Satanism isn't a religion because their members do not believe in heaven, hell, god, or satan. They use the symbol of satan to serve as a stark contrast to Christian teaching. The main idea is that Christian teaching mortifies the flesh, making the spirit all important, but to a satanist, the flesh is all there is, so mortifying it is to mortify human existence entirely. Therefore, the counterpoint Satanism provides is total respect and total expression of human nature.