It is well known that Sartre describes his form of existentialism as atheistic, and much of the rhetoric of Sartrean existentialism draws off the image of God's absence from the world. There are nevertheless, I argue, deep grounds for thinking that the coherence and well-groundedness of Sartre's thought requires that his phenomenological ontology take finally the form of an onto-theology: Sartre's ontology runs into difficulties concerning the origin of the for-itself and the unity of being; an onto-theology like Schelling's, which avoids the ‘ontological optimism’ that Sartre objects to in Hegel, both releases Sartre's ontology from its difficulties and furthers Sartre's central philosophical purposes.