The conversion to Christianity by the Slavs along with the creation of the Slavonic alphabet which enabled them to express themselves religiously and philologically, was one of the most significant achievements of the Byzantine civilization. This task began in the years of Michael III Bardas when the patriarch was Photios and went on into the following century. The first to be converted were the Moravian Slavs. Since the middle of the 9th century their sovereign Ratislav had sent an embassy to Byzantium and asked for a mission of Byzantine bishops and teachers to teach the Christian religion in the language of his people. Ratislav, however, turned to Byzantium for political reasons also. As he drove the German suzerainty away he came into the vicinity with the Bulgarians who had been allies of the Franks since 863. This alliance constituted a threatening "pincer" for the Moravians, so in a movement of distraction they turned to Byzantium.
The emperor Michael III and the patriarch Photios realized the benefits that Byzantium would get by the expansion of its religious and political influence and assigned this important mission to two brothers from Thessalonike: Constantine-Cyril and Methodios, who in the past had assumed similar activities. They departed in 864 for Moravia having with them slavonic translations of liturgical texts in a new alphabet, the so-called glagolitic. The first phase of their mission (864-867) was crowned with absolute success.
The conversion of the Moravians was followed by that of the Bulgarians, which was imposed, however, by force of Byzantine arms. In 864, the sovereign of the Bulgarians, Boris, was baptized taking the name Michael. For this conversion to the Christianity had occurred under military pressure, so the sovereign of Bulgaria, in 866, did not hesitate to change his tactics and asked for a mission of clerics from Rome and the kingdom of the Franks. The Pope, who was interested in the developments in the Balkans was on the lookout to restore Rome's -having been driven out by the iconoclast emperors- recognition in the area. So he rushed to send clerics and the patriarch Photios was infuriated. The anger of the patriarch was so great that in the council of 867 he condemned the irregular actions of the Latin clergy and primarily the creed of filioque.