Atheism is a belief system, just like religion. Belief in the non-existence of God is unsupported by any evidence. After all, how do you prove a negative?
Rational skeptics, or Agnostics, whatever they're called--that is, those who believe only that which has been proven by science--do not believe in either the existence or non-existence of God.
For the record, I'm a Catholic, slighly observant. I choose to believe in the existence of God. Just as an atheist chooses to not believe in the existence of God. Neither proposition can be proven. Well, not quite. I suppose God could strike down those atheists who say, "strike me down if you exist."