The Union of Brest-Litovsk
Ever since 1386, the lands that would become modern Ukraine were part of the Roman Catholic kingdom of Poland and Lithuania. Subject to over two centuries of Roman Catholic influence and manipulation, the Orthodox Church in this region gradually grew weaker. The new ruler of Poland-Lithuania, the ardently Roman Catholic King Sigismund III Vasa (r. 1587–1632), ordered the Jesuits to increase their propagandizing efforts among the Orthodox in this region, which was known in Western Europe as Ruthenia.
In 1589 Patriarch Jeremias II of Constantinople, on his return home from Moscow, tried to bring order and reform to the Church there, but some of the bishops resented his interference. Meanwhile, the Poles were promising the Orthodox bishops privileges equal to those of the Polish bishops, including being seated in the Polish Senate, if they would acknowledge the supremacy of the Roman Papacy.
In 1596, at the Council of Brest-Litovsk, nine of the eleven Ruthenian Orthodox bishops formally accepted union with the Roman Church, with the agreement that the Orthodox would be allowed to maintain all their liturgical rites and customs. This arrangement, built upon the decisions of the Council of Florence in 1439, was contemptuously rejected by many of the Orthodox faithful.
The Orthodox who resisted the forced imposition of this “Unia” arrangement were given much support in their struggle by the Cossack brotherhoods—groups of vigilantes and frontiersmen which formed in defiance of the Polish overlordship of the Ukraine.
In 1619 Patriarch Theophanes of Jerusalem secretly consecrated seven Orthodox bishops in Kiev, in defiance of the Unia. This enabled the Orthodox to reestablish some semblance of regular Church life, especially in eastern Ukraine. But the Unia would continue to hold sway in western Ukraine, and eventually in the traditionally Orthodox lands that would be absorbed by the Austro-Hungarian Empire.
The Uniate Christians were steadily subjected to Latinizing influence. The hierarchical, clerical, and academic leadership of their Church was dominated by the discipline and doctrine of the Roman Papacy.