Simple. In all realms except theocracies, prevent priests from holding council positions.
Then watch either close friends become priest, or even more likely have 1 character be in the council and another character from the same player be the elder of the religion. This is one reason why penalty/reward systems built into the game mechanics can be so tricky, because once they exist a sizeable part of the player base will look for ways to get all the positives and avoid all the negatives.
Really All I see this doing is encouraging an explosion in realm sponsored religions. The state will use the religion to further their control with threats against region lords from the elders, and the religion will use the state to try and force every region lord to belong. While that would be fun in a few realms, if it becomes the de facto standard things are somewhat less fun.
