The 9th-century Byzantine missionaries Saints Cyril and Methodius 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.
The language was standardized for the mission of the two apostles to Great Moravia (the territory of today's western Slovakia and Czech Republic; see Glagolitic alphabet for details). For that purpose, Cyril and his brother Methodius started to translate religious literature to Old Church Slavonic, allegedly based on the Slavic dialects spoken in the hinterland of their hometown, Thessaloniki, in today's Greece.
As part of the preparation for the mission, in 862/863, the Glagolitic alphabet was created and the most important prayers and liturgical books, including the Aprakos Evangeliar (a Gospel Book lectionary containing only feast-day and Sunday readings), the Psalter, and Acts of the Apostles, were translated. (The Gospels were also translated early, but it is unclear whether Sts. Cyril or Methodius had a hand in this.)
The language and the alphabet were taught at the Great Moravian Academy (Slovak: Veľkomoravské učilište) and were used for government and religious documents and books between 863 and 885. The texts written during this phase contain characteristics of the Slavic vernaculars in Great Moravia.