How do immigration and birth rates affect religious demographic shifts in America?
Executive summary
Immigration and birth rates are the two demographic levers most strongly shaping which faiths grow or shrink in the United States: immigration disproportionately increases the representation of non-Christian faiths and bolsters Catholic ranks in particular, while differential fertility and generational retention determine growth within groups already present [1] [2]. Together with religious switching and assimilation, these forces explain much of the recent shift away from a majority-Christian country toward a more religiously diverse, and in important segments more secular, society [1] [3].
1. Immigration as a supply-side reshaper of religious markets
Immigration changes the religious composition largely by bringing into the U.S. higher shares of adherents to religions that are underrepresented in the existing population: adherents of Islam, Hinduism and Buddhism make up a larger share of new arrivals than they do of the resident U.S. population, and immigrants account for large shares of those non-Christian groups [1] [4]. At the same time, immigrants remain a substantial source of Christians in the United States—Pew finds a majority of foreign‑born adults identify as Christian even as their Christian share fell between 2007 and 2023–24—so migration both diversifies and sustains Christianity in different ways [3] [5].
2. Birth rates: who reproduces matters more than overall fertility decline
National births are near record lows, but fertility varies sharply by religious commitment: more religious Americans have higher fertility than the nonreligious, creating a widening “religious‑secular” fertility gap that favors growth among devout groups absent offsetting switching [6]. Demographic models used by Pew and others show that differential fertility rates, when combined with migration and switching, substantially affect long‑run shares: groups with higher net reproduction require less immigration or conversion to maintain or grow their shares [1] [7].
3. Assimilation, religious switching and family transmission blunt simple expectations
Immigrants do not always reproduce the religious profile they had on arrival: levels of religious practice and affiliation change after migration, influenced by family size, time in the U.S., and social integration—research finds the number of children is a key predictor of post‑migration religious attendance, and generational patterns show second‑generation immigrants often diverge from their parents’ religiosity [8] [3]. Pew’s scenario work likewise stresses that switching—people leaving or joining religions—can dwarf the effects of fertility and migration in shaping future shares, meaning immigration can slow but not fully reverse secularization absent other forces [1] [7].
4. Projections: immigration delays, redirects, but doesn’t erase broader trends
Long‑range demographic projections from Pew and the Congressional Budget Office show that net immigration will be the primary driver of population growth in coming decades even as overall births decline, and that immigration disproportionately bolsters non‑Christian groups’ size relative to what they would be without migration [9] [1]. However, modelers caution that the trajectories are sensitive to switching, shocks (war, policy shifts) and enforcement regimes—Congressional Budget Office updates note that policy changes affecting removals and migration flows alter population size and therefore the pool from which religious change can occur [9] [10].
5. Uneven patterns, political stakes and competing narratives
The religious effects of migration are not uniform: Catholics have been reinforced culturally and numerically by Latin American immigration even as their internal ethnic and social profile changes, while refugees and low‑skilled migrants tend to be more religious overall whereas high‑skilled immigrants skew more secular—patterns that fuel competing narratives about whether immigration “saves” religion or accelerates secularization [11] [12] [6]. Commentary and advocacy sometimes overstate single causes: some analysts emphasize immigration as a bulwark of Christianity, others highlight the rise of the unaffiliated among immigrants and natives alike—Pew’s own reports show both dynamics are occurring [2] [3] [13].
6. Bottom line: interaction, not a single cause, explains religious change
Immigration supplies people whose affiliations differ from the established population, birth rates determine which affiliations persist across generations, and switching and assimilation mediate both—together these interacting processes produce the observed decline in Christianity’s majority share alongside growth in non‑Christian faiths and the religiously unaffiliated [1] [3] [6]. Projections and policy matter—changes in migration policy, refugee flows, or cultural transmission could speed, slow or redirect these trends—but current evidence points to a multifaceted process in which immigration and fertility are necessary but not sufficient explanations [9] [7].