Old Church Slavonic

Old Church Slavonic or Old Slavonic 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. Until the reforms of Patriarch Nikon of Moscow between 1652 and 1666, Church Slavonic was the mandatory language of the Russian Orthodox Church.