Old Church Slavonic or Old Slavonic was the first Slavic literary language. Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and using it in translating the Bible and other Ancient Greek ecclesiastical texts as part of the Christianization of the Slavs. It is thought to have been based primarily on the dialect of the 9th-century Byzantine Slavs living in the Province of Thessalonica (in present-day Greece). Old Church Slavonic played an important role in the history of the Slavic languages and served as a basis and model for later Church Slavonic traditions, and some Eastern Orthodox and Eastern Catholic churches use this later Church Slavonic as a liturgical language to this day. As the oldest attested Slavic language, OCS provides important evidence for the features of Proto-Slavic, the reconstructed common ancestor of all Slavic languages.

Old Church Slavonic - Wikipedia