Last time, I wrote about trashing the cleric class, giving that spell list to magic users, and adding in some mechanics for mystic rituals. If you haven't read that article, please do. This post won't make much sense otherwise.
Anyway, a really obvious question comes up, what if just-some-guy performs a ritual?
Well, just-some-guy has to make a Saving Throw vs Spell with a -1 per-spell-level penalty. He is, however, entitled to a bonus based on his ability score modifiers for Charisma, Intelligence, and Wisdom (these could of course be another penalty). Failure results in something like a permanent, irreversible Charm Person being laid upon the would-be Sorcerer. He/she would always view everything about the deity/entity/spirit in the best possible light. (This is where incongruously enthusiastic Slime God cultists come from.) Failure for spells of Level 3-4, results in a Geas (GM's choice as to what it's about) and a Charm Person being brought down on the guy/gal. Failure on spells of fifth or higher levels, result in complete removal of the unfortunate would-be Sorcerer's mind; he/she is now simply a meat-puppet controlled by the entity. Rolling a 1 on any of these Saving Throw checks, results mind-shattering insanity.
Magic Users don't have to make this check for spell rituals of a level they can cast, because their rudimentary training is sufficient to shield their brains from alien influence. (Drawing forth the strange energies of the Feywode seriously rattles the old brain-box.) If, however, a Magic User is attempting a spell ritual at a level he/she is unable to normally cast, the Saving Throw check applies.
And now I have contrived a mechanical reason for weird, slavish cultists following the inscrutable demands of alien entities. Enjoy.