I've actually heard of it. Though interesting, not going to bother.
Manicheans, Albigensians and Bogomils- all decried as "heresies" by the Catholic church, and all very cool and worth reading up on.
It's interesting to note that the word "catholic" (here pron. c'th-OLL-ic ) translates roughly "of the people"- and that's how the Church saw themselves, as a monopoly, and anything that threatened that monopoly was crushed without mercy, and often in quite "unchristian" manner.
Mont Segur, and other towns like it. The Albigensian Crusade. That was (arguably) the start of the fall of the Church as "holy" in the eyes of much of Europe, at least by any educated who did not buy into the dogma.
But in AM, don't confuse the big G with the Church- one is power, the other merely taps it in their own fashion (and to their own ends, in their own fashion). And opposed to the "god is the universe" view many espouse today, the god of the AM church is much more concerned purely with humanity and worshipers. The wilderness, certainly the fae and even less civilized folk - not so much.
Hohum... You could go with Terry Pratchet's view in Small gods... there are PLENTY of gods out there, all with their own influences and strengths waxing and waning with worship.
While it might sound silly when you consider it is from a book of Comedic Fantasy... Think again about the implications?
Each "god" would have a might-score IE be finite and thus not impossible to best. They would also be oposed by other gods that might have other agendas. And best of all... They'd most likely be unable to touch the world directly unless in a very strong Regio of some sort.
This path basically makes all gods into very powerful faeries. Folllowers might still get to partake of their "might and glory" but in a less powerful way.
A good place to look for ideas would be the old game Popolus from Bullfrog.
Certainly true. But at the sametime it is a mistake to think this the only possible reaction of the church. Even if being less spectacular or bloody the church also took in a lot of lay religiosity, adapting practices and dogma to changes in religiosity, using and expanding on non-dogma traditions. Whole dissident movements becoming sanctioned from Rome and integrated into the papal hierarchy.