Hello Koval!

There's plenty of information for you in the guides section of this forum, there are quite a few people who try to keep the WRE pagan.

If a character comes of age, there is a certain chance he follows a certain religion. For the WRE that is 10% chance for pagan, 90% for christian. This is the average trigger. Another trigger is the religion of your faction leader. He gives a 75% chance that the new family member will follow his religion. That's the only way I know to influence family members' religion.