Background Numerous studies have been done of the religious affiliations, beliefs, and practices of the Millennial generation (North Americans born between 1980 and 1995). Many of these studies assume that their findings will be useful to denominations and congregations wishing to attract more young people. But at least one influential social theory would imply the opposite: that religious decline is inevitable as societies modernize and secularize. Purpose The current essay suggests a useful addition to the religious decline theory, the work of Karl Mannheim and his followers on the sociology of generations, and applies it to the major studies of generational change and continuity in religious practice among young adults in present-day North America. Methods In total, 35 academic books, 43 articles in refereed journals, and 22 denominational research reports are included in the analysis. Only works studying the religious beliefs, practices, and affiliations of the Millennial generation in the United States and Canada, and based on data collected through approved social science research methods are included. Results The essay first summarizes the findings of the studies, noting similarities and differences in religious decline across the various social locations of race, gender, and country. Next, it summarizes the causes that the various studies have postulated for this decline, relating them to Mannheim's argument that the “fresh encounter” with one's childhood religious worldview which occurs in young adulthood is a key determinant of whether a person accepts or rejects his/her childhood religious socialization. A final section evaluates the effectiveness of the suggestions that some of the studies make for stemming or reversing religious decline in young adulthood, in the light of Mannheim's theory. Conclusions and Implications A key determinant of religious affiliation, belief, and practice in adulthood is the fresh encounter that young adults have with the culture and worldview transmitted to them in childhood. The lifecycle stage of emerging adulthood poses a challenge to denominations and congregations since it is often the age group that is least connected with organized religion. The essay concludes with suggestions for further research.