Americans are an enterprising people, and we start churches like we start businesses. (It is not always possible to tell the two apart.) There are more than twice as many distinct religious communities as McDonald’s restaurants in the United States, and eight times as many religious congregations as ZIP codes. The diversity of American creeds and the comity among their adherents is remarkable: The West Texas city in which I was raised was dominated by white Baptists and brown Catholics, but we had everything from staid Methodist congregations to foot-washing Primitive Baptists, holy-rolling Foursquare and Full Gospel churches, a tiny congregation of Latin-loving sedevacantists led by a discalced Franciscan (try that on a...