The Ruthenian Uniate Church (Belarusian: Руская уніяцкая царква, romanized: Ruskaja unijackaja carkva; Ukrainian: Руська унійна церква, romanized: Ruśka unijna cerkva; Latin: Ecclesia Ruthena unita; Polish: Ruski Kościół Unicki) was a particular church of the Catholic Church in the territory of the Polish–Lithuanian Commonwealth. It was created in 1595/1596 by those clergy of the Eastern Orthodox Church who subscribed to the Union of Brest. In the process, they switched their allegiances and jurisdiction from the Ecumenical Patriarchate of Constantinople to the Holy See.

The church had a single metropolitan territory — the Metropolis of Kiev, Galicia and all Ruthenia. The formation of the church led to a high degree of confrontation among Ruthenians, such as the murder of Archeparch Josaphat Kuntsevych in 1623. Opponents of the union called church members "Uniates". Catholic documents today no longer use this term due to its perceived negative overtones.

By the time of the Union of Brest, East Slavic-speaking inhabitants of the Polish–Lithuanian Commonwealth and surrounding areans were generally known as Ruthenians. They largely adhered to a Byzantine-derived Orthodox Christianity, whereas most Poles had embraced Roman Catholicism; Lithuanians largely remained pagan to the late Middle Ages before their nobility embraced the Latin form upon the political Union of Lublin (1569) with the Poles. The eastward expansion of the Grand Duchy of Lithuania had been facilitated by amicable treaties and inter-marriages of the nobility when faced with the external threat of the Mongol invasion of Kievan Rus'. Ethnically, the Catholics of the Commonwealth were Poles, Germans and Lithuanians.

During the Protestant Reformation of the 16th century, both the Catholic Church in the Commonwealth and the Ruthenian Church underwent a period of decay.^{[citation needed]} The Ruthenian Church was the church of a people without statehood. The Poles considered the Ruthenians as a conquered people.^{[citation needed]} Over time, the Lithuanian military and political ascendancy did away with the Ruthenian autonomies.^{[citation needed]} The disadvantageous political status of the Ruthenian people also affected the status of their church and undermined her capacity for reform and renewal.^{[citation needed]} Furthermore, they could not expect support from the Mother Church in Constantinople or from their co-religionists in Moscow. Thus the Ruthenian church was in a weaker position than the Catholic Church in the Commonwealth.^{[citation needed]}

Both the Catholic and the Ruthenian churches suffered from the policy of nominations to higher benefices by the King, the indifference of the nobility, and a low state of clerical education and discipline.^{[better source needed]} The monarchs used nominations to bishoprics as rewards to faithful civil servants.^{[better source needed]} After Metropolitan Joseph II Soltan (1509–1522), the names of the great families are missing among the nominees to the bishoprics. While the great families could have obtained the nominations had they cared, since they did not, the nominees came from the poorer gentry and from the burghers.^{[better source needed]} Prelates continued to live the style of life they were used to as laymen: they took part in raids and carried on trade and money lending. The Ruthenian Church had no cathedral chapters to make up for the deficiencies of the bishops.

The level of education of the Ruthenian peasantry had been falling during the sixteenth century.^{[better source needed]} This was one of the main reasons for ecclesiastical decay and one of the impediments to renewal. For the common people, their religion was ritualism;^{[citation needed]} attendance was often limited to baptism and church burial.

Poles regarded Ruthenians as a conquered people. As such, Ruthenians became a second class people in society, their culture backward compared to the other ethnic groups in the Commonwealth. This delayed the church in recovering from the predations of the Reformation. While the Ruthenian nobility had equal rights with the Polish nobility, by the fifteenth century their ranks had been thinned by war and waves of emigration to the east. The Poles who took their place came to control the sejm. If the Ruthenian aristocracy wanted to profit from its equality, it had to become Catholic and Polish. Intermarriage played a great role in the assimilation of the Ruthenian aristocracy; usually the Catholic faith prevailed. As a result, few Orthodox aristocratic families were left in Galicia or Podilia.^{[better source needed]} By the second half of the sixteenth century, Ruthenian nobility had little reason to feel discriminated against. They had kept their wealth, had access to the highest offices, and were socially accepted as equals with the Catholic nobility. By absorbing the Polish form of Western culture, they were also the first to be lost for the Ruthenian people.^{[citation needed]} With the loss of the elite, the Ruthenian Church and people increasingly lost leadership, representation in the government, and benefactors for church-sponsored programmes.^{[better source needed]}

While the Catholic Church in the Commonwealth had successfully resisted the appeal of the Reformation, the Ruthenian church continued to decay. The Ruthenian elite looked externally for aid.^{[citation needed]} The Patriarch in Constantinople could send neither aid nor teachers.^{[citation needed]} Protestant aid was unacceptable to many of them. They therefore turned to the Pope in the hope that he would curb the excesses of the Polish Catholics against Catholic Ruthenians.^{[citation needed]} In this way, they also hoped that acceptance of the Ruthenian hierarchy into Catholic communion would also lead to acceptance of the Ruthenian elite into the political structure of the Commonwealth.^{[citation needed]}