Answer:
The Byzantines sent wave after wave of christian missionaries to the Slavic lands, which sorrounded the Byzantine Empire.
Two of the most important missionaries were Saint Cyril and Saint Methodious. They were brothers, and converted many slavs to Orthodox Christianity, specially those living in Moravia. They also invented the cyrilic alphabet to give a script to the Slavic languages, and help spread christianity faster. Hence the name of the alphabet.
