Photios I of Constantinople
During the 9th century, the Emperor in Constantinople encouraged missionary expeditions to nearby nations including the Muslim caliphate, and the Turkic Khazars. In 862 he sent Saints Cyril and Methodius to Slavic Great Moravia. By then most of the Slavic population of Bulgaria was Christian and Tsar Boris I himself was baptized in 864. Serbia was accounted Christian by about 870. In early 867 Patriarch Photios I of Constantinople wrote that Christianity was accepted by the Kievan Rus', which however was definitively Christianized only at the close of the following century.