In my current game I am the king of Sweden and have reformed the Norse faith. I have access to the Jomsvikings holy order and have raised them to help out in a holy war against a Slavic duchy. However, in either sieges or battles the holy order does not fight, in the same way as if I was attacking a member of my own religion. Is this intentional? Which religions will Norse (or other pagan) holy orders fight against, or under what conditions will they fight generally?
From some searching about, this appears to be intentional. Holy Orders will willingly fight Infidels (people of different religious groups) and Heretics (people of heresies of your religion). They will not fight people with whom you have Religious differences (people within your religious group who are not Heretics of your religion). For the Jomsvikings, this means they will fight any and all Christians, Muslims, Jews and Zoroastrians and they will also fight followers of the Old Norse heresy. They will not fight other Pagans (Slavics, Romuva, Suomenusko, Tengri, etc.)