The widespread assumption in American politics that core social identities—such as religion, race, and ethnicity—shape politics but are, themselves, largely impervious to political influence. This book challenges this conventional wisdom by looking at religion, a social identity whose political relevance is routinely discussed in both academic and non-academic circles. The book demonstrates that individuals' partisan identities, which solidify in young adulthood, can influence religious decisions later in life. The findings refute the claim that America's current polarization is solely the product of religious sorting into the political parties, with seculars supporting the Democrats and the devout joining the Republican ranks. Instead, partisans also help produce this religious-political polarization, as Democrats select out of organized religion and Republicans select into it. The book sheds light on religion’s role in politics by highlighting politics' role in religion and offers an important step in understanding how politics influences group identification.