CATHOLIC AND ORTHODOX
Initially a distinction could be drawn between Poles and Lithuanians in that the Lithuanians, who had the larger territory - including, at least in theory, Kiev - were more tolerant of Orthodoxy. But there was a continual process of Polish-Lithuanian rapprochement encouraging an ever tighter identification with Catholicism. The pressure was felt especially by the Orthodox aristocracy. The advantages of conversion were considerable, both materially in terms of political power and culturally, as Poland was open to the cultural and intellectual evolutions that were occurring in Europe. The peasantry on the other hand, and the artisan class, clung stubbornly to Orthodoxy. So an Orthodox peasantry was confronted with a Polish and increasingly 'Polonised' Ruthenian landlord class ('Ruthenian' - or 'Rusian' - being the term used to refer to former subjects of Kievan Rus). In addition, Poland had welcomed large numbers of Jews, fleeing persecution in Germany. For the Poles the Jews were useful to fulfil socially necessary tasks that were beneath the dignity of the noble class - merchant, tax collector, tavernkeeper, money lender, landlord's agent - all roles that made them very unpopular among the peasantry.
The peasants, however, had an escape route. They could flee westward into the area properly called the Ukraine, the borderlands, and join up with the Cossacks:
This map shows the 'wild fields' - the area out of all Polish government control - in the seventeenth century, superimposed on the contours of modern Ukraine in its pre-2014 boundaries (the striped area shows territory in modern Russia).
I'm told that the word 'Cossack' corresponds to a Turkish word, kazakh, as in Kazakhstan, meaning 'free man', meaning that they weren't serfs. They were formed into self-defending military companies and from the early sixteenth century the Polish and Russian governments saw an advantage in their existence as a buffer against the Tatars. In 1526, the Polish government agreed to formally recognise the 'registered Cossacks' as a rather unpredictable part of the Polish defence force. But there were also Cossacks in the area 'beyond the rapids' - the 'Zaporizhia' - wildest of the wild lands in the Southern area of the Dnieper river. These were unregistered and from the point of view of the Polish and Russian governments quite unpredictable.
Meanwhile, in the mid fifteenth century, Constantinople, like most of the rest of the Orthodox world, had succumbed to the Ottoman Empire. But before that, in an attempt to secure Catholic support against the Ottomans, the Patriarch of Constantinople, together with other leading Orthodox hierarchs including the now Moscow based Metropolitan of Kiev, had agreed in the Council of Florence-Ferrara, to a union with Rome. The result was a revolt both in Constantinople and in Moscow. Moscow broke the connection with Constantinople, dissolved the Kiev metropolitanate and created a new metropolitanate of Moscow. Constantinople in turn, now returned to Orthodoxy but under Ottoman domination, created a new Metropolitanate of Kiev, albeit, given the continued vulnerability of Kiev, based in Vilnius, capital of Lithuania.
On the one hand one could say that Moscow was now out of communion with the rest of the Orthodox world. On the other hand, Moscow was now the only remaining Orthodox state - all the other Orthodox communities were either subject to the Muslim Ottoman Empire or, in the case of the Ruthenians, to the Poles (Rumania, though incorporated into the Ottoman Empire still had a relatively high degree of autonomy). Moscow was brought back into the wider Orthodox family when Constantinople recognised the metropolitanate in 1589, allowing it the title of Patriarch. This was shortly after the death of Ivan IV, the 'Terrible', and before the early seventeenth century invasion from Poland - the 'time of trouble'.
THE 'UNIATES 'AND THE KIEV ACADEMY
As, in Poland, the Counter-Reformation got underway, and as they lost the protection of their own Ruthenian aristocracy, the pressure on the Orthodox clergy became ever harder to bear. As a result, a number of the higher clergy proposed joining the Catholic Church but being allowed to continue to use the Eastern liturgy and to have a married clergy - both necessary conditions for maintaining the loyalty of the peasantry. The result was the 'Union of Brest' of 1596. Although they were destined to become an important force in Ukrainian history, their immediate situation was uncomfortable - regarded with suspicion by the Catholics and with positive hatred by the Orthodox.
Partly in response to the Uniates a very remarkable development occurred - the formation in 1632, under Cossack patronage, of the 'Greek Slavonic Academy' of Kiev. The leading figure was a Moldavian nobleman who had become a monk in the Kiev Caves Monastery, Peter Moghila, who became Metropolitan of Kiev in 1633. This could be seen as the first modern theological school in the Russian world but although it was established as a bulwark for Orthodoxy, it was miles far removed from Orthodoxy as it was understood in Russia. And here we enter into a paradox of Catholic/Orthodox church history.
The Catholic West was fascinated by the classical thought and culture of Greece and Rome, hence Thomas Aquinas's intensive use of Aristotle, but they had very little access to it. It was preserved in Constantinople but preserved, so to speak, in amber - regarded as a precious heritage from their ancestors which, nonetheless, had been superseded by Christianity. Plato and Aristotle were of interest for the purity of their language and as models for the process of reasoning but the actual philosophical questions they posed were now settled. By Christianity. So when Constantinople shared Christianity with the Slavs they didn't share their classical culture, whether because they thought the barbarians couldn't assume it or simply because they didn't think they needed it because they had something better. (2)
(2) I discuss this in my essay Plato and Christianity - http://www.peterbrooke.org/politics-and-theology/plato/east-and-west.html
But with the decline and fall of Constantinople, a huge treasure trove of classical literature passed over to the West and was a major influence on the Renaissance and on its shadow twin, the Reformation. None of this affected Russia. Russian Orthodoxy could be called Christianity without Classicism. But it had a huge effect on Poland and consequently on Peter Moghila and his Kiev academy. From a Russian point of view the academy had a very Catholic character. The instruction was in Latin, the method Scholastic and such exotic things as philosophy, rhetoric, classical literature and poetics were taught.