For religion, there are various ways to do it, depending on how strong you want to be and how the nations/cultures spread.
You can do it KenNES Gods and Man where belief in god(s) creates an all-powerful god. More gods means less power but more uses. So you can have players start with a chosen god/parthenon and have them spread.
You can also do it like culture spread, where people of a similar culture are much more likely to have the same religion. Also, look for religion/culture compatability. Such as North's End of Empires.
Finally, you can just have Religious Str in each country and various bonuses there. (Pilgrimage ect)
don't forget to give poweful religions their own stats under their description! Powerful religions have incomes, and can direct diplomacy and pilgrimage sites from a hierichy (depending on the religion)