It's the "local traditions" mechanic from Attila, a dumbed down version from previous interterritorial religion osmosis. It basically means that it doesn't matter what you do, there'll always be like 10 influence from the religion that province had at the start of the game.

There's a mod for that issue in the workshop, just search "1212" and "local traditions".

Be aware that this makes it way more easy for any faction (yours included) to steamroll without public order issues, or for judaism to rise because that 1 influence from the jewish building tree will be all the religious influence at play if there isn't a religious building of your own or a faction-wide modifier like Easter in christianity.

I had to install it to make a Reconquista faction playable, because they get a x2 modifier in the non-state religion presence malus which makes half the iberian peninsula an eternal public order pain in the as, specially with those early buildings that can't compete with 10 muslim influence and -24 religious turmoil.