There seems to be very little player agency in the fervor of their religion. I'm not saying it should be easy, but there should be more tools available, which could even have potential to go wrong.
In this case, I suggest that the head of a faith be able to single out a particularly virtuous follower to celebrate. This should cost cost piety, and maybe even prestige, to increase the fervor of the religion, based on how good an example of the faith the chosen person is. There would need to be a limit on the frequency of this ability's use, and potential for it to go wrong. Perhaps if this person ever does anything religiously improper (or is discovered to have done so) then this backfires and costs fervor.
In this case, I suggest that the head of a faith be able to single out a particularly virtuous follower to celebrate. This should cost cost piety, and maybe even prestige, to increase the fervor of the religion, based on how good an example of the faith the chosen person is. There would need to be a limit on the frequency of this ability's use, and potential for it to go wrong. Perhaps if this person ever does anything religiously improper (or is discovered to have done so) then this backfires and costs fervor.