sAtAnIsM
The following is basic info, and this doesn't necessarily reflect the views of the Church of Satan. I'm not a member/representative of the CoS, nor am I a Satanist. However, I respect their beliefs and did the best I could at putting it in a way everyone could understand. If you want any more info, or more detailed info, then go to www.churchofsatan.com.
To start, I think I need to clarify something: satanism is NOT devil-worhipping. Sure, that's what it'll say in dictionaries... because dictionaries want simple definitions, ones they can put in 20 words or less. But satanism is not a simple philosophy.
Another reason that satanism has been basically an underground philosophy is that they don't really advertise it; people have to come to them to find more about it. So what is it exactly?
To me, satanism stands for individualism, rebellion, non-conformity, and embracing everything that Christians consider a "sin." Satanists try to live life to the fullest. They mind their own business and do not preach. Nor do they make sacrifices, because there is no one to sacrifice to! In that way, I believe that Satanists are atheists too. The main difference as I see it is the satanist's belief in magic. But, put to rest any notions you might have about Satanists running around killing people. They love and embrace life. Serial killing is not promoted or advocated by LaVey (founder of the CoS) or the CoS. All those stories you hear about on the news about "Satanists" killing animals are not true because those misguided people aren't satanists; they're usually devil-worshippers.
I do not believe in Satan as a deity, and I don't believe that any true LaVeyan satanist could consider Satan to be real. I think that LaVey intended for Satan to be an archetype for the ancient Hebrew god who represented sin, individualism, rebellion, pride, etc.
Additional Information:
Brief Info on the Church of Satan and Anton LaVey
A Chaplain's Handbook -- A Historical Text Outlining the Religious Requirements and Practices of the Church of Satan
Texts as copyrighted by Lavey:
The Nine Satanic Statements
The Eleven Satanic Rules of the Earth
The Nine Satanic Sins
Pentagonal Revisionism: A Five-Point Plan
