I think it's a stretch to call atheism a religion, and even more of a stretch to say Religion is only a set of beliefs. While that is somewhat true, There is more to religion than that
religion n.
1. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
2. A personal or institutionalized system grounded in such belief and worship. The life or condition of a person in a religious order.
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion.
I used to think I was agnostic, but then I decided that I was being wishy-washy and finally decided that I don't believe in a God(s) or an afterlife. If that makes me an atheist, that's fine, but it's not a set of beliefs that I choose in order to be an atheist.
Don't get me wrong, I still have morals and I do believe in right and wrong. But Right & Wrong DO NOT equal Good & Evil. I was raised as a christian, and I got a lot of good things from it, but it just always seemed like a lie/crutch/excuse to me. And not a single person I went to church with seemed sincere about it. They would do wrong ("sin") things with the thought that they would be forgiven for it by default. They used religion as an excuse to do wrong. I just can't buy into it.