Sagan's (and many non-believing scholars') problem with the concept of God is not that he is not provable, it's that you can't apply the scientific method to God at all. Not only can it not be proved, it can't be disproved (as a whole).
There are?
Back to the scientific method. You can apply the scientific method to gravity, and show relations that prove calculus works as we express it.
I don't know enough about metaphysics to say anything here.