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.
Cyril was ordained as a priest and served as an official at the Hagia Sophia church where he developed a close relationship with the Patriarch of Constantinople, the bishop Photios. The brilliant scholar quickly became the bishop's librarian. Cyril became a teacher of philosophy at the Magnaura university in Constantinople where he gained the epithet “Constantine the Philosopher”.
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. The trip, unfortunately, ended in failure if it had intended to convert the Khazars to Christianity as the Byzantines only managed to baptise around 200 of them. The Khazaria state eventually adopted Judaism instead. Cyril did bring back souvenirs, though, said to be the relics of the exiled 1st century CE Bishop of Rome, Saint Clement.
Prince Rastislav of Great Moravia requested that Emperor Michael III and the Patriarch Photius send missionaries to evangelize his Slavic subjects. His motives in doing so were probably more political than religious. The Emperor quickly chose to send Cyril, accompanied by his brother Methodius. The request provided a convenient opportunity to expand Byzantine influence. Their first work seems to have been the training of assistants.
Although he succeeded in setting up many new churches, unfortunately for Cyril, Frankish bishops in Moravia who were pushing the case for the rival western half of the Christian church opposed his missionary work at every step. Conservative church clergy were also against the holding of services (or even disseminating religious literature) in any language outside the traditional trio of Latin, Greek, and Hebrew.
The new Pope Adrian II gave Methodius the title of Archbishop of Sirmium (now Sremska Mitrovica in Serbia) and sent him back to Pannonia in 869, with jurisdiction over all of Moravia and Pannonia, and authorisation to use the Slavonic Liturgy. Methodius now continued the work among the Slavs alone.
Feeling his end approaching, Cyril became a Basilian monk, was given the new name Cyril, and died in Rome fifty days later.
The East Frankish rulers and their bishops decided to remove Methodius. The archiepiscopal claims of Methodius were considered such an injury to the rights of Salzburg that he was captured and forced to answer to East Frankish bishops: Adalwin of Salzburg, Ermanrich of Passau, and Anno of Freising. After a heated discussion, they declared the deposition of the intruder, and ordered him to be sent to Germany, where he was kept prisoner in a monastery for two and a half years.
Rome declared emphatically for Methodius, and sent a bishop, Paul of Ancona, to reinstate him and punish his enemies, after which both parties were commanded to appear in Rome with the legate. The new Pope John VIII secured the release of Methodius, but instructed him to stop using the Slavonic Liturgy.
Methodius was summoned to Rome on charges of heresy and using Slavonic. This time Pope John was convinced by the arguments that Methodius made in his defence and sent him back cleared of all charges, and with permission to use Slavonic. The Carolingian bishop who succeeded him, Witching, suppressed the Slavonic Liturgy and forced the followers of Methodius into exile. Many found refuge with Knyaz Boris of Bulgaria, under whom they reorganised a Slavic-speaking Church. Meanwhile, Pope John's successors adopted a Latin-only policy which lasted for centuries.
Pope Stephen V exiled the disciples of the two brothers from Great Moravia in 885. They fled to the First Bulgarian Empire, where they were welcomed and commissioned to establish theological schools. There they and scholar Saint Clement of Ohrid devised the Cyrillic script on the basis of the Glagolitic. Cyrillic gradually replaced Glagolitic as the alphabet of the Old Church Slavonic language, which became the official language of the Bulgarian Empire and later spread to the Eastern Slav lands of Kievan Rus'. Cyrillic eventually spread throughout most of the Slavic world to become the standard alphabet in the Eastern Orthodox Slavic countries. Hence, Cyril and Methodius' efforts also paved the way for the spread of Christianity throughout Eastern Europe.
