Atheism doesn't make a claim that there is no God. Atheism simply does not accept the claim that there is a God. I am not saying that God doesn't exist or impossible, I'm just saying that at this time I'm not convinced there is one. The default position there is no God. Someone some time ago made a claim: There is God. So the two responses are: I believe you (makes you a theist) and I don't believe you (makes you an atheist). As an atheist I am not saying: God cannot exist. Saying that would force me to present an evidence or to defend my claim. That's why the burden of proof is on those who make the claim that God exists. I am simply skeptical about evidence that has been presented so far by those who claim to "know" there is God. There is nothing arrogant about saying I don't believe you. I do agree with you, however, that saying "There is no God" without presenting evidence is a bit arrogant.
Probably nothing - I will seize to exist. And if there is God - I am not afraid, I am trying to live my life as a decent human being anyway, regardless of God or no God.