Are you remaining tribal, as if you go feudal your vassals automatically join your wars, as in you raise their levies. As a tribal it's a bit more complicated. To answer your question, no, only your own religion has the modifier to defend against holy wars. However they can still join you normally, if your relations are good enough.

Gotcha, ty for clarifying! I was asking primarily in a feudal context, as I'm ideally planning on feudalizing before GHW become a concern.

Short answer, if you are a multi-pagan realm all the vassals of your religion in the realm will join as members of the holy war while the rest will not pick a side. So if you are tribal that means you can just call them in normally after the war has started. If you are feudal then you still get their levies as per normal. Which is why it can be a good idea to set up the region of your realm which will have vassals of your religion first.

Regardless I wish you luck. Having just hit year 800 or so in my own multi-pagan empire game (I know own all of Europe and have only unreformed pagans in it, it's very exciting) I know the pain of getting past the Crusade era.