Christianization of the Rus'. The Kievan period
Orthodox Christian Constantinople's greatest mission outreach was to areas known as Kievan Rus that are now the states of Ukraine, Belarus, and Russia. Christianity was introduced into Kievan Rus by Greek missionaries from Byzantium in the 9th century. In 863–869, Saint Cyril and Saint Methodius translated parts of the Bible into Old Church Slavonic language for the first time, paving the way for the Christianization of the Slavs. There is evidence that the first Christian bishop was sent to Novgorod from Constantinople either by Patriarch Photius or Patriarch Ignatius, circa 866-867 AD.
By the mid-10th century, there was already a Christian community among Kievan nobility, under the leadership of Greek and Byzantine priests, although paganism remained the dominant religion. Princess Olga of Kiev was the first ruler of Kievan Rus to convert to Christianity, either in 945 or 957. Undoubtedly influenced by his Christian grandmother and by a proposed marriage alliance with the Byzantine imperial family, Olga's grandson Vladimir I (c. 956-1015) prince of Kiev, from among several options, chose the Byzantine rite. Baptized in 988, he led the Kievans to Christianity.
This date is often considered the official birthday of the Russian Orthodox Church. Thus, in 1988, the Church celebrated its millennial anniversary.
The Kievan church was originally a metropolitanate of the Patriarchate of Constantinople and the Ecumenical patriarch along with the Emperor, appointed the metropolitan who governed the Church of Rus'. The Metropolitan's residence was originally located in Kiev. As Kiev was losing its political significance due to the Mongol invasion, Metropolitan Maximus moved to Vladimir in 1299; his successors, Metropolitan Peter and Theognostus, moved the residence to Moscow by the 14th century.