Satanism does, obviously, have roots in Christianity. But this does not make Satanism a form of Christianity, any more than Christianity is a form of Judaism. Satanism is not just a "Christian heresy", any more than Christianity is just a "Jewish heresy." Most Satanists do not believe in Christian theology. Most Satanists have their own interpretations of who/what "Satan" is, and do not believe in the Christian God. (There are many different kinds of Satanism, with a variety of different interpretations of "Satan.") Most forms of Satanism derive ideas from other sources besides just Christianity, just as Christianity derived ideas from other sources besides Judaism.

Some wiccans define all non-Christian religions as forms of Satanism.

No, Satanism, be it Modern or Traditional,(Modern being the atheistic branch, Traditional being the theistic branch), is not a sect or form of Christianity. We may have some of our beliefs roots or at least relations to Christianity, but we are not Christians any more so than Jainists or atheists are.

I beg to differ. Most Theistic Satanists I know are quite vocal about their belief in both the Biblical God and Satan. It also seems paradoxical to call oneself a Theistic Satanist and not believe in deities...

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"The Lord can make you tumble, the Lord can make you turn, the Lord can make you overflow...but the Lord can't make you burn." - Randy Newman

Why wouldn't Satanism be a form of Christianity seeing as Lucifer was a fallen angel?

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"If God were alive today, he'd be an atheist" - Kurt Vonnegut

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Theistic Satanism does indeed have strong roots in Christian theology. Just about every other sect of Satanism does not recognize Christian dogma. The term "Satanism" is used merely in its capacity as an archetype. Many others choose to use Prometheus, Melek T'au, En.Ki, etc.

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"The Lord can make you tumble, the Lord can make you turn, the Lord can make you overflow...but the Lord can't make you burn." - Randy Newman

I thought it was prince EA and not EN.KI ?