Theists - how certain are you that a God exists?

[robvalue]

I agree. Strong (not gnostic) atheism is wholly justified by the extraordinary nature of the claim, and the total absence of credible evidence. It's a very logical belief.

Gnostic atheism can also be justified, but it's a matter of defining exactly what you mean by "know". Under some definitions, it is also justified in my opinion, although I'm not fond of trying to argue that point.