That's certainly true, whatever classical druids really were. I'm willing to suspend disbelief enough to accept a secret tradition existing out in the countryside (the pagus), particularly if said tradition really did have powerful magical secrets to hand down.
I'm also willing to believe in dark underground traditions such as that of Guorna the Fetid.
Educated Roman city dwellers worshiping Mercury in the manner of a thousand years earlier is one step too anachronistic for me. However, this does sound exactly like the sort of stories a Medieval might tell about "pagan" antecedents.
Or at least vaguely Christian, with Gnostic or Hermetic leanings. That sounds like the religion of a sorcerer to me and such philosophies were certainly available in the 8th century.
I like the idea of the Diedne as Golden Bough style pagans, with all the trappings of 19th century belief. I was going to develop them along those lines long ago, but with a Gallic flavor rather then the Ye Ole Misty Isles background of the current edition.
It is indeed amusing to frame it that way! The Schism War then becomes like the friction among late 19c/early 20c Hermetic groups, as they break apart and snipe at each other, except our sniping is enhanced by CrIg!
This brings to mind the possibility of Steampunk Ars Magica, featuring the Hermetic Order of the Golden Dawn, in which House Crowley and House Waite go at each other....
(BTW, I usually do frame it almost that way: Even Mythic Europe has myths and folklore that simply are not true. Since nearly the beginning of my time with AM, I have seen the "Roman" traditions of the Order lacking any continuity with real Roman traditions, in the same way that in our world, Rome and Britain were not actually founded by heroes of the losing side of the Trojan War. Mercurian magi in Rome? Myth. Magi who do not explicitly attempt to challenge their mythologized history are never going to discover this, the same way that our own myths about history do not give way easily. AM5 canon took a very different and certainly interesting approach.)
The Schism War ended with the event that was intended to end it. The old romans who created the Curse of Thoth ritual (HoH:TL 76) never included a warning: "Do not sacrifice a tribune taken by treachery" since it was obvious to them what would happen. With the war in the balance (HoH:TL 41) the inner Magvillus Council enacted the ritual with the thirty-seven most senior House Guernicus magi. The great covenant of Magvillus and everyone & everything in it at the time was reduced to ashes. With no clue what happened the rest of the Order decided the event was either an Act of God or an act of the Diedne. And either way it was time to stop fighting.
Literally.
Naples has miles of underground labyrinths, and quarries, and storerooms, and catacombs, etc, dug out over millennia, due to the highly convenient volcanic tuff underneath the city. Rome is similar.
Until the 20th Century there was a sort of Cult of the Dead still active under Naples, as Christian families adopted bones from old catacombs to act as focus for prayers for relatives who died far from home. Until the local Bishop forbade the process.
It is hard to imagine sufficient alliances to make a difference. Yes, Merinita and Bjornar were close, and might have been allies if the opposition were not overwhelming, but Diedne had a history of making enemies.
More likely, as I think @Ezechiel3571 suggests, the treachery (ritual of Thoth and whatnot) of Tremere and Guernicus was exposed, turning everybody against them. That would probably not make them friends of the Diedne, but if not a friend, the enemy of an enemy is a useful aid. This could eradicate Tremere and Guernicus.
That is very conceivable. Loch Leglean, rather than Hibernia, was Diedne's stronghold. The victory was close as canon tells the history, and it is not a stretch to have them survive holding Loch Leglean, and if they could do that, maybe Hibernia too.
I find it unlikely that they would take Stonehenge as well though. The opposition from their old enemies in Ex Miscellanea would be too strong.
But absolutely, Hibernia+Loch Leglean is very plausible. To continue on that thread, maybe the Diedne then allied with the Order of Odin (or whatever hedge wizard gave rise to the myth of the Order of Odin), to form the Order of the North. After a couple of generations, spreading Hermetic theory and Parma Magica in the new Order, they would be a real match even for the recovering Order of Hermes. A century of Cold War follows.
Allowing any group which has access to their roots of power (Parma Magica and their unified theory) to exist can easily become a death blow for the Order. It is what has allowed them to grow powerful, expand, and effectively control the magical side of the area they inhabit. While expanding and growing powerful they have created both a fair amount of enemies and those who resent them.
If the Diedne would not surrender and rejoin the Order, then the rest of the Order had little choice but to destroy them. To do anything less would result in an eventual alliance of their enemies with access to the secrets of their power.
So for Diedne to win, the rest of the Order would have to be destroyed or absorbed into them.
In theory Diedne could find allies outside the order, bringing Parma Magica to the table, and possibly actually founding an order of Odin in the North with which the Order now has an uneasy truce...
That's what the Oath dictates, yes, but it is a rather simplistic view on politics and warfare.
The war was close, and with every house present in every tribunal, it was fought everywhere. It is quite plausible that, defeating the Diedne on the Continent would see the martial houses of Flambeau and Tremere, and maybe Tytalus, nearly eradicated. If, now, Diedne prevails in the Loch Leglean and Hibernia, do you see Jerbiton, Bonisagus, and Crimon rising to arms to go chase them out?
Diedne too would be weakened, and while Europe is quite small (as Timothy Ferguson points out in his podcasts), a surviving fraction of Diedne would hardly be able to conquer it all.
No, at near extinction, most magi would have more serious matters to attend to than upholding the Oath.
At some point the war would stop being a war for dominance and start being a was for survival, and as John Ringo wrote- in war for survival simply surviving is a form of victory.
The Diedne made a pact with the Manx Dragon, and relocated the Domus Magnus in 1011. The supposed fall in 1012 was a hoax. The Isle of Man was also a good vantage point for a new alliance with the Norse nobility and wizards, and they started building new covenants on Iceland, Greenland, and Vinland. It took two hundred years to fully integrate their magic with the Norse traditions, but now they are ready to strike back.
History shows it's a bit more nuanced. At a certain point one has to settle for the nil all draw.
It's fair to say the rise of Protestantism is the same kind of all encompassing existential threat to the Catholic church's power base as Diedne's survival would be to the Order.
The Catholics tried, but at a certain point had to give up and concede Protestantism had to be tolerated.