[gfodor]

I would guess most atheists would not say that they are certain a God does not exist. They certainly wouldn't say they can prove it to be the case. The path that leads you to atheism is the lack of evidence in favor of an unfalsifiable claim, that God exists. So the claim "God does not exist" is kind of meaningless, since it relies upon the definition of God, which is something atheists do not have a definition for since they do not have a belief about what that word means.

[Rooster61]

Speaking of weasel words, you use the word definition in place of concept. Atheists do have a concept of God. Otherwise the argument of what an atheist is would not only be moot but would in fact never arise as an argument.

[zaccus]

How can you be certain that 0 times any other number will always equal 0? Have you tried multiplying it against every number? Of course not, that's impossible. Does that count as uncertainty?

So I'll put it this way: I am as certain that there is no God (narrowly defined as an omnipotent being) as I am that 0 times any other number will always equal 0. The reason is because omnipotence is a logically inconsistent concept, and is thus impossible. That's as certain as I can be about anything.