The Uniate Church, commonly known today as the Greek Catholic Church, institutionally came into being on 9 October 1596, when the then-metropolitan of Kyiv and many Orthodox bishops from the Grand Duchy of Lithuania (the state comprising Belarusian and Ukrainian lands, which formed a commonwealth with the Kingdom of Poland) signed an agreement known in history as the Union of Brest (Brest is now a city in Belarus close to the Polish border).

Under the agreement, the signatories pledged allegiance to the pope in Rome and submitted to the jurisdiction of the Holy See. The agreement stipulated that the newly converted Orthodox hierarchs and their flock would retain the Old Church Slavonic language and the Eastern rite in liturgy and religious practices, the Julian (Old Style) calendar, administrative autonomy, as well as married clergy.

On the Belarusian lands, the Uniate Church comprised some three-quarters of the population until 1839, when it was abolished by Russia, which in the second half of the 18th century partitioned the Polish-Lithuanian Commonwealth along with Prussia and Austro-Hungary and came into possession of a larger part of Belarusian and Ukrainian ethnic territories. Uniate believers were administratively converted to Orthodoxy (some of them chose Roman Catholicism), while clergy who refused were exiled to Siberia or interior Russia.

On the Ukrainian lands, under the influence of struggles led by Ukrainian Cossacks with the Lithuanian-Polish Commonwealth in the 17th century, Orthodoxy was not so eager to surrender its positions to the Uniate Church as in neighboring Belarus. In time, with the exception of Lviv, all leadership positions returned from Uniate to Orthodox clergy.

In 1689, the Orthodox Church in Ukraine, loosely connected with the Patriarch of Constantinople, lost its semi-independent status and become linked to Moscow's Orthodox Patriarchate, which began to appoint metropolitans in Kyiv and exert religious, political, and cultural influence on Ukraine.

Russia abolished the Uniate Church in Ukraine in 1839; Uniates formally existed only in the area of western Ukraine controlled by Austro-Hungary. The revival of the Uniate Church in western Ukraine (under Polish control) in the first half of the 20th century was connected with the activities of Greek Catholic Metropolitan Sheptytskyy.

After western Ukraine was incorporated into the Ukrainian SSR after World War II, Stalin initiated the so-called Lviv Council in 1946, which voided the 1596 Union of Brest. Ukrainian Uniates were converted to Orthodoxy, while priest who disagreed ended up in prison or went underground.

During the Gorbachev era of the Soviet Union, underground Ukrainian Catholic Church leaders began to reemerge. In 1989, the Soviet government allowed the Ukrainian Greek Catholic Church to register its parishes. The Uniate leader exiled in Rome -- Metropolitan Myroslav Cardinal Lyubachivskyy -- returned to Ukraine in 1991.

There has been no meaningful revival of the Uniate Church in Belarus in the post-Soviet era. A small group of nationally conscious Belarusian intelligentsia pledged to reestablish the Uniate Church in the early 1990s, advertising it as a "national" church -- that is, independent from Moscow-sponsored Orthodoxy and Warsaw-sponsored Roman Catholicism -- but found only a handful of adherents.

Apart from Ukraine, significant Uniate communities exist among ethnic Ukrainians in Canada, the United States, and Poland.