The nature of God is such that it cannot be proven. Otherwise, we lose the choice to believe.
Belief is not a choice.
For example, you don't have the choice to believe that you exist. You either believe that, or don't.
If I wake up one day and say today I'm choosing to believe in the Flying Spaghetti Monster, I'd be just pretending to believe in Him. Of course if I actually truly believed in His Noodly Appendage, then I wouldn't have the choice to simply un-blieve in Him anytime I like. Otherwise I would just be pretending.