Old Church Slavonic (OCS) or Old Slavonic (/sləˈvɒnɪk, slæˈvɒn-/ slə-VON-ik, slav-ON-) is the first Slavic literary language and the oldest extant written Slavonic language attested in literary sources. It belongs to the South Slavic subgroup of the Balto-Slavic branch of the Indo-European language family and remains the liturgical language of many Christian Orthodox churches.
Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and undertaking the task of translating the Gospels and necessary liturgical books into it 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).
