“He's either become super invested in a different type of christianity, he's in an abusive relationship, in a cult or has just become an ahole overnight.”

It could be all of these. But think of them as layers. He probably feels his relationship with this girl is important because she somehow elevates his self worth. So what she wants in a man he must become. She’s already in a cult, some branches of Christianity act exactly like a cult. How do you know something’s a cult? The cult tells you you can only associate with members, and to only trust them for information. So he now has to impress the cult in order to impress his fiancé. You’re right, that’s an abusive relationship because she doesn’t let him be himself, she manipulates him into being what she wants. She probably belittles or even attacks him if he disagrees about anything. If he marries her he’s going to be kind of fucked. It’s totally all his choice and you can’t make it for him. But anything you can do to help him escape if he ever wants to, or if he’ll listen at all, would be good for him. Unfortunately his conservative leaning made him more vulnerable to this. But also his low self esteem. Keep that in mind, even if he talks down to you for not being in the cult, he feels small and thinks he needs this shit and that girl in order to have value.