Maybe the one guy's father is called Christian. So he's Christian's (son) and just likes a good bar fight if there's an opportunity. He doesn't care about the fanfiction.
They will never learn cause they're not actually fighting over religious differences. Their religion is their tribe and they're fighting the other tribes over land, resources and "honor".
That's an interesting question since so much of Judiac literature wasn't finalized until the 4th century. And there are hints that Christianity feed back into Judaism. Like for example the ordering of the Hebrew Bible, the fall of the Book of Enoch from a privileged position, the development of a full afterlife, and things about the Messiah prophecies.
I heard one of the X-Men movies had a line lifted from a popular fan creation on YouTube. Yes it was the one about the Juggernaut. So maybe that is more like what happened.
Poor Baha'i, no one even considers them worth mocking. Someone carve out a Baha'i homeland in Iran so people will remember they exist and our descendents will hate us for adding yet another skydaddy faction.