Most of the time it's just Bacchus' boiz, which I let off with a branding because satanic cults are cool and I might get into it at some point myself, but two other cults are proving themselves a problem. I don't even have to lift a finger to convert new provinces, as somewhere along the line I picked up a bloodline that increases my religion's conversion rate even more, so now my augur/seeress spends all their time hunting heretics. So I'm well into the late game as an immortal Roman Emperor, with reformed Hellenism (proselytizing, civilized, temporal, can't remember the second doctrine I picked) and prettymuch all my provinces are converted.