Alexander Schmemann, A History of the Orthodox Church
At the end of the seventh century came a new invasion from a Turkic people, the Bulgars, who asserted their authority in provinces settled by Slavs and began a struggle against the empire that lasted many years. As occurred later with the Varangian conquerors of the eastern Slavs, they themselves became Slavicized. Almost at the very gates of the capital, a mighty Bulgar-Slavic state was gradually formed by a nearly uninterrupted war that lasted throughout the whole eighth — or “iconoclastic” — century.
We do not know how this new and threatening Slavic problem would have been solved for Byzantium if a development had not taken place in the second half of the ninth century that marks the real beginning of the Slavic chapter in the history of Orthodoxy: the “translation” of Christianity into Slavic by two brothers who became Byzantine saints: Constantine (who later received the monastic name of Cyril) and Methodius.
There is an immense literature on the first teachers of the Slavs. Historical research on their activity is complicated by confessional hostility. Which holds the honor of first encouraging Slavic Christianity — Constantinople or Rome? There are disputes on these matters, but the answers are unimportant by comparison with the immense significance of the heritage of Cyril and Methodius in the destiny of the Eastern Church.
They belonged to the intellectual elite of Byzantium, which was grouped around Patriarch Photius in Constantinople in the second half of the ninth century. Constantine was a philosopher, scholar, and linguist, and important missions to the Arabs and Khazars, who lived in southern Russia on the near side of the Dnieper, had been entrusted to him. Natives of Thessalonica, a city with a large Slavic population, the brothers had in all probability known the local Slavic dialect from childhood.