Satanism is a different religion to Christianity. It has almost entirely different beliefs and a different history. Its usage of words is different to that of Christianity. Satanism is not the worship of the Christian Satan, because then Satanism would be part of Christianity. In order to believe in the Christian Satan, you have to be a Christian. Satanists are not Christians, their beliefs are different. The Satan of Satanism is different to the Satan of Christianity. Just like when the Christians say "prophet", they do not mean Muhammad or Zarathustra, they mean a Christian prophet because the others are not from their own religion. When Satanists say "Satan" they do not mean the Christian Devil. Satanists do not believe that Satan fell from heaven, because, that is a Christian belief. There is no heaven to fall from, no God to do battle with, nor a hell to fall to. These are all ideas made up and used by Christians, and are not part of Satanism.
This can't be repeated enough. Satanism is a different religion to Christianity the same way that Islam, Buddhism and Pantheism are all different to Hinduism. They're not only different a little bit, they're simply completely different to each other. They're not comparable. When asking Satanists about their religion, start with finding out what they believe. Because firstly and foremost, Satanism is not Christianity and Satanists do not believe in the Christian Bible and its stories, not Islam or its Koran and not Hinduism or its pantheon of daft gods.
You don't have to take my learnéd word for it. The present High Priest of the Church of Satan, Peter Gilmore, explained it in 1992:
“This material sums up the rudiments of Satanic philosophy. It certainly has nothing to do with the prevailing Judeo-Christian outlook of altruism and self-sacrifice, and can thus seem quite alien and frightening to one brought up in that worldview [...]. There are no elements of devil worship in the Church of Satan. Such practices are looked upon as being Christian heresies. Believing in the Christian world-view of God vs. the Devil and choosing to side with the Prince of Darkness is pointless to the Satanist as neither exists [...]. Satan is not a conscious entity to be worshipped, rather it is a name for the reservoir of power inside each human to be tapped at will.”