What do Atheists believe?

Dilbert creator is quoted as saying the following about atheists:

This brings me to atheists. In order to be certain that God doesn’t exist, you have to possess a godlike mental capacity – the ability to be 100% certain. A human can’t be 100% certain about anything. Our brains aren’t that reliable. Therefore, to be a true atheist, you have to believe you are the very thing that you argue doesn’t exist: God.

It's an interesting train of thought that lead me to conclude that atheists don't actually not believe in God, it's just that they don't WANT to believe in God....

Why they don't want to is probably the question that is harder to answer.