The brothers Cyril and Methodius are commonly known as the ‘Apostles of the Slavs’.
They jointly pursued the Christian missionary work of the so-called ‘Slavic peoples’ in the 9th century, which is why they were given the designation “Apostles of the Slavs”. They created the first script for the Old Slavonic language – the Glagolitic alphabet.
The work of the two brothers from Salonica (Macedonian, Solun) had a great influence on the cultural development of the Slavs. In the Orthodox Church they are venerated as saints with the attribute apostles.
In most of the literature on Cyril and Method (Macedonian, Kiril i Metodij), however, the background of the two brothers is seldom examined. Often they are mistakenly and succinctly referred to as Greeks, since their homeland is within modern Greek borders since 1913.
However, it is the case that the brothers took as a basis the Slavic language that was spoken in their homeland (in the area of Thessaloniki). Thus, the first literary language of the Slavs is based on the dialects of Macedonia.
We find a reference to this historical fact in the work “Selected Writings. VI, Early Slavic Paths and Crossroads. 1, Comparative Slavic Studies. The Cyrillo-Methodian Tradition” by Roman Jakobson.
Jakobson provides unquestionable information about the language of the brothers, which he describes as the ‘native Macedonian dialect’.
So we read on page 34 in his book:
Therefore the Slavic apostles, Constantine-Cyril and Methodius, were able to use their native Macedonian dialect for missionary work in Moravia. Moreover, their teaching in this language were intended not only for Moravians but for all the Slavic lands, as was expressly stated at the beginning.
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Source: Selected Writings. VI, Early Slavic Paths and Crossroads. 1, Comparative Slavic Studies. The Cyrillo-Methodian Tradition by Roman Jakobson – Google Books Links