Excerpts from Books and Wikipedia
"The threat to European Christendom posed initially by the fifth century Germanic invasions, and by the seventh century by the rapid rise of Islam, forced Christianity onto the defensive. Monastic foundations consolidated Christian learning and thought, and it was from these that the first wave of revivalist Christian missionaries emerged in the seventh century.
The split between Roman Catholic and Byzantine, or Eastern Orthodox, Christianity saw Central and Eastern Europe transformed into a competitive arena for accumulating converts. . . . They translated the scriptures into a language known as Old Bulgarian, or Old Church Slavonic, developing as they did the Cyrillic alphabet, which is still used in Russia and Bulgaria today." [DK Timelines, p. 143] "The emperor of Bulgaria Boris I was baptized in 864 and was named Michael. Bulgaria fall under the influence of Byzantine Empire." [Wikipedia: Timeline of Middle Ages]