The missions of Saints Cyril and Methodius transformed the religious and cultural development of Central and Eastern Europe during the 9th century. Sent by Michael III (reign 842 to 867) from the Eastern Roman Empire (“Byzantine Empire”) to evangelize the Slavic populations of Great Moravia, the brothers recognized that the spread of Christianity required a liturgical and literary tradition accessible in the local Slavonic language. Around 862 to 863, Constantine (later Cyril) and Methodius developed the Glagolitic alphabet, enabling the translation of the Gospels, liturgical texts, and ecclesiastical writings from Greek into Old Church Slavonic.

Following the deaths of Cyril (869) and Methodius (885), their disciples continued their work in the First Bulgarian Empire under Boris I of Bulgaria (reign 852–889). Literary centers at Ohrid and Preslav became major hubs of Slavonic scholarship, where a new script based largely on Greek uncial forms emerged - later known as the Cyrillic script in association with Saint Cyril. Strengthened by the Council of Preslav (893), which elevated Slavonic as a liturgical and administrative language, the Cyrillic tradition spread across Eastern Europe and Eurasia and remains in use today among numerous Slavic and non-Slavic languages.