Well, I for one can prove, beyond reasonable doubt, that G-d exists...
First though, you can't question or discount someone's belief anymore than they can set it before you as concrete fact.
When I was growing up, I went to religious school at my synagogue every Sunday. One week the Rabbi took questions from the students. It just so happened that there was a thunderstorm outside that day at that moment.
Eventually he got a question that asked if G-d really existed (these question were all hand written and pre-submitted).
After he had read the question allowed... but before he could answer it, there was a bright flash of lightening, followed by a loud rumble of thunder. Let's put it this way... after that he did not bother answering the question.