It is easy to misunderstand Satanism, because Satanism misunderstands Satanism.
Taken on the surface, a Satanist would be a follower or devotee of 'Satan', also known as Lucifer or the Devil in most Christian belief systems. And yes, I know that many Satanists and other occultists argue over that. But just go with it for a moment.
Satan was the angel who fought with God and was cast out, to become regent of Hell.
So it would seem clear that Satanists are against God, and in opposition to Christianity. Some Satanist sects do indeed do things that would make it seem so, such as their priests wearing Catholic priestly garb (Roman collar reverse-tabbed) and performing 'Black Masses' where the "Lord's Prayer" is recited backwards, etc. An inverted cross is the sacred symbol of the religion, and a sacrament is desecrating wafers that have been previously consecrated by an actual Catholic priest. That's pretty much anti-Christian.
On the other hand, there are sects of Satanists who see themselves as more followers of Lucifer, the light-bringer, the one who offered Eve something that we all now know and value - knowledge of good and evil. Satan was not the 'bad guy' but was painted that way by Christians, according to these Satanists. Satan was just trying to help humanity and informing us that having Free Will means we must be free to exercise it without fear of Holy reprisal (judgment day), or it isn't really Free Will. Their Satan is just pointing out the lies fed to humanity by the Christian God and His followers.
And then there are members of other cults or religions (take yer pick) such as Astrum Argentum and so on, who embrace and blend the concepts of Satanism with their own, exemplified in England by men who belonged to such organisations as well as The Hellfire Club, people like Oscar Wilde and Aleister Crowley. Many of them take great delight in tweaking the noses of offended Christians, so they'll wear devil costumes, complete with horns and long tails, and such. Most of them don't dislike Christians as much as they kind of pity them, considering them weak-minded dupes or sheep.
When someone tells me that they are a Satanist, I don't really know anything - so much depends on what they mean by it. Satanism covers a lot of territory. What kind of Satanist? What group? What are your core beliefs? And etc.
"Do as though shalt shall be the whole of the law" is the single commandment of Satanism. It is used by Wiccans too - with the "An it harm none" bit stuck to the front of it. Most Pagan religions, however, celebrate Free Will in whatever manner they see it most applicable. This is in distinct contract to religions that put a God or Gods on top, and everything else below that. Satanists celebrate only one Holy Day - your own birthday.
