James Luther Adams (1901–1994), an American professor at Harvard Divinity School, Andover Newton Theological School, and Meadville Lombard Theological School, and a Unitarian parish minister, was the most influential theologian among American Unitarian Universalists in the 20th century.

Paul Johannes Tillich was a German-American Christian existentialist philosopher, Christian socialist, and Lutheran theologian who was one of the most influential theologians of the twentieth century. Tillich taught at German universities before immigrating to the United States in 1933, where he taught at Union Theological Seminary, Harvard University, and the University of Chicago.