He teaches twentieth-century American history and has special interests in religion, ethnoracial history, and American intellectual and cultural life. His first academic monograph, Tri-Faith America: How Postwar Catholics and Jews Held America to its Protestant Promise (Oxford University Press, 2011, now in paperback), charted the decline of the idea that the United States was a “Christian nation” and the subsequent rise of the notion that the country was premised on something called “Judeo-Christianity.” The book then showed how acceptance of America’s “Judeo-Christian” premise altered the country, in places like education, collegiate fraternities, suburbs, and the census, all during what were the supposedly conformist 1950s.