Malevolent au where young Arthur got indoctrinated in a cult. And the weird way jarthur plays out under that circumstance.
Because, to be frank, Arthur does seem like the type to be susceptible to joining a cult. Rather, he's someone who could be easily targeted.
Maybe, in this au, Bella and Faroe both died in childbirth. Yes, Arthur blames himself for not being there, but bear with me. He didn't get to spend time with Faroe. All he knew was that he was afraid of having a child, raising a child, being responsible for a child. He couldn't ever imagine doing it on his own. When the intrusive thoughts hit him again, he catches himself wondering whether he's relieved he doesn't have to deal with it. He knows he didn't want a married life, so that means he must be glad they both died? He can't tell intrusive thoughts apart from regular ones, this poor soul.
So, see, he has enough sources for guilt and doubt, but he hasn't been hit hard enough to get that hatred for the "god has a plan" agenda.
A married life wasn't exactly what he wanted, but it was, at the very least, something stable. Something he could belong to. Something he could use to define who he is. Now he's off track again. If only there a was a tight knit community who accepted him and made him one of their own.
It would be very funny if it's not a cult that worships a powerful, existing god, but a regular irl-like off-brand christian cult. That's just centered around a guy who's good at making shit up.
And if John still ended up in Arthur's head, Arthur would start freaking out thinking it's his cult's god's divine messenger.
And John would go with, it because it granted easy control and submission.
