the reformation comes after centuries of schism, plague, climate change and the collapse of medieval civilisation. The Shattering of Europe into the thousand shards of protestantism comes as a result of generations of degeneration, no one alive in the 16th century remembered a time when Europe was United. No-one in the middle ages could even imagine what it would be like if it wasnt.
There were heresies which were around for centuries and in defined territories, and they didnt break from the church. The word Catholic meant Catholic, and all sides considered themselves Catholic, there was no concept of nation or nationalism, Europe was in CK2 terms, one realm with no kingdom authority and no executive or government but one realm. It is not inevitable that a heresy would break completely from the Church, it was inconceivable. It would not happen, neither the orthodox nor the heretic could even imagine that it might nor would he suffer it if he did.
On the whole if it sat for centuries with defined borders it wouldnt sunder, it would be come old news. It would stop being heresy, education or dying passions would bring the heretic to compromise, the thing of difference would become a non-issue where both posistions are equal in orthodoxy as a question or controversy but not as a heresy, etc the conflict would no longer be heated or vast enough to compromise a heresy and difference between the two orthodoxies would either no longer be great enough or important enough to cause division within the church.
The Friars as a good example, rather than being a heresy, they became an accepted but different movement within the orthodoxy, or pretty much all biblical interpretation issues, peterist etc, that when raised in the early church were named heresies but by the middle ages merely questions of little to no importance.
Arch-Heresies being of course a clear exception, but as moslems are the only good example during the time covered by the game and theyre already dealt with its not worth worrying about.
The major heresies of the middle ages are not compatible with such ideas, being revolutionary as in the albergesians or anarchic and passion driven as in the lollards. The only and best way to model the victory of heresy, although so unlikely a thing would be more effort than gain to do so, would be to have it replace the orthodoxy, not stand aside it and even then only a petty few would qualify.
It might be conceivable that if the Albergesians stand powerful and permanent they could stand a papal pretender to the throne of st-peter, using Papal mechanics rather than anti-pope mechanics as from what i understand the anti-pope mechanic wouldnt work for antipopes at all and especially not different religion anti-popes. but maybe it would who knows.
but it probably wouldnt be possible to have it not have such mechanic while weak. So the game mechanic purpose of becoming a heretic meaning being stripped of the privileges of europe and the church would be broken if from the start all had access to everything. Or would otherwise break the balance and sense of such mechanics.
