Hi,
Not clear that the Diedne care.
Not clear that being Christian is incompatible with being a Diedne. Also, the Schism War was not waged to convert the Diedne to Christianity. It is not clear that the Diedne were any less Christian to begin with than your average Tremere or Tytalus. The Diedne were more secretive, so continuing to be secretive is hardly a loss.
Not clear that the Diedne care. They never tried to convert the Order before, or convince them to wage an anti-Crusade. Furthermore, Diedne magic depends less on vis than that of other magi. If the Diedne believe in metempsychosis, as the Druids were said to, then big Longevity Rituals are less necessary: You're going to come back. They emphasize spontaneous over formulaic magic, so are less likely to cast big rituals. They need a library to boost Arts, AL and Phil, but can do fine without much of a lab.
So if the Divine encroaches, and makes life more difficult for their enemies, the Diedne should care, why?
Ah, now that's a good question! My preference leads toward initiations that help them make the most of their next (or past) lives, toward abilities that help boost their magic when burning enemies alive (preferably in a wicker cage, naturally), and toward abilities that benefit spontaneous magic.
Not necessarily either. Do the Diedne care about dominating or rejoining the order? I think the real question here is what the Diedne wanted, as a House, before the Schism War. If such an objective existed, it probably had little to do with the Order, which is probably why they were aloof to begin with. Adding a new objective to destroy the Order does make sense, either for vengeance, for justice or for simple pragmatism.
Or being part of something other than the Order. I'm not sure that the Diedne have the skills to, say, best the Tytalus at their own game. Or remain entwined in the Order without a bunch of paranoid Tremere noticing. Or actively deceive House Guernicus for 200 years. Acting against the Order at a great distance, ok. But actually running the Order makes the other magi utter idiots.
I don't think it's necessary to have any Diedne survivors, and if there are survivors I don't deeply need them to be organized. But if the Diedne have survived as a House, I prefer them to remain in Mythic Europe and prefer them associated with the Church. Where better for scholars to hang out? If you do not want magi casting spells on you, who better to be than a churchman? And if your Parma resists such a spell, won't they likely attribute it to your obvious holiness? And if you do cast a spell (without words or gestures, of course, unless you have managed to pick up Holy Magic), isn't that probably just a minor miracle?
This works fine if you like your Diedne infernal, or prefer them as secular as any other magus. It works less well if Diedne must have the Pagan flaw. But if the Diedne can handle a bit of syncretism, they might actually influence Christian practice, even as they themselves evolve.
So when the Church starts making life difficult for magi, this is just the Church doing what it does, and has absolutely nothing to do with the Diedne, who are long gone with nary a trace. Nothing to see here, folks. And if there seems to be an uptick in book-burning, that's regrettable, even barbaric, but hardly unusual. And if heretics are burned, and not just their books, that's certainly not a repackaged Diedne ritual, just a sign of the times.
Anyway,
Ken