That's actually... a pretty good idea. I just wanted "spell-like abilities" instead of actual spells - but I guess I could just make "those spells" less flashy in practice. I'll have to work on that.
I have to admit, I cannot claim any originality in this suggestion. It is how RuneQuest, which Legend is a variant of, has been doing this for years. In fact, if you want a whole raft of groups and spells tied to them by theme and purpose, get "Cults of Glorantha" for MRQ2 (Mongoose RuneQuest). It uses this rule set and offers many example religions and sorcery orders to use as inspiration. And I believe its still only 1 dollar for the pdf!