Doubt is not the opposite of faith; it is one element of faith.

It is the most centered act of the human mind. It is tempting to conflate revelation with the norm but we must keep in mind that revelation (whether original or dependent) is not an element of the structure of systematic theology per se but an event. This is because the Christian message claims a priori that the logos “who became flesh” is also the universal logos of the Greeks.

Theologically he is best known for his major three-volume work Systematic Theology (1951–63) in which he developed his "method of correlation" an approach of exploring the symbols of Christian revelation as answers to the problems of human existence raised by contemporary existential philosophical analysis. Among the general public he is best known for his works The Courage to Be (1952) and Dynamics of Faith (1957) which introduced issues of theology and modern culture to a general readership. Paul Johannes Tillich (August 20 1886 – October 22 1965) was a German-American Christian existentialist philosopher and theologian.