There is a European folklore motif about the Black School of magic. Students went, even aspiring priests and ministers, and, when they were done, the devil took one of the students in payment for the whole class.
I think it has to do with the ability to read and write. That was seen as very powerful magic by the great masses who couldn’t do either. Priests, who could read and write and presumably communed with holy and possibly unholy powers, were seen as authorized wielders of magic.
Lots of folklore also sees churches as places where trafficking with supernatural powers occurs. On Sundays, it is hallowed and safe, but it is a terribly dangerous place on Saturday nights, where entities dangerous to humans hold services.
I’m one of the very few who thought the Dresden Files TV show was actually better than the books. But the books are darned good, so no prob! They depend fairly heavily on order; you shouldn’t just pick 'em up any whichway, but read them in formal canonical order.
Fritz Leiber’s Fafhrd and Gray Mouser stories are very good. Alas, they don’t get better as they go on: they get worse. Notably. Pronouncedly. Strongly. The first stories are brilliant. The next few collections are…good. Then…less and less and less, until the very last offered an ugly whips-and-chains lesbian bondage and sadomasochism exercise. Very sad. But, for goodness sake, explore the early stories! You won’t find better!
Just a note here: I read the Dresden books out of order for a bit, then finally found a copy of Storm Front and went thru them in order. And I found going out of order to be awesome! I was constantly reading something that made me go “Aha! That’s what they were talking about in that other book!” or something similar. I find lineareality to be over-rated most of the time, tho and am one of a small(ish) number of people who enjoy a “show don’t tell” type of narrative structure.
The term hedge wizard comes from “hedge witch”, a term used in folklore for the cunning folk or local herb-doctors who also use spells and charms to heal the sick.
In fantasy literature, a hedge wizard or hedge magician is generally a wizard of low ability, usually self-taught or with a low education background as opposed to the common examples of being apprenticed to a mentor or studying though a structured educational system. Some fictional backgrounds identify them more with rural than urban backgrounds. In the novels of Mercedes Lackey, the term is derogatory, describing a character as incompetent, uneducated, of lower social standing or of lesser power. It is similar to calling someone a hack writer or a slob but specific to practitioners of magic in these stories.
In role-playing games and video games a hedge wizard is usually a weaker wizard encountered when still at a lower level. With a limited number of spells and lacking in power they are normally easy to defeat. In the Ars Magica roleplaying game, a hedge wizard is any mage not of the Order of Hermes, who claims the largest monopoly on power.
IIRC, Ars Magica hedge wizards occupied an odd place in the system, being playable as either a companion character or as your magician character. While in theory the hedge wizard should be less powerful than the hermetic mages, the gap could be overcome to a large extent with a careful build. Also, since the game centers around hermetic strongholds, the hedge wizard inevitably ends up associated with a “school” of sorts, even though he doesn’t study the hermetic arts.