One might say that there are two kinds of atheist -- those who believe there is no God and those who don't believe in God.  I suppose the first could be said to have "atheism" as a sort of religious opinion, but those who are just not convinced there is a God have it as simply an opinion based mainly on lack of good reason to think there is one.  Detailed looks at history and at physics and astronomy and the evolution of life is sufficient to show that the heavens most certainly do not declare the glory of God.

It entertains (and irritates me) no end that theists persist in insisting atheism is a religion.  If they would get over that they would be more persuasive but as it is it just convinces me they are not thinking at all clearly.  I suppose what is going on is they want to see it as some sort of choice we make in religions, but that is not the case at all -- our beliefs are either rational or based on indoctrination from childhood, and some overcome the indoctrination and some don't.

I noticed the comment above that belief in God in many cases is based on fear of death -- either for ourselves or our loved ones.  This is powerful, and I can see where it would create a strong desire to believe, even in spite of there being little evidence.