If we shouldn't expect evidence in either case then the probability of God's existence is just the prior, right? How could P(God) be above .5? I can't imagine thinking that the existence of an omnipotent, omniscient and benevolent being who answers prayers and rewards and punishes the sins of mortals with everlasting joy or eternal punishment was a priori more likely than not.

I wonder what variety of first cause argument he's making. Even if everything must have a cause that does not mean there is a first cause and the existence of a first cause doesn't mean the first cause is God. Aquinas made two arguments of this variety that actually try to prove the existence of God, but they require outdated categories and concepts to even make.