I know some folks really love their atheism, but talking about adding it to something like CK2....It's just stupid.
We are playing in a time period when religion (the Abrahamic faiths) were starting to bring people together. Paganism didn't work if you wanted to unify a large group of people, like a kingdom. There were many gods, compared to Christianity's ONE god. One god, and through that god kings were given the power to rule over everyone else, or so they said. I'm sure many monarchs and people from all sorts of walks of life would roll their eyes during sermons or while having to deal with the clergy, but that's probably as far as it went. As a king, the church establishment was part of what gave you your power as a king.
I remember learning of how Scotland wanted to be anointed with a particular Holy Oil so badly, the mark of truly being recognized as a king. They were denied more than once by the Pope, but did eventually get it. Scotland had a king, but it's like he wasn't REALLY seen as a king. A country needed that centralization that a king and kingdom brought. With atheism, that wouldn't have worked in those times. It would have been more like paganism, the grand unity would be lost. French, English, German, whoever, you were all Christians, and all part of something bigger than your kingdoms.
Imagine telling people about your ideas of atheism.... You'd in a sense be saying that you wanted no part in this grand operation that was Christian Europe, because you knew better and there was certainly no god so everyone should just stop the charade. You'd get no where. If you were trying to establish some sort of Atheistic movement, you'd be stopped immediately. They would try to convince you to stop talking that way, or to just keep it to yourself or anything to get you to keep you from getting yourself killed.
I'm an atheist myself I suppose, but I can see the purpose of religion and what it did for medieval times. I think it's extremely outdated and should be discarded NOW, but back then it was almost essential. I'm speaking of Christianity here, I don't really know how deeply the Islamic establishment and their secular powers mixed.
"By the 13th century it was expected that a king would become king by being crowned and anointed. As well as being placed on a throne, he would also be anointed with holy oil by an archbishop and have a crown placed on his head. The holy oil was believed to make the king sacred by making him into a new person through the grace of God. The kings of England and France had used this ceremony for centuries. By the 13th century, however, if a kingdom did not already use coronation and anointment, it had to ask the pope to allow its kings to be crowned and anointed. The king of Norway was given permission in 1247. Requests to the pope for Scottish kings to be crowned and anointed were made in 1221, 1233 and 1251, but were rejected due to English protests that this would go against the king of England‟s claim to be overlord. The pope finally gave permission for coronation and anointment in 1329."