The only true sign of
intelligence you will encounter from another human being.
A theist, or believer in God, will tell you that he ABSOLUTELY exists. An atheist (non-believer) will say he ABSOLUTELY doesn't.
Well,
the last time I checked, it's been like 2000 years since God was reported to have talked to anybody, making it pretty difficult to prove. Besides which, all religion is predicated on faith (See also: Not supposed to be proven).
Prove he doesn't exist? Possibly
the only thing harder to do. Besides the enormity of the task, there's the fact that anyone who ever took a logic class knows and that is that you can't prove a negative. You can only prove (sometimes) that something hasn't happened YET. It may still the next time.
So... you have a theist, an atheist and an agnostic... None of them really knows the truth about the nature of existence.
But only the agnostic is smart enough to admit it.