I believe that we couldn't possibly know for sure that a God exists unless he gives some concrete unmistakable proof of his existence. Of course, I cannot disprove the existence of a God, because for all we know, he could exist. There is really know way to accurately prove or disprove anything with regards to religion because we do not know enough. Did God make humans or did humans make God? I don't think we can ever be 100% sure. There is just too much that we humans are incapable of understanding.
So I'm Agnostic, and I will always be Agnostic. Well, at least until God proves himself.