Social identities, as those pertaining to religion, may change over time as adolescents make the important transition into young adulthood. This 4-year longitudinal study examined developmental changes in religious affiliation, identity, and participation across the transition from adolescence to young adulthood among 584 individuals (from M (age) = 17.9 years to M (age) = 22.1 years; 55 % female). We also investigated whether changes varied as a function of individual (i.e., gender and ethnicity) and contextual (i.e., college type and residential status) factors, as well as the association between religiosity and well-being (e.g., meaning and purpose in life, depressive symptoms). The results indicated a significant decline in reported affiliation with a particular religious group or faith for all youth. The change in a psychological sense of religious identity varied by gender, and the change in religious participation differed by ethnicity, but other individual-difference factors generally played minor roles in the changes in religiosity across time. Religiosity was more consistently linked with a greater sense of meaning and purpose than with fewer depressive symptoms across the transition to adulthood, suggesting that it may be particularly important for eudaimonic well-being. Overall, the findings suggest that youth generally experience a decrease in religiosity as they transition to young adulthood, but this rate of change may vary between individuals. The results have important implications for the way in which religion is viewed and lived out by young adults in the United States.