I think I hear you now: The PC benefits; what's in it for the NPC?thank you, but I read the wiki before I posted my question.
My post's entire point was, wiki only mentions effects religion has on the PC, and the fact that you the PC can convert your slaves to your religion.
But, why would you bother converting your slaves if it has no effects on them?
well... I guess it would adjust your penalty/bonus for interacting with them. as you get a bonus for rolls to convince them to do stuff if the same religion and a penalty if enemy religion.
Take for example church of the moon.
It gives influence bonuses and tax discounts if the PC follows this religion.
Are there any effects at all on slaves if they follow this religion?
...And I don't have an answer to that.