The union was strongly supported by the king of Poland and grand duke of Lithuania, Sigismund III Vasa, but opposed by some [[bishop]]s 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 Little Russia or the 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.