My Vassals keep turning into faithless heathens. How do I fix that?

I recently got the Monks and Mystics DLC.

I started playing as Seljuq Sultan of Rum. I was pretty confident that I'd be able to knock out the ERE way before Ottomans even appear.

But then this strange thing happened. Many of my vassals converted to another religion.

I am a very broadminded guy, I was happy to let them play with their new God until I could pass the religious title revocation law but those impudent wretches are now forming factions, making preposterous demands.

I wanted to demand their religious conversion. It seemed like a good plan, Since I was their liege for about 23+ years (Giving me the long reign opinion boost) and had good traits like just and diligent. But no sir, they are all "true believers" as the modifier tells me and I can't even make that demand. Not even after wasting good gold on gifts.

I had a few loyalists on council so luckily I was able to pass the religious title revocation law but now I am nervous. All of the heathens are being completely unreasonable and are predisposed to refuse my royal command for revoking their titles. I could force the issue, thereby expediting other general revolts too. Not to mention, ERE would want in on that as well to get back as much lands as they could, halting my conquest of the City of Dreams by decades, even centuries.

How do I convert the heathens back? or failing that, destroy their houses, stem and root without any major repercussions? And most importantly, how do I find the traitor who is behind this whole conversion madness?

UPDATE: I think I may have found out why did they become heathen. It might be action of some secret religious society which they were all members of. Now I just need to find out who's the scoundrel forming such socities and send him to his God.

Solution 1:

Not so much into M&M, but yes with secret cults those who change religion get "true believers" modifier. And why do many of them change religion is, well, because that's how they made this mechanics to work. I think many objected because it is a bit silly. It happens more often than not that new religion starts as the cult and gets mass conversion spree. Which means that new sect does faster and unchangable conversion, much faster then another strong existing religion and against strong and established religion. I saw many complaints in forums against this, since it would make more sense for new cult to gorow slowly.

For me it makes no sense, and as usual with every new DLC half of the mechanincs for new stuff is pretty much broken and not properly tested or balanced. So it's matter of preference in the end. But expect this will happen in your other games too in some kingdoms.