True enough. That's certainly the problem with trying to "prove" God to skeptics. It's also the problem facing atheists, which was my original point.
Theologian Karl Barth had a good discussion on the difficulty of "proving" God. The difficulty lies in the finding evidence (facts within the universe) about a God who is "bigger than" the universe. It's an example of the blind men and the camel -- and for this reason God's revelation of Himself is the necessary means by which He can be known.
"Church Dogmatics"?