Wikipedia
Old Church Slavonic (, ), also known as Old Bulgarian and Old Church Slavic (; often abbreviated to OCS; self-name , slověnĭskŭ językŭ), was the first Slavic literary language. The 9th-century Byzantine missionaries Saints Cyril and Methodius of Slavic, Greek descent, or both, are credited 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 (now in Greece). It 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.