I know it's easy to keep the vikings pagan, and even though historical realism is hard to obtain, and the only reason for it is fun, it seems a good enough reason to me


Also, if you send in your religious agents, I assume they spread the word, so to speak. I guess that makes it easier for one self, when/if you decide to conquer the province, as part of the population already have the same beliefs, but are there other effects?