Cyril and Methodius spoke 'native Macedonian dialect'
The brothers Cyril and Methodius are commonly known as the 'Slav Apostles'.
Together they carried out the Christian missionary work over the so-called 'Slavic peoples' in the 9th century, which is why they were given the name "Slavonic Apostles". They created the first script for the ancient Slavic language - the Glagolitic alphabet.
The work of the two brothers from Salonica (Macedonian, Solun) had great influence on the cultural development of the so called Slavs. In the Orthodox Church they are worshiped as saints with the attribute of apostles.
Most of the literature on Cyril and Method (Macedonian, Kiril i Metodij) rarely examines the real background of the two brothers. They are often mistakenly and succinctly referred to as Greeks since their homeland has been within modern Greek borders since 1913.
However, the brothers took the Slavic language as a basis, which 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 hint in the work "Selected Writings. VI, Early Slavic Paths and Crossroads. 1, Comparative Slavic Studies. The Cyrillo-Methodian Tradition" by Roman Jakobson.
Jakobson undoubtedly provides information about the language of the brothers, whom he calls 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.
Literature: Selected Writings. VI, Early Slavic Paths and Crossroads. 1, Comparative Slavic Studies. The Cyrillo-Methodian Tradition von Roman Jakobson - Google Books Links
Source: Makedonien.mk - German translation/Deutsch