The religion system of CK2 doesn't really work with the backstory of LI. Specifically, the system seems to be set up to reflect the Abrahamaic religions with the penalties for non-state religion in provinces and the usage of missionaries. However, this isn't really a fit for the various Classical religions present in the mod, or the timeline which seems to have featured more religious regionalism than OTL.
My idea is to have the Hellenistic religions (Solar, Greco-Roman and Mithraic) act like the Indian religions in vanilla CK2, so they will have less effective missionaries but fewer interfaith relations penalties.
Additionally, a new Reassert Authority CB could be added, which allows a ruler to vassalise de jure vassals by force- allowing the ERE to quickly reclaim their provinces without changing the religious setup.
Finally, religions without strong missionary traditions should not be able to use Holy Wars.
My idea is to have the Hellenistic religions (Solar, Greco-Roman and Mithraic) act like the Indian religions in vanilla CK2, so they will have less effective missionaries but fewer interfaith relations penalties.
Additionally, a new Reassert Authority CB could be added, which allows a ruler to vassalise de jure vassals by force- allowing the ERE to quickly reclaim their provinces without changing the religious setup.
Finally, religions without strong missionary traditions should not be able to use Holy Wars.