^"legal and ontological distinctions" are irrelevant. CK 2 only deals with religion in terms of its political importance. The political divide between what we now call Catholicism and Eastern Orthodox centers around the Bishop of Rome (The Catholic Pope) claiming temporal authority over all Christendom. That began some 200 years before the first starting date. Christmas day 800 AD is when that split truly became irreversible with the Bishop of Rome crowning a Frankish warlord as Western Roman Emperor. So by 867 AD the die is well and truly cast.
This is 100% of the reason that I won't implement Chalcedonian as a united religion in EMF. If EMF were way more focused upon its vanilla map option, then the 769/CM bookmark would likely warrant unification and a dynamic schism, but it is definitely a shit-load of work for a map option for EMF that I can barely support as it is (since I don't play w/o SWMH) and is much less commonly used than EMF+SWMH.
EDIT: Note that when I say "SWMH," these days in terms of my own personal play, I'm referring to miniSWMH. [Unrelated to my point, but I figured that I ought to specify since I don't talk a lot about actually playing CK2.]
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