1911 Encyclopedia Britannica
In the article Slavs (under Languages ) will be found a fairly complete account of Old Slavonic in its first form, as it is taken as representing, save for a few peculiarities noticed in their place, the Proto-Slavonic. The reasons are there given for believing it to be the dialect of Slavs settled somewhere between Thessalonica and Constantinople and represented now by the Bulgarians and Macedonians.
After the language had been fixed by the original translations of the New Testament and other Church books it was no more consciously adapted to the dialects of the various peoples, but was used equally among the Croats (whose books were accommodated to the Roman use and written in Glagolitic), Serbs and Russians. These insensibly altered them to make the words easier and allowed their native languages to show through; and the same was the case with the Bulgarians, whose language soon began to lose some of the characteristics of O.S. Hence our earliest MSS. already show departure from the norm which can be established by comparison; about a dozen (8 Glagolitic) MSS. and fragments afford trustworthy material dating from the 10th and 11th centuries, but even then the S. Slavs were weak in distinguishing i and y, the Russians mixed up Ä… with u, Ä™ with ja and so on; but in the actual texts great conservatism prevailed, whereas any additions, such as colophons or marks of ownership, betray the dialect of the writer more clearly, and such scraps and a few deeds are our earliest authorities for Servian and Russian. But the Church language as insensibly modified continued to be the literary language of Croatia until the 26th century, of Russia until 1700, and of Bulgaria, Servia and Rumania until the early part of the 10th century, and is still the liturgical language of Dalmatia, the Balkans, Russia and the Ruthenian Uniates.
Its literature was enriched in the second generation by the works of Clement, bishop of Ochrida, and John, exarch of Bulgaria, and other writers of the time of Tsar Simeon, but it is almost all ecclesiastical in character. Perhaps the most interesting book in Church Slavonic is the Russian chronicle, but that has many old Russian forms. Otherwise certain translations of Greek Apocrypha are of importance, especially when the Greek original is lost, e.g. the Book of Enoch; other Apocrypha in Church Slavonic are said to have been written by Jeremias, a Bogomil priest, but they are probably derived from Eastern sources. The Slavonic text of the Bible is not of importance for textual criticism, as the translation was made late, and even so has never been studied from that point of view. The whole Bible was not finished till the 15th century, some of the less necessary books being translated from the Vulgate.
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Chisholm, Hugh, General Editor. Entry for 'Old Slavonic'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/o/old-slavonic.html. 1910.