To expect an atheist to have a *positive* belief about the non-existence of God is silly.

As an atheist, I go as far as to say: "No worthy evidence has ever been produced. Because I have no evidence in favor of, I maintain that there is no such thing as God. I could be wrong--but I believe there is no God."

The only epistemologically truly knowable thing is self-referential knowledge. Otherwise it's always possible we are in a matrix.