The Union of Krevo (1385), which joined Poland and the Grand Duchy in a confederation, hinged on Lithuanian Grand Duke Jogaila's conversion from paganism to Roman Catholicism and his subsequent marriage to twelve-year-old Queen Jadwiga of Poland. Thus he became Wladyslaw II Jagiello, king of Poland. Poland and Lithuania were later united into a single state, the PolishLithuanian Commonwealth, by the Union of Lublin (1569).

When Roman Catholicism became the official religion of Lithuania shortly after Jagiello's conversion, the Lithuanian and Belorussian nobilities began converting from Orthodoxy to Catholicism and assimilating Polish culture (including the language), a process accelerated by the Union of Lublin. As a result, the Belorussian peasantry was ruled by those who shared neither their language nor its religion, Eastern Orthodoxy.

The Union of Brest (1596), which united the Roman Catholic Church with the part of the Orthodox Church that was within the Polish-Lithuanian Commonwealth, was viewed favorably by both the Polish king, Sigismund III, and a number of Orthodox bishops, clergy, and faithful. The new Uniate Church (see Glossary) acknowledged the supremacy of the Roman Catholic pope and accepted articles of Roman Catholic religious doctrine. In return, the Uniate Church retained its traditional Orthodox rites and customs as well as a measure of autonomy in nondoctrinal matters; it was also given the same rights and privileges as the Roman Catholic Church. However, fear of the new church's becoming Latinized and Polonized (see Glossary) caused many of the Orthodox faithful to reject the union, and the Orthodox Church continued to exist alongside the Uniate Church in an often bitter struggle.

In the aftermath of the Union of Brest, both civil and religious authorities persecuted the Orthodox Church and supported the Uniates in their takeover of Orthodox property. Social conditions deteriorated, there was a large-scale revolt against Polish landowners in 1648-54 (coinciding with the Khmel'nyts'kyi rebellion in Ukraine), and many Belorussians fled to the Ukrainian steppes to join the Cossacks (see Glossary). There was little economic development in Belorussian lands, and the vast majority of the Belorussian population lived on subsistence agriculture.

Data as of June 1995