Agnosticism - the lack of knowing whether or not God exists.
The claim that "God does not exist", is just as much a claim of knowledge as the theist belief that "God exists".
You do well to remember that absence of evidence is not evidence of absence. You cannot conflate atheism with agnosicism. The agnostic makes no claim. You as as an atheist have made the claim. Now, justify your claim epistemilogically and ontologically.
That would be "strong" atheism, whereas "weak" atheism is simply the lack of belief in god(s). I subscribe to the latter, myself...I don't *know* that a god or gods do not exist, but I have no belief that they do.