Religions
From Crusaderkings
The following religions exist within the game.
1. CATHOLICISM
2. ORTHODOX CHRISTIANITY
3. SUNNI ISLAM
4. SHIA ISLAM
5. JUDAISM
6. PAGAN
Note that while the game was originally modeled to simulate Christian medieval kingdoms (and these are the only "official" playable ones) there exist mods to play as all territories and religions.
One issue that does exist with religion is the fact that Jewish provinces presently have no means of converting over when imported to future chronological games.