconfiguring the contours of religious freedom, institutional arrangements display considerable shifts of religious boundaries across Western European nation-states.

6 Reconfiguring Religious Boundaries

This article has reviewed and synthesized various fields of research that have emerged in response to public controversies over migration-driven religious diversity and its challenge for national solidarity and social cohesion in Western Europe. Focusing on the new presence of Islam in Christian-heritage societies, it has drawn on a relational boundary paradigm to ask whether, as often presumed in comparison with North America, religious heterogeneity constitutes a bright boundary in Western Europe that potentially undermines societal integration. The answer to this question is far from obvious. Mutually reinforcing micro-level mechanisms of prejudice formation and religious acculturation seem to sustain relatively bright symbolic boundaries that juxtapose Christianity and secular European values on the one hand, and Islam and religious fundamentalism on the other. To a certain extent, micro-level mechanisms of discrimination and religious (self-)segregation also reproduce social boundaries, although Muslim religiosity seems increasingly decoupled from labor market achievement and educational attainment, thus allowing for the blurring of social boundaries. Repeated interactive sequences of Muslim claims-making and policy responses to religious diversity, finally, have resulted in considerable shifts of institutional boundaries, putting Islam on slightly more equal terms with Protestant and Catholic churches while reaffirming the limitations of religious freedom. It seems fair to conclude, then, that symbolic, social, and institutional boundary processes associated with religious diversity, far from being fully aligned, have followed quite distinctive logics.

This conclusion evidently faces limitations. To start with, foregrounding similar patterns and trends across Western European countries, it hardly captures cross-national variations of boundary processes under consideration. Furthermore, the article—along with the literature reviewed here—largely focuses on the Muslim population and hence neglects various non-Christian and Christian religious migrant groups that contribute equally to new religious diversity in Western Europe. These include Buddhists, Hindus, and Sikhs from South Asia as well as various currents of evangelical Christianity from Africa, East Asia, and Latin America (e.g., Vertovec 2000; Baumann and Salentin 2006; Gallo 2014). Indeed, systematic comparisons among different new religious minorities, and studies of their complex interreligious relationships (see for example, Yurdakul 2016; Nagel and Peretz 2022), stand out as major research lacunae. Yet, given the sheer size and public visibility of Muslim minorities, it is safe to assume that the research publications reviewed in this article do capture crucial boundary processes set off by migration-driven religious diversity in Western Europe.

The largest limitation is that highly specialized research literature remains mostly silent on how the three macro-level boundary dynamics dissected above concatenate over time. One could imagine three different scenarios, all of which find some, albeit variable, support in scattered empirical evidence. In the first scenario, symbolic and social boundaries would stay largely disconnected from institutional boundary shifts. Cultural polarization and discriminatory equilibria would, in other words, remain unaffected by institutional accommodation of Muslim claims for public recognition. The weak effects of integration policies upon the acculturation of Muslims (Ersanilli and Koopmans 2011), and the strikingly similar Muslim employment gaps across vastly different institutional settings (Connor and Koenig 2015) may point in this direction. In this scenario, Muslims in Western Europe may end up facing a similar boundary configuration to Afro-Americans in the USA, for whom policies of anti-discrimination and affirmative action since the Civil Rights Act have coexisted with endemic racist exclusion in education, employment, and housing (for comparison see Foner 2015).

In the second scenario, persisting symbolic and social boundaries would eventually roll back institutional boundary shifts. It is noticeable that in all four countries under consideration Christian religious rights continue to find greater support among the majority population than Muslim religious rights (Statham 2016). An increasing appeal of right-wing populist parties that, amongst other things, exploit symbolic boundaries vis-à-vis Muslim immigrants, may overturn accommodative legislation, notably in those countries in which the constitutional judiciary has relatively weak autonomy. Policy developments in France, where the Republican concept of laïçite has been redefined in expansive terms, applied not only to public but also to private domains (see Hennette Vauchez and Valentin 2014) and harnessed for the struggle against “Islamist separatism” (as in the fiercely debated 2021 separatism bill), perhaps come closest to this scenario.

In the third scenario, institutional boundary shifts would slowly lead to the blurring and shifting of symbolic and social boundaries. Providing new opportunities, resources, and frames for Muslims in their day-to-day struggles against prejudice and discrimination, institutional openings may help to achieve not only equal citizenship but also full-scale socio-cultural membership. It should be emphasized that this scenario of boundary transformation comes with extended contestations and conflict; not accidentally, Muslims tend to perceive more discrimination when they are socio-culturally integrated and live in contexts with more inclusive integration policies or highly secularist definitions of national identity (Trittler 2019; Yazdiha 2019). Some of the empirical evidence cited in this article—the recent decline in anti-Muslim prejudice (Bell et al. 2021), the partial decoupling of Muslim religiosity from other attitudes and from socio-economic outcomes (e.g., Connor and Koenig 2013; Röder 2014; van Klingeren and Spierings 2020), native support for (certain) Muslim religious rights (Carol et al. 2015)—can indeed be interpreted as initial signs of an increasingly path-dependent process of slowly shifting symbolic and social boundaries—a process that aligns with more general slow-moving trends of Western societies toward greater cultural inclusivity (see Bloemrad et al. 2019).

It would certainly be premature to fully bank on the third scenario, for two main reasons. For one, the state of research on slowly unfolding macro-processes of boundary formation is far from satisfactory. Some valuable panel surveys notwithstanding, there is not much research that traces symbolic and social boundary processes over larger temporal horizons, largely because of the lack of suitable data; the need for reliable longitudinal data on (religious) discrimination is particularly urgent. There is even less systematic research on how and when symbolic, social, and institutional processes of religious boundary formation concatenate over time. By inventing research designs precisely targeting this problem, the literature on Muslims in Europe could not only overcome existing fragmentation but also enrich the boundary paradigm theoretically. The other reason not to embrace the third scenario too easily lies in the contingent nature of social processes. They remain highly sensitive to disruptive events—economic crises, domestic political instability, international conflict, and the like—that can derail path-dependence and reactivate latent boundaries. But these two caveats notwithstanding, the third scenario would conform with broader sociological theories that emphasize that modern societies are no longer integrated through shared values or religious homogeneity, but allow for the cultivation of religious (and all kinds of other) differences. Attesting to the integrative capacity of law in modern society, the reconfiguration of religious boundaries in response to ongoing international migration would thus undo some last remnants of the confessional age in which Western nation-states were born.