Paul Tillich(1886-1965) was, along with Karl Barth
, one of the greatest theologians of the twentieth century. Serving as a chaplain in the army during the first world war he was shocked, and became very pessemistic about the likely course of European culture.
Thus, he became one of the founding members of Religious Socialism, and and interested in Marxist philosophy. Between 1919-1933 he worked in universities, but like Barth, as a critic of Hitler, he was forced away. He then went to work in American universities.
The central theme of his theology is a desire to link the events of everyday life with the revalation given through Christ. He drew influences from many spheres, including expressionism, psychoanalysis and Kierkegaard. He wrote many books, the most famous of which are "The Courage to Be", and "The Dynamics of Faith". He completed the massive "Systematic Theology" just before his death, and it has become a very influential work.