"The second commandment is like the first: Thou shalt love thy neighbor as thyself."
St. Mark 12:31
These two brothers were born in Thessalonica of a senatorial family. Saint Cyril was sent to Constantinople to study, where he became known as the Philosopher; but it was the Holy Church that he desired to serve, and he was ordained a priest. While Cyril was still young, the Patriarch of Constantinople recommended in the year 848 to the reigning Emperor to place him at the head of a mission which was to be sent to the Khazars of the eastern Danube region. Their king desired to learn of Christianity and had requested missionaries. Cyril asked for the time to learn the Turkish language which this people spoke, and after only a short while was ready to preach. The prince of the Khazars received Baptism and the entire nation soon followed his example. Cyril founded churches and furnished them with excellent ministers, then returned to Constantinople, refusing all presents offered him by his converts.
He was next missioned to Bulgaria with his younger brother Methodius, who was a monk of eminent sanctity. This nation, which had migrated like the Khazars from the east, had settled in the Moldavian region and a part of Hungary; they had been exposed to Christianity by some Greek prisoners, and the sister of their king had become a Christian by the good offices of the empress Theodora. When Saint Methodius, an excellent artist, was delegated to paint in the palace for the king, as he requested, a scene which could frighten the beholders, he chose for subject the Last Judgment. The king was so impressed that after being instructed he was baptized, in 865, with forty-eight of his followers. The nation followed their leaders after a brief revolt had been promptly quieted.
The two brothers preached also in Moravia, invited there by the pious king of that nation, whom they baptized with most of his subjects. It was there that Saint Cyril invented a Slavonic alphabet, translating the Bible and other writings from Greek and Latin into the language of the Slavic peoples. In 867 the missionaries went to Rome, where Saint Cyril, who was ill, died soon afterwards.
Saint Methodius was named bishop of Moravia and Pannonia or Hungary. Difficulties were not lacking, but he remained there until 880, when he returned to Rome to justify his conduct, which certain enemies had accused to the Pope. The Pope cleared him before the adversaries, and settled some questions regarding the language to be used in worship. He permitted the Mass to be said in the Slavic language, not to the exclusion, however, of Latin. Saint Methodius also baptized the king of the Bohemians, and again many of the king's subjects followed his example. The Saint lived for about 20 years after his brother's decease; the exact date of his death is unknown.
Les Petits Bollandistes: Vies des Saints, by Msgr. Paul Guérin (Bloud et Barral: Paris, 1882), Vol. 3