Old Church Slavonic (словѣ́ньскъ ѩꙁꙑ́къ, slověnĭskŭ językŭ), also known as Old Slavonic, was the first Slavic literary language.
Historians credit two 9th-century Byzantine missionaries, Saints Cyril and Methodius, with standardising the language, as well as using it in translating the Bible and other Ancient Greek ecclesiastical texts (texts relating to the Christian Church or its clergy) as a part of the Christianisation of the Slavs.
It's often thought to have been based on the dialect of the 9th-century Byzantine Slavs who were living in the Theme of Thessalonica (θέμα Θεσσαλονίκης, théma Thessaloníkis, now part of present-day Greece).

The language played an important role in the history of the Slavic languages. It served as a basis and model for later Church Slavonic traditions. Some Eastern Orthodox and Eastern Catholic churches use the later Church Slavonic (црькъвьнословѣньскъ ѩзыкъ, crĭkŭvĭnoslověnĭskŭ językŭ), also known as New Church Slavonic, as a liturgical language to this day.
Old Church Slavonic provides important evidence for the features of Proto-Slavic, the reconstructed common ancestor of all Slavic languages.

More to come! :-)