Like it says in the title, I'm curious how people view the Diedne and the Schism War.
My take, to start us off, actually starts with the Infernal. Hell . . . recruits. Some faeries - generally malevolent - have been drawn over the line between Realms. (For that matter, there's probably at least one demon who uses a Faerie regio to provide "visits to Hell" for diabolists.)
Then we get to House Tremere. They are probably aware that some of the entities their tradition used to deal with have become aligned with the Infernal. Perhaps the corruption of House Tytalus got started because someone called on a spirit that had transitioned.
Now we get to House Diedne. Diedne essentially brought in an entire order of magician-priests. They were the largest house by far, they stayed aloof, and just about every magic tradition at least dabbles in theurgy, if only to ward away hostile influences. Also, House Diedne was determinedly non-Abrahamic.
Hell recruits. What if some of their pantheon ended up aligning with the Infernal? Would they even notice? Noticing, would they care? The spirits in question had been theirs to call on since time immemorial.
Under this idea, Tremere was probably the first to notice that Diedne was "dealing with demons". Given their own institutional experience, they probably guessed how it happened, and pointed it out to the leaders of Diedne so that they could clean house internally. At this point, Diedne would have rebuffed Tremere for trying to shove Christian sensibilities down their throats and get rid of their time-honored gods and spirits. (In my view, "aloof" easily evolves into "insular, provincial, small-minded, and narrow-minded" when it comes to whatever you're aloof from.)
And at that point, Tremere internal logic would say, "Doesn't matter if they're too corrupt or just too proud/stupid to listen now, they're going to be corrupt and a massive threat to the Order Real Soon Now. If they won't take good counsel and prevent their self-inflicted disaster, we have to make sure it doesn't spread."
Whether Tremere or Diedne bears more blame for the disaster that followed would depend on saga-specific circumstances - how high-handed or condescending were the Tremere when giving the Diedne the heads-up? How often were the Diedne-specific Infernals being called on? How long did it take the Diedne to reject Tremere counsel? Had any Diedne moved on to calling upon demons that weren't from their culture? Etc.