Union of Brest
The Union of Brest refers to the 1595-1596 decision of the (Ruthenian) Church of Rus', the Metropolia of Kiev-Halych and all Rus', to break relations with the Patriarch of Constantinople and place itself under the Pope of Rome in order to avoid being ruled by the newly established Patriarch of Moscow. At the time, this church included most Ukrainians and Belarusians, under the rule of the Polish-Lithuanian Commonwealth. The hierarchs of the Kievan church gathered in synod in the city of Brest to compose the union's 33 articles, which were then accepted by the Roman Catholic pope. At first widely successful, within several decades it had lost much of its initial support. In Austrian Galicia, however, the church fared well and remains strong to this day, most notably in the Ukrainian Greek Catholic Church.
The union was strongly supported by the king of Poland and grand duke of Lithuania, Sigismund III Vasa, but opposed by some bishops and prominent nobles of Rus' and perhaps most importantly by the nascent Cossack movement for Ukrainian self-rule. The result was "Rus' fighting against Rus'" and the splitting of the Church of Rus' into Greek Catholic — also known as Uniate — and Orthodox jurisdictions.
A large area in the southwest of Russia became absorbed by Lithuania and Poland after the destruction of Kievan power by the Tartars. This southwestern part of Russia was commonly known as Rus which in Latin became Ruthenia; this is the territory that is present day Ukraine. In 1386, the kingdoms of Poland and Lithuania were united under a single ruler. The monarch of the united realm was Roman Catholic, and a substantial minority of the population were Russian and Orthodox. These Orthodox were in a difficult situation because the Patriarch of Constantinople, to whose jurisdiction they belonged, could exercise no control in Poland, as the former Byzantine capital had fallen to the Muslim Turks. The bishops were appointed not by the Church but by the Roman Catholic king of Poland.
The authorities in Poland always tried to make the Orthodox submit to the pope to reunify Christianity. With the arrival of the Jesuits in 1564, pressure on the Orthodox increased. The state of the Ruthenian Church was poor; clergy were uneducated and the bishops were without the funds they needed to properly run the Church. Many priests were ordained without basic training and new rites were developing that were neither Latin nor Greek in their character. Constantinople was now under Muslim rule and Moscow had recently been elevated to a Patriarchate. The Ruthenian bishops were stuck between a population converting to Roman Catholicism on the West and a rising Muscovite force in the East.
At this synod six out of eight Orthodox bishops — including the Metropolitan of Kyiv, Michael Ragoza — supported the union, but the remaining three bishops from the extreme west of Ukraine and Eastern Poland (Lviv', Lutsk, and Przemyśl) would not join the union until later (1700, 1702, and 1693 respectively). The Cossack forces of Ukraine felt the Union was a bretrayal to the Polish rulers and united with the Russian Empire to fight against Poland and all who supported the Empire, including the Greek-Catholics. In 1620 Patrarch Theophanes II of Jerusalem arrived in Kyiv and ordained an Orthodox hierarchy for the Ruthenian Church and thus there emerged a situation of both Orthodox and Eastern Catholic bishops coexisting in the same territory in Ukraine from this point onwards.