The point you are missing is whether the atheists claim to KNOW that God does not exist.<quoted text>
Then you do not know what you are talking about.
Atheists deny the existence of (a) god. Period. It doesn't mean they have to strap a bomb on their chest and run into the St. Paul's cathedral.
Agnostic: maybe there is a god, maybe not, but there is no proof one way or the other.
But they do not attach themselves to a religion.
You can reject a belief in God without being 100% sure whether God exists or not. Such people are called agnostic atheists. I consider myself one of them.
You need to understand that claims of knowledge and belief are different. Read up on this more before replying.