What would have been the fate of the Polish Lithuanian Commonwealth had the Ukrainians and Byelorussians under there control converted to the dominant faith, maybe there languages adopting latin alphabet? Maybe this happens by late 16th/early 17th centuries.
According to Prof. Andrew Wilson's excellent book,
The Ukrainians: Unexpected Nation?, one of the strengths of the Ruś Church (that is, the Orthodox Church in Kievan Ruś itself) according to the Ukrainian historiographical tradition was that despite its Orthodoxy, it was fluid enough to accommodate influence outside of the Orthodox tradition (therefore, to Ukrainian historians, the Nikonian reforms in the Russian Orthodox Church, while often billed as bringing the ROC in line with Greek Orthodox practice, was in reality an attempt to bring into the ROC a "Ukrainianized" Greek Orthodoxy). Of course, there's two traditions here of Eastern Christianity, as already been observed:
In theory Belarus and Ukraine WERE mostly Catholic by late 18th century (Byzantine Rite Catholic), of course Union of Brest was hardly noticed by Ruthenian peasants (they were not aware that they suddenly became Catholic) but same thing happened in Prussia, when peasants without even noticing it switched from Catholicism to Lutheranism. In Austrian Galizia, when Union was not delegalised, vast majority of Ukrainians remained Byzantine Rite Catholic, same would happen if Ukraine remained part of PLC.
“The union was not accepted by all the members of the Ruthenian Orthodox Church in these lands, and marked the creation of Greek Catholic Church and separate eparchies that continued to stay Orthodox among which were Lviv eparchy, Peremyshel eparchy, Mukachevo eparchy and Lutsk eparchy that at first accepted the union but later oscillated back and forth.” It seems that by mid-XVII at least Eastern Ukraine was predominantly Orthodox (hence appeal to Moscow during Khmelnitsky Uprising) and even in the mid-XVIII the Orthodox had been prevailing at least in some areas of Right Bank Ukraine and Belorussia.
My own idea would be simple, and within that tradition mentioned above:
>At an early enough stage during the disintegration of Kievan Ruś, keep the notion in what is now Bielaruś (Belarus) and Ukraine going that they are an inheritor of the Ruśian tradition (even with Muscovy doing its thing) and thus they are part of a Ruthene (Русины, Rusynŷ)
narod. This means that, unlike OTL, the Ruthenians (what we now call Belarusians, Ukrainians, and Rusyns) must either avoid Polonization altogether or only allow a limited form of it, and some of its traditional institutions from the Grand Duchy of Lithuania (such as the use of the Правда Русьская/
Pravda Ruśskaja law code, for example) must be maintained.
>The Common East Slavic written tradition known as Chancery Slavonic or
Old Ruthenian must be maintained as the written language of the Ruthenian people, which would mean a continued use of Cyrillic. Now, there could be a possibility of fusing the chancery language (which was in large part based on the speech of Vilnius/Viľńa) with the popular speech, making it seem to us like a hybrid of the other East Slavic languages apart from Russian, with the adaption of Church Slavonic terminology (as the "high" language at the time) and some Polish elements, and that would be left for historical linguistics to figure that one out. Latin script would thus be accorded a secondary status, at first, and eventually take on an appearance (with some exceptions) that approximates OTL Czech and Slovak orthography. Even if Latin script use becomes more widespread, the Cyrillic script would still be seen as
their script by which the language is written, especially for religious purposes.
>If Catholic, then among the Ruthenes will still be in the Byzantine Greek Catholic tradition; they can't be pushed into the Roman Rite tradition because then that would make them cling more to Orthodoxy. Rather, I'd see it more as a fluid type thing, with Greek Catholic and Greek Orthodox in a manner similar to the ROC's Nikonian/Old Believer split - that's bound to happen here anyway. The Union of Brest would remain important here for the development of the Ruthenian Greek Catholic Church, but I would argue that the Ruthenian Church here would also appeal to
Laetentur Caeli as their method of accepting union with Rome while maintaining autonomy. At the same time, those Ruthenians who wish to maintain Orthodoxy will probably need to learn from the Reformation the need to ensure a renewal of liturgical practice and a need for theologically-sound trained clergy - hence the Mohyla Academy needs to be a permanent phenomenon that is spread throughout the Ruthenian Orthodox Church.
