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.

Karl Paul Reinhold Niebuhr (1892–1971) was an American Reformed theologian, ethicist, commentator on politics and public affairs, and professor at Union Theological Seminary for more than 30 years. Niebuhr was one of America's leading public intellectuals for several decades of the 20th century and received the Presidential Medal of Freedom in 1964. A public theologian, he wrote and spoke frequently about the intersection of religion, politics, and public policy, with his most influential books including Moral Man and Immoral Society and The Nature and Destiny of Man.