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.

Christian existentialism is a theo-philosophical movement which takes an existentialist approach to Christian theology. The school of thought is often traced back to the work of the Danish philosopher and theologian Søren Kierkegaard (1813–1855) who is widely regarded as the father of existentialism.