Slavic language based primarily on the Macedonian (South Slavic) dialects around Thessalonica (Thessaloníki). It was used in the 9th century by the missionaries Saints Cyril and Methodius (Cyril and Methodius, Saints), who were natives of Thessalonica, for preaching to the Moravian Slavs and for translating the Bible into Slavic. Old Church Slavonic was the first Slavic literary language and was written in two alphabets known as Glagolitic (Glagolitic alphabet) and Cyrillic (Cyrillic alphabet) (the invention of Glagolitic has been ascribed to St. Cyril). Old Church Slavonic was readily adopted in other Slavic regions, where, with local modifications, it remained the religious and literary language of Orthodox Slavs throughout the Middle Ages.

The language as it appeared after the 12th century in its various local forms is known as Church Slavonic; this language has continued as a liturgical language into modern times. It continued to be written by the Serbs and Bulgarians until the 19th century and had significant influence on the modern Slavic languages, especially on the Russian literary language that grew out of a compromise style incorporating many Church Slavonic elements into the native Russian vernacular.