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.

In 862 the Slavic Prince Rostislav of Moravia sent a request to Constantinople for missionaries who could help him strengthen Christianity among the Slavs. (It must be stated that the motives impelling him were not solely religious, but also political. By strengthening Christianity among the Slavs he was fortifying his own national independence against the new historical colossus of Germanism, which was forming at the time). The choice, probably made with the help of Patriarch Photius, naturally fell on the two brothers, and in the middle of 864 they arrived in Velehrad, the capital of Moravia.

Alexander Schmemann, A History of the Orthodox Church

Alexander Schmemann, A History of the Orthodox Church

Here, along with their purely missionary activity, they began their great work of translating Christian writings into Slavic. From a legalistic point of view, they were working in a region that had been part of the sphere of influence of the Roman Church from ancient times.

These were years of intense struggle between Pope Nicholas I and Patriarch Photius. To regularize their situation, or perhaps because of a summons from Rome, which had become disturbed at the growth of Greek influence among the western Slavs, the brothers quickly went to the Western capital, where Nicholas’ successor Hadrian greeted them ceremoniously and affectionately. The Slavic Gospel was placed on the altar of St. Mary as a sign of papal blessing, and the Slavic liturgy was performed in many Roman Churches.