One can't help but discuss religion, or religious theory and structure at any rate, when discussing this, so I hope the following doesn't offend anyone: The "Do As Thou Wilt" that is the primary tenet of the Church of Satan doesn't serve to bind it as an organization, as you might imagine - EVERYONE in a group with that as their only law believes THEY have a connection to "The Source" or whatever. The only way to earn rank in a group like that is through merit - service to the group, impressive demonstrations of skill, etc. I don't know if that code of behaviour would work for what you're talking about - usually cult leaders work off of having their flock believe they have some special connection to "the source" or some such. In which case, he'd likely have at least few rules for them, and possibly some of them would be just for them to prove they obey and believe in him.
I know its ironic, but a "Satanic" cult usually isn't Satanic, at least by their church doctrine. And if you think back on most cults that have ended up being recorded for the abuse of their members, and thus worthy of the term "cult", most of them have been usurptions of some other, more valid faith. The Jonestown victims were (trying to be) Christian. As were the Branch Davidians. The Church of $cientology (or, at the very least, the corruption within that organization) is a demonstration of the perfect way to use the good ideas in Dianetics in an exactly backwards way against people. And the Hale Bopp-ers? Well, who knows exactly - but neither they nor Do (their leader) were Satanists.
What I'm getting at is that probably the best way to work it, IMHO, would be to have the cult leader use a Good or at least neutral faith and say that he has received special revelation and has been chosen to lead a new following (probably indicating it to be separate from the older "corrupt" following.) He might start with some sort of minor ability with an arcane spellcasting class, using that to convince his new flock of his "truth". (Possibly his spellbook would be disguised as a special book of Holy study that was given unto him.)
I can't remember where I've seen them, but there ARE rules somewhere for empowering deities based on their followings - if I remember correctly (and admittedly I might not, as I haven't seen them in a while and didn't have a purpose for them in mind when I did) with as few as 15 devoted followers, a "god" can begin to bestow 1st level spells to a single Cleric. Maybe someone else remembers where the rules on that are?