Atheist is just another word for "arrogant". How can we as inconsequential human beings say for a fact there is no God. When we don't know anything, really.
Atheism means that you don't believe there are any deities, since there is no evidence suggesting there are. It does not mean that you
know none exist*. Just that you see no reason to believe in them, and so you don't.
* Yes, there are atheists who do claim to "know" this, but IMO they are almost as delusional as the people who claim to "know" that deity X
does exist (although the latter is certainly even more unfounded).
Even Mr. Atheism, Richard Dawkins, doesn't claim to know there is no God. Here's his scale, on which he places himself (and I would place myself) as a 6, "de facto atheist":
1. Strong theist. 100 per cent probability of God. In the words of C.G. Jung: "I do not believe, I know."
2. De facto theist. Very high probability but short of 100 per cent. "I don't know for certain, but I strongly believe in God and live my life on the assumption that he is there."
3. Leaning towards theism. Higher than 50 per cent but not very high. "I am very uncertain, but I am inclined to believe in God."
4. Completely impartial. Exactly 50 per cent. "God's existence and non-existence are exactly equiprobable."
5. Leaning towards atheism. Lower than 50 per cent but not very low. "I do not know whether God exists but I'm inclined to be skeptical."
6. De facto atheist. Very low probability, but short of zero. "I don't know for certain but I think God is very improbable, and I live my life on the assumption that he is not there."
7. Strong atheist. "I know there is no God, with the same conviction as Jung knows there is one."
Anyone who would place themselves as a 1 or a 7 is unjustifiably sure of their thing, IMO.