While Manichaeism and Existentialism would seem to be two very different topics, they are intricately connected in Mailer’s work. Though as a young man Mailer maintained atheistic beliefs, his ideas shifted in the mid-1950s, and his evolving theory of existentialism became intricately tied to his developing spiritual ideology, which by the 1960s was shaping most of his writing. While he borrowed ideas from famed existential theorists like Kierkegaard and Sartre, Mailer formulated his own unique brand of existentialism, one that included the possibility of a God. The crux of existentialism in Mailer’s mind, as this chapter explains, was the ability to face down the unknown with courage, which in turn meant confronting the Manichaean idea that an imperfect God was constantly at war with the Devil.