Satan first is mentioned in the Book of Job, used by the Jew to describe an angel of their god who acts in the role of quality controller, testing the people of Israel through adversarial hardships such as famine, disease and invasion.
When the Christian religion came into existence, they used Satan, Satanic, and Satanists to mean anything that was contrary to their religion, thus a Jew, Muslim, Cathar, Pagan or Scientist could be dumped in the same bucket as a follower of Satan. Satanism never existed until Christianity invented it, and there was no movement that identified itself as Satanists until the 1880’s.
From around the 1740’s Satanism became associated with an extreme form of hedonism by wealthy individuals in the Hellfire movement. In the 1880’s Satanism as a movement came to media attention after lurid stories appeared about black masses involving upside-down crosses, stolen consecration wafers, sexual orgies, and reversed versions of the Christian masses in several locations in France.
For about another hundred years Satanism was pursued by small groups of people pursuing a similar form of Satanism as that in France in the 1880’s. In the 1960’s Anton Lavey decided to create an atheistic version of Satanism and defined his interpretation of Satanism in the Satanic Bible and other publications, forming the Church of Satan. Arguments within the Church of Satan saw several further groups emerge such as Temple of Set under Dr Michael Aquino.
The centralistic and militant form of atheism expressed by Church of Satan including Social Darwinism caused Satanists to form alternative versions of Satanism, the most successful being Satanic Temple in 2014.
It has only been since the 1960’s that Satanists have collectively taken ownership of Satanism away from Christianity and Hollywood, forming their own definitions of what it means to be a Satanist, and self-identifying as Satanists. Regardless of if the Satanist follows a spiritual or a rational version of Satanism, all agree that the path embraces the primacy of the will and choice of self over group conformity, and a love of life.
Modern Satanists separate themselves from those that either promote or pursue Christian or Hollywood definitions of Satanism of harming people and animals by referring to them as Reverse Christians. Since the era of Book of Job, the idea of Satan and Satanism has been an evolving process, and it is only in the last 60 years that Satanism has been clearly defined into a positive version contrary to that of Hollywood and Christianity.
There is currently an epic fight going on between Satanists such as Satanic Temple and those such as Christians over the ownership and definitions of what it means to be a Satanist, a battle that is being won thanks to modern Satanists taking an active role in defining their Satanism rather than allowing outsiders to define it for them.