Americans are increasingly polarized by a variety of metrics. The dimensions, extent, causes, and consequences of that polarization have been the subject of much debate. Yet despite the centrality of religion to early discussions, the analytical focus on America's divides has largely shifted toward partisan identity, political ideology, race, and class interests. I show that religion remains powerfully implicated in all dimensions of American polarization, and sociologists must once again make religion more central to their analyses. After outlining research on American polarization, focusing on the role of religion, I survey findings within the burgeoning literatures on cultural transformation processes, (White) Christian nationalism, complex religion, and Americans’ attitudes toward science in order to underscore the centrality of ethno-religious identities, religious demography, and religious institutions for both shaping and exacerbating various forms of polarization. Lastly, I propose an agenda for elucidating religion's ongoing role in understanding polarization beyond public opinion research at the macro-, meso-, and micro-levels. Though polarization research has been dominated by political scientists, leveraging religion in our analyses—not merely as a sui generis variable, but as a site of complex social behavior—facilitates novel sociological contributions to these literatures via our relative attention to multiple levels of analysis, theoretical eclecticism, and methodological fluidity. Expected final online publication date for the Annual Review of Sociology, Volume 48 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.