I'm on my fifth playthrough of the game, 597 hours in total. The first four playthroughs were as various Pagan and Muslim faiths. I never once saw anything about a heresy appear in game.
This time around I decided to start as a Catholic and establish a new Christian faith. I've already been given two separate prompts asking me if I want to convert to the Cathar heresy, and then later my new Christian faith was declared a heresy and about six random rulers throughout Europe decided to convert to my faith without any input from me.
Seems like heresies are a big deal when you start as a European Catholic but I had no idea they even existed before this. I tried reading up on it and it seems like heresies occur when Fervor is too low.
Does anyone know if heresies can occur in other religions? I usually don't pay close attention to Fervor, so I'm not sure if it's a Christian-exclusive mechanic, or if Catholicism just happens to be one of the only faiths with such a swingy Fervor score.
That makes sense. I'm curious to see what kind of heresies would appear for various Pagan or Eastern faiths.
From what I read, the Fervor threshold for heresies is 40%. I also saw a mod online that causes Fervor to gravitate toward a lower value, so that most faiths' Fervor doesn't stick at 90%-100% anymore. I'll have to look into it some more. Thanks!