The brothers Constantine, better known by his monastic name Cyril, and Methodius were originally from Thessalonica, a city that in the 9th century had a strong Slavic presence. This exposure gave them familiarity with Slavic culture from an early age.  

Constantine quickly advanced at the court of Constantinople, where he met many intellectuals, including the future Patriarch Photius. He and his brother were entrusted with three diplomatic missions on behalf of the empire: the first to Baghdad, the capital of the Arab Caliphate; the second to the Khazars; and the third to the Slavs.  

The mission of Cyril and Methodius to the Slavic lands was prompted by a letter sent by Rastislav of Great Moravia to Byzantium. It is believed that Moravia had already been converted to Christianity by the neighboring German clergy. Rastislav, seeking to establish an independent ecclesiastical hierarchy free from Frankish influence, turned to distant Constantinople for assistance.  

Byzantium responded to the call, aiming to counter Western influence in the Danube region. The two brothers were tasked with developing an alphabet for the Slavs to facilitate effective preaching—this became the Glagolitic script. The reaction from the Frankish bishops, fearful of losing their influence in the East, was swift. Cyril and Methodius were summoned to Rome by Pope Nicholas I. However, instead of condemning them, the Pope seized the opportunity to take control of the Slavic conversion mission and establish ties with the emerging Moravian and Pannonian churches, even at the cost of alienating the Frankish clergy.  

After Cyril’s death in Rome in 869, the Pope sent Methodius on a second mission to Moravia and Pannonia, this time under Roman authority. The German response was harsh, and Methodius frequently found himself entangled in geopolitical and jurisdictional conflicts that led to his imprisonment and restrictions on his activities.  

In his final years, he dedicated himself to translating the Bible into the Slavic language, a groundbreaking achievement for the time, which was completed in the 880s. Methodius died in Velehrad, Moravia, in 885.