From the LCMS website:

Cyril (826-69) and Methodius (c. 815-85) were brothers who came from a Greek family in Thessalonica. The younger brother took the name “Cyril” when he became a monk in 868. After ordination, Cyril became librarian at the church of Holy Wisdom (Hagia Sophia) in Constantinople. In 862 the brothers were sent by the emperor as missionaries to what is now the Czech republic, where they taught in the native Slavic tongue. Cyril invented the alphabet today know today as “Cyrillic,” which provided a written language for the liturgy and Scriptures for the Slavic peoples. This use of the vernacular established an important principle for evangelical missions.