stylistic:style_transfer, structural:summarization, information:classification, linguistic:translation
History

It is generally held that the language was standardized by two Byzantine missionaries, Cyril and his brother Methodius, for a mission to Great Moravia (the territory of today's eastern Czech Republic and western Slovakia; for details, see Glagolitic alphabet). The mission took place in response to a request by Great Moravia 's ruler, Duke Rastislav for the development of Slavonic liturgy.

As part of preparations for the mission, in 862/863, the missionaries developed the Glagolitic alphabet and translated the most important prayers and liturgical books, including the Aprakos Evangeliar, the Psalter, and the Acts of the Apostles, allegedly basing the language on the Slavic dialect spoken in the hinterland of their hometown, Thessaloniki, in present-day Greece.

Based on a number of archaicisms preserved until the early 20th century (the articulation of yat as / æ / in Boboshticë, Drenovë, around Thessaloniki, Razlog, the Rhodopes and Thrace and of yery as / ɨ / around Castoria and the Rhodopes, the presence of decomposed nasalisms around Castoria and Thessaloniki, etc.), the dialect is posited to have been part of a macrodialect extending from the Adriatic to the Black Sea, and covering southern Albania, northern Greece and the southernmost parts of Bulgaria.

Because of the very short time between Rastislav 's request and the actual mission, it has been widely suggested that both the Glagolitic alphabet and the translations had been "in the works" for some time, probably for a planned mission to the Bulgaria.

The language and the Glagolitic alphabet, as taught at the Great Moravian Academy (Slovak: Veľkomoravské učilište), were used for government and religious documents and books in Great Moravia between 863 and 885. The texts written during this phase contain characteristics of the West Slavic vernaculars in Great Moravia.

In 885 Pope Stephen V prohibited the use of Old Church Slavonic in Great Moravia in favour of Latin. King Svatopluk I of Great Moravia expelled the Byzantine missionary contingent in 886.

Exiled students of the two apostles then brought the Glagolitic alphabet to the Bulgarian Empire, being at least some of them Bulgarians themselves. Boris I of Bulgaria (r. 852–889) received and officially accepted them; he established the Preslav Literary School and the Ohrid Literary School. Both schools originally used the Glagolitic alphabet, though the Cyrillic script developed early on at the Preslav Literary School, where it superseded Glagolitic as official in Bulgaria in 893.

The texts written during this era exhibit certain linguistic features of the vernaculars of the First Bulgarian Empire. Old Church Slavonic spread to other South-Eastern, Central, and Eastern European Slavic territories, most notably Croatia, Serbia, Bohemia, Lesser Poland, and principalities of the Kievan Rus' – while retaining characteristically Eastern South Slavic linguistic features.

Later texts written in each of those territories began to take on characteristics of the local Slavic vernaculars, and by the mid-11th century Old Church Slavonic had diversified into a number of regional varieties (known as recensions). These local varieties are collectively known as the Church Slavonic language.

Apart from use in the Slavic countries, Old Church Slavonic served as a liturgical language in the Romanian Orthodox Church, and also as a literary and official language of the princedoms of Wallachia and Moldavia (see Old Church Slavonic in Romania), before gradually being replaced by Romanian during the 16th to 17th centuries.

Church Slavonic maintained a prestigious status, particularly in Russia, for many centuries – among Slavs in the East it had a status analogous to that of Latin in Western Europe, but had the advantage of being substantially less divergent from the vernacular tongues of average parishioners.

Some Orthodox churches, such as the Bulgarian Orthodox Church, Russian Orthodox Church, Serbian Orthodox Church, Ukrainian Orthodox Church and Macedonian Orthodox Church – Ohrid Archbishopric, as well as several Eastern Catholic Churches, still use Church Slavonic in their services and chants.