Within Jean Paul Sartre's atheistic program, he objected to Christian mysticism as a delusory desire for substantive being. I suggest that a Christian mystic might reply to Sartre's attack by claiming that Sartre indeed grasps something right about the human condition but falls short of fully understanding what he grasps. Then I argue that the true basis of Sartre's atheism is neither philosophical nor existentialist, but rather mystical. Sartre had an early mystical atheistic intuition that later developed into atheistic mystical experience. Sartre experienced the non-existence of God.