Gabriel Honoré Marcel was a French philosopher, playwright, music critic and leading Christian existentialist. The author of over a dozen books and at least thirty plays, Marcel's work focused on the modern individual's struggle in a technologically dehumanizing society. Though often regarded as the first French existentialist, he dissociated himself from figures such as Jean-Paul Sartre, preferring the term philosophy of existence or neo-Socrateanism to define his own thought. The Mystery of Being is a well-known two-volume work authored by Marcel.

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.