I am an atheist because there is absolutely no reason to believe an all-powerful deity created the Universe. There just isn't. Religion and its roots clearly demonstrate that it's all a big scheme. It has devolved into nothing more but a ploy to obtain money. Even before that, it did nothing but cause unnecessary violence.
The fact that somebody can even convince themselves that there's such a thing as Satan, or Hell, or Heaven for that matter, is astounding. What is so hard to grasp about life? We are born, we live, and then our body stops functioning, then we die and aren't conscious anymore. There doesn't need to be a magical world that we get transported to forever after.
That said, perhaps there is some form of spiritual force in the universe, it's impossible to be certain. Then again, it's also impossible to be certain that there isn't a flying teacup orbiting the Earth that is invisible and completely undetectable. But I remain confident that we simply are, and that we were not created by a cosmic overlord who created a set of rules for us to follow, lest we burn in fire forever.