As far as I know, Marv did not believe in God. He had little tolerance for any form of psychic comfort that involved what he considered to be self-delusion, setting a curiously high standard for a man who led a double life until his 60’s. (It seems pretty common for us human beings to cherish beliefs that reveal our blind spots and shadows, and Marv was no exception.)
But he did believe in Pooh, and in the urgency of being in this life, as it is now. And if I have ever shown any courage, it has come out of trusting that in this belief, he really did know what he was talking about.