To understand Satanists, we must first understand Satan. Just like the fictional characters Dr. Strange or Lex Luthor, Satan has an origin story.
The first mentions of Satan appear in the Hebrew Bible, from which much of the Christian Old Testament is derived. However, there's a lot of uncertainty among religious scholars regarding what the authors meant when the word "Satan" appears in the Old Testament. The definition can vary depending on how the Biblical Hebrew term for Satan (שָּׂטָן) is translated and interpreted. In some cases, the term simply means "opponent" or "adversary" and clearly indicates a human figure, not a supernatural one. In other cases, it suggests Satan is "the accuser," or part of a heavenly legal system. There is no consensus on which references to Satan indicate human adversaries and which ones indicate a supernatural enemy of God [source: Stokes]. While the concept took many forms over the centuries, the idea of Satan representing an outsider who opposes established values is the common thread woven through all his incarnations.
The Christian New Testament contains a much clearer evolution of Satan as a single, supernatural evil being in opposition to God. He often appears at God's behest, to test humans so they display their true faith [source: Farrar and Williams]. This version of Satan is sometimes referred to as "the Satan of the Scriptures."
However, even in the New Testament there is a great deal of confusion about who Satan is. Scholars must look closely at different translations of the Hebrew and Aramaic words for "evil one" or the proper names of certain demons, like Abaddon, Beelzebub or Belial, and try to guess whether they refer to Satan or a more generic evil in context. When referencing the Bible solely, it's difficult to determine what Satan looks like, where he came from or what his goals might be.
So, where did the version of Satan with the horns, pitchfork and unbreakable contract come from? That Satan is the result of early Christianity's effort to surpass the popularity of earlier religions, plus some epic mythology created by a famous poem.