One of the most significant recent changes in American religious congregations is the shift toward more enthusiastic worship practices. Despite how clear and significant this trend is, the processes by which worship services have become more enthusiastic remain unclear. I address this question using linear decomposition techniques with data from the National Congregations Study—a nationally representative repeated cross-sectional survey of American religious congregations. Drawing from theories of population ecology, organizational adaptation, and institutions, I identify two distinct processes by which the population of congregations is incorporating more enthusiastic worship practices: (1) new congregations that are disproportionately nondenominational or evangelical are entering the population with highly enthusiastic worship practices, and (2) older congregations across all traditions are changing their worship to incorporate more enthusiastic practices. Because most congregations are older, the second process accounts for most of the change in the population. These findings help advance our understanding of how the population of religious congregations is changing in the 21st century.