The most realistic religion on the planet. It preaches belief in oneself as the ultimate god. While true Satanism has nothing to do with Christianity(except to point out Christianity's obvious flaws), it does reference it a lot in order to clarify its own belief system.
Basically the main difference between Chritianity and Satanism is that Christianity preaches that you should harm no one, not even your enemies. Satanism, on the other hand preaches that you should harm no one unless they cause, or intend harm to you, in which case, you should kick their fucking ass.
The only real flaw in Satanism is it's name. If it were called something else, it might be more widely accepted by other people.
Satanism is a great religion, but would be a lot more popular if it was called something else, like maybe Humanism...