Paul Tillich in the context of Lutheran theologian

Existentialism is a family of philosophical views and inquiry that explore the human individual's struggle to lead an authentic life despite the apparent absurdity or incomprehensibility of existence. In examining meaning, purpose, and value, existentialist thought often includes concepts such as existential crises, angst, courage, and freedom.

Existentialism is associated with several 19th- and 20th-century European philosophers who shared an emphasis on the human subject, despite often profound differences in thought. Among the 19th-century figures now associated with existentialism are philosophers Søren Kierkegaard and Friedrich Nietzsche, as well as novelist Fyodor Dostoevsky, all of whom critiqued rationalism and concerned themselves with the problem of meaning. The word existentialism, however, was not coined until the mid 20th century, during which it became most associated with contemporaneous philosophers Jean-Paul Sartre, Martin Heidegger, Simone de Beauvoir, Karl Jaspers, Gabriel Marcel, Paul Tillich, and more controversially Albert Camus.

Formal principle and material principle are two categories in Christian theology to identify and distinguish the authoritative source of theology (formal principle) from the theology itself, especially the central doctrine of that theology (material principle), of a religion, religious movement, tradition, body, denomination, or organization. A formal principle tends to be texts or revered leaders of the religion; a material principle is its central teaching. Paul Tillich believes the identification and application of this pair of categories in theological thinking to have originated in the 19th century.

As early as 1845 the Protestant theologian and historian Philip Schaff discussed them in his The Principle of Protestantism. They were utilized by the Lutheran scholar F. E. Mayer in his The Religious Bodies of America in order to facilitate a comparative study of the faith and practice of Christian denominations in the United States. This is also treated in a theological pamphlet entitled Gospel and Scripture by the Commission on Theology and Church Relations of the Lutheran Church—Missouri Synod.

Rollo Reece May (April 21, 1909 – October 22, 1994) was an American existential psychologist and author of the influential book Love and Will (1969). He is often associated with humanistic psychology and existentialist philosophy, and alongside Viktor Frankl, was a major proponent of existential psychotherapy. The philosopher and theologian Paul Tillich was a close friend who had a significant influence on his work.

May's other works include The Meaning of Anxiety (1950, revised 1977) and The Courage to Create (1975), named after Tillich's The Courage to Be.

Transtheism refers to a system of thought or religious philosophy that is neither theistic nor atheistic, but is beyond them. The word was coined by either theologian Paul Tillich or Indologist Heinrich Zimmer.

Zimmer applies the term to Jainism, which is theistic in the limited sense that gods exist but are irrelevant as they are transcended by moksha (that is, a system that is not non-theistic, but in which the gods are not the highest spiritual instance). Zimmer (1953, p. 182) uses the term to describe the position of the Tirthankaras having passed "beyond the godly governors of the natural order."