"Hail Satan?" (2019)

[Belacqua]

There were no self-identified Satanists before LaVey. Satanism, as a philosophy, was not a thing before LaVey. Before the CoS, Satanist was used like an insult, comparable to "pagan" or "heathen." If you're so puzzled by all of this, visit the CoS website. Read The Satanic Bible and The Satanic Scriptures.

A useful source that might be less self-regarding than the site of a particular sect:

Satanism, Magic and Mysticism in Fin-de-siècle France; R. Ziegler

on Amazon here

Maybe you're using the term Satanist in a very specific sense, to refer only to a certain type who claim that they are the true Satanists and those who came before don't deserve the title. This would be in keeping with different versions of other religions who say they are the true and only group that deserve the title, while their predecessors and competitors are fakes. 

I'm using it in the way that the sources I refer to above use it: more generally for groups of people who worship Satan. Of course there are variations, but I'm not devoted enough to one flavor to deny others use of the title.