Since the topic has come up I'd like to share an idea for this I've never seen before.
If you're a region lord and a priest, you get a new option "found schism". You can only found a schism of a religion you belong to.
A schism of an existing religion functions similar to its parent religion with a few exceptions.
First, anyone in the parent religion can switch to the schism at a temple of the parent religion. The idea is they've gone to the temple to profess their faith to the reformation.
Second, region lords with temples will have the option "Rededicate temple" which can change an existing temple to be of the new schism. Rededicating a temple will cause unrest but not as much as closing or destroying one. In addition when a temple is rededicated a random percentage of peasants will convert to the schism.
This link between the parent religion and schism works both ways allowing for an ongoing conflict between the competing sects. In the case of multiple schisms all sects are to be considered on equal footing. For example if religion B is created as a schism of religion A and Religion C is created as a schism of religion B, a member of Religion A can join either B or C as well as rededicate temples.
