Oops, sorry -- I doubt I will write up any more of it than I have noted above. I did create it in eTools and it works pretty slick. But tested in actual play, it could be a while.
Regarding spell lists: I may discover that instead of the entire spell list, I want each cult "type" to have its own unique themed spell list. It could work kind of like cleric domains, actually.
When I was building sample characters, I found myself drawn away from blasting type spells and more toward spells that would a) help the cultist be sneaky, b) help the cultist charm/sway/compel/intimidate other people, c) help the cultist (temporarily) be "more like" the thing they venerate, d) bring appropriate summoned help. So Hypnotism yes, Magic Missile no.
Maybe that would be a productive area of discussion. The "types" of cults I have so far as noted above seem to be demon worshipping cults (when WotC's Big Book of Devils comes out that will be helpful for the LE/devil side of things), Cthulhuesque "things man was not meant to know" cults (aberration focus), death/undeath cults, and draconic cults.
I think there are general spells that any cult would find beneficial and fitting for the purposes of controlling/intimidating others, and being sneaky. So let's start with those. Looking at the first two levels of spells as a starting point, a cultist spell list might look like this:
1st--
Bane
Cause Fear
Command
Comprehend Languages (for those forbidden tomes)
Doom
Endure Elements
Obscuring Mist
Protection from Good/Law etc.
Disguise Self
Hypnotism
Confusion, Lesser
Obscure Object
Charm Animal
Charm Person
Pass without Trace
Speak with Animals
Summon Monster I or Summon Undead I (come up with a single suitable monster for each cult -- fiendish are fine for demonic cults, but what about a Cthuluesque cult?)
2nd--
Death Knell
Desecrate (maybe only for undeath cults)
Summon Monster or Undead II
Summon Swarm
Detect Thoughts
Daze Monster
Tasha's Hideous Laughter
Darkness
Hypnotic Pattern
Invisibility
Misdirection
Command Undead (maybe develop similar for Aberrations, Demons, etc. depending on the cult)
Ghoul Touch (undead only)
Scare
Alter Self
Darkvision
Augury
Calm Emotions
Enthrall
Undetectable Alignment
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And then there are lots of interesting spells from other sources that would fit right in to a particular cult.
Ultimately, though, as this is an NPC class, maybe what I want is the freedom to pick and choose any spell I want from whatever source I want, depending on the situation?