Satanism and atheism have one common principle: The idea that you can do what you want to without fear of retribution from a higher authority.
Satanism and humanism, mixed together, would be a good way to describe atheism. Satanism says, "do as thou will" while humanism says "man is God/man is the center of infinity". So, in reality, atheist is Satanism plus humanism (or a better way to put it would be: Atheism = Satanism + Atheism, or possibly even Satanism + Humanism + Evolutionism/Darwinism = Atheism).
You guys judge if it is true or false... don't judge the person.