The god proposition is supported not by fact, but by faith. At the end of the day, the final word that the religiously-disposed have is to say that “It is so because I believe it to be so,” before covering their ears. For them, this trumps everything.
This is the hook that caught Descartes when he confronted the issue, and then backed off very quickly. “I think,” he said, “Therefore I am.” This is fine. He is self-aware therefore he is sure he exists. He cannot be entirely sure that he exists as he perceives himself or that anything that is around him is as he perceives it, but he does make a very convincing argument, based on the progression of rational logic, that it is so (and thus takes several hundred pages to confirm what any pragmatist already knows. But that’s an aside.) However, when confronted by the idea of God, God must exist, he says “Because he cannot imagine a world in which he does not.” Oops.