Although Patriarch Photius of Constantinople reported sending a missionary bishop to the Rus' in the aftermath of this attack, the mass conversion of Rus' to Christianity would not come until 987-988, when Grand Prince Volodymyr of Kyiv agreed to be baptized and decreed that his lands should convert to Christianity. The Baptism of Rus' marked the founding of the Church province known as the Metropolia of Kyiv. Eparchies were created and bishops appointed to them as the jurisdiction grew in size and in territory.
Throughout the period of Kyivan Rus', bishops continued to be appointed from Constantinople to administer spiritual matters in its territory. As the Great Schism developed from the mid-11th through the 13th centuries, the Church of Kyiv remained under the formal jurisdiction of Constantinople, though with considerable local autonomy. In 1240, however, Kyiv fell to the Mongols, and a period of chaos ensued.
By the mid-14th century the last surviving remnant of an independent Rus', the Kingdom of Rus', had come under the rule of an expanding Poland, and eventually the Polish-Lithuanian Commonwealth. Latin Rite Catholic dioceses were created, political privileges were denied to the Ruthenian gentry and clergy, and efforts were made to convert the populace from Orthodoxy to Catholicism.
It was against this backdrop that the inheritors of the Kyiv Metropolia diverged into three streams. The ecclesiastical provinces to the northeast, in Muscovy (today's Russia), declared jurisdictional independence and established their own Patriarch of Moscow in 1448, which was ultimately recognized by Constantinople in 1589. Within today's Ukraine, however, opinion was split between those who felt that a revival of the Orthodox Church in union with Constantinople was paramount, while others sought accommodation with the Catholic Poles and the creation of an Eastern Rite Catholic Church. It was this third group, known at that time as the "Uniates," that was dominant at the dawn of the 16th century. Metropolitan Michael Rahoza of Kyiv and other hierarchs of the Church of Rus' agreed to a union with Rome, known as the "Union of Brest", in 1595–1596. This became what later would be called the Greek Catholic Church — a church under the authority of the Pope, Catholic in doctrine, but observing the liturgical practices and customs of the Eastern Rite, including married clergy. For several decades thereafter, Greek Catholics were in the ascendancy, and St. Sophia Cathedral became the seat of a Greek Catholic Metropolia of Kyiv.
There remained many in Ukraine who did not agree to the Church's acceptance of Catholic doctrine. These included the "brotherhoods", groups of lay well-to-do townspeople who sought to cultivate academic and theological learning. Among their leaders was the magnate Kostiantyn Ostrozhs'kyi, who founded an academy and who had previously supported the publication of the first printed Slavonic Bible in 1581 (one of which is in the permanent collection of the UHEC). This ultimately led to an "illegal" re-establishment of an Orthodox episcopal hierarchy in 1620.
The Polish king Władysław IV, responding to a variety of political pressures, legalized the Orthodox Church in 1632 by signing the "Measures for the Accommodation of Citizens of the Kingdom of Poland and the Grand Duchy of Lithuania of the Ruthenian Nation and the Greek Faith", and in 1633 Petro Mohyla became the first Orthodox Metropolitan of Kyiv to be recognized by the Polish Crown.