The two brothers were born in Thessalonica, in present-day Greece – Cyril in about 827–828 and Methodius about 815–820. Cyril was reputedly the youngest of seven brothers; he was born Constantine, but was given the name Cyril upon becoming a monk in Rome shortly before his death, according to the Vita Cyrilli ("The Life of Cyril"). Methodius was born Michael and was given the name Methodius upon becoming a monk at Mysian Olympus (present-day Uludağ), in northwest Turkey. Their father was Leo, a droungarios of the Byzantine theme of Thessalonica, and their mother was Maria.
The exact ethnic origins of the brothers are unknown, there is controversy as to whether Cyril and Methodius were of Slavic or Byzantine Greek origin, or both. The two brothers lost their father when Cyril was fourteen, and the powerful minister Theoktistos, who was logothetes tou dromou, one of the chief ministers of the Empire, became their protector. He was also responsible, along with the regent Bardas, for initiating a far-reaching educational program within the Empire which culminated in the establishment of the University of Magnaura, where Cyril was to teach. Cyril was ordained as priest some time after his education, while his brother Methodius remained a deacon until 867/868.
Mission to the Khazars
About the year 860, Byzantine Emperor Michael III and the Patriarch of Constantinople Photius (a professor of Cyril's at the University and his guiding light in earlier years), sent Cyril on a missionary expedition to the Khazars who had requested a scholar be sent to them who could converse with both Jews and Saracens. It has been claimed that Methodius accompanied Cyril on the mission to the Khazars, but this may be a later invention. The account of his life presented in the Latin "Legenda" claims that he learned the Khazar language while in Chersonesos, in Taurica (today Crimea).
After his return to Constantinople, Cyril assumed the role of professor of philosophy at the University while his brother had by this time become a significant player in Byzantine political and administrative affairs, and an abbot of his monastery.