I keep seeing "Atheism isn't a belief system" posted by atheists on this forum. Agree/disagree? What is it?

It is a belief system, because if you are an Atheist, then you most likely trust science to give you your answers. This means that you must believe in science in order to trust any conclusions that it may come to. You must have to believe in something; it is impossible to be indifferent towards this matter.