The Orthodox Saints site has this interesting account of the first Christian teachers to Slavs, who also devised the alphabet for Slavic language.
"Holy Equals-to-the-Apostles Methodius (885) and Cyril (869), first teachers of the Slavs. The two saints were brothers, born in Thessalonica. St Methodius, the elder brother, served as a soldier for ten years before becoming a monk. Cyril was librarian at the Hagia Sophia in Constantinople; then he too became a monk.
"Their first missionary work was not among the Slavs: When the king of the Khazars (a Mongol people who then inhabited much of what is now Russia) petitioned the Emperor Michael to sent teachers to instruct his people, the Emperor chose Cyril and Methodius as his emissaries. They converted the Khazar king to the Christian faith, along with many of his nobles and commoners.
"When King Rostislav of Moravia likewise sought teachers of the Christian faith, Cyril and Methodius were again sent forth. This time they devised an alphabet for the Slavic language (based on Greek but adding characters to indicate sounds particular to Slavonic), and used it to translate many of the Greek service books into the language of the people. (In theory, the Orthodox people have always been privileged to hear the Church's services in their own tongue, though often attachment to dead languages has prevented this ideal from becoming reality.) Both brothers were repeatedly attacked by Germanic priests of the region, who opposed the use of the common tongue in the liturgy. At different times, both brothers were forced to appeal for exoneration and protection to the Pope of Rome, who supported them warmly each time.
"After the two Saints reposed, attacks on their work continued, and their disciples were eventually driven from Moravia. The disciples, fleeing southward, found a warmer welcome among the southern Slavic peoples, and their work bore much fruit in Bulgaria (including modern-day Serbia) and other countries. And, of course, the alphabet that they devised, called Cyrillic after St Cyril, remains the standard alphabet of both the Slavonic service books of the Church and the Slavic languages of today."