Well Ed, if you want to think that atheism, disbelief in Zeus, and bowling are belief systems, go ahead. I think that makes "belief system" a rather useless term (not that it's terribly useful anyway).
I never brought up Zeus or bowling. That was you, Merlyn.
atheist - a person who denies or disbelieves the existence of a supreme being or beings.
atheism - the doctrine or belief that there is no God.
Maybe atheism is a disbelief system!