In order to state categorically and absolutely "there is no God" one must know for sure He's not sitting on some rock on the back side of Jupiter's 3rd moon. In fact, to make the claim "there is no God" one would have to be able to make an exhaustive list of everything that exists in the cosmos and show god is not one of those items. That, of course, would mean the person making the statement and the list is All-Knowing, and, therefore, God himself. It is a basic axiom of logic that one cannot deny his own existence.
The statement "there is no God" is therefore self-defeating, and logically impossible.
One may say he is 99.9% sure god doesn't exist, or he hasn't seen evidence or he is unsure. (agnostic) But, no one can ever truly make the absolute statement "there is no god" (atheist)
