The spread of Christianity
Internally, the 8th and 9th centuries saw the gradual assimilation of the Bulgars by the Slavic majority. There are almost no sources that describe this process, but it was certainly facilitated by the spread of Christianity, which provided a new basis for a common culture. Boris I of Bulgaria (852–889) was baptized a Christian in 864, at a time when the conflict between the Roman church and the Eastern church in Constantinople was becoming more open and intense. Although Boris’s baptism was into the Eastern church, he subsequently wavered between Rome and Constantinople until the latter was persuaded to grant de facto autonomy to Bulgaria in church affairs.
The spread of Christianity was facilitated by the work of Saints Cyril and Methodius, who had invented an alphabet in which to write the Slavic language (known as Old Church Slavonic or Old Bulgarian) and almost completed the translation of the Bible (most parts of both the Old and the New Testament) into the vernacular of the land. They also developed a Slavonic liturgy in Moravia. When Moravia committed to Rome and expelled the disciples of Cyril and Methodius, many of them resettled in Bulgaria, where they were welcomed by Boris and undertook the translation of church books and the training of priests. St. Clement and St. Naum are credited with preparing more than 3,000 priests at the religious educational centre (in effect the first Slavic university) they established on the shores of Lake Ohrid (Okhrid) in Macedonia.
Bulgaria’s conversion had a political dimension, for it contributed both to the growth of central authority and to the merging of Bulgars and Slavs into a unified Bulgarian people. Boris adopted Byzantine political conceptions, referring to himself as ruler “by the grace of God,” and the new religion provided justification for suppressing those boyars of Bulgar origin who clung to paganism and the political and social order with which it was linked. In 889 Boris, whose faith apparently was deep and genuine, abdicated to enter a monastery. When his eldest son, Vladimir, fell under the influence of the old boyars and attempted to reestablish paganism, Boris led a coup that overthrew him. After Vladimir was deposed and blinded, Boris convened a council that confirmed Christianity as the religion of the state and moved the administrative capital from Pliska to the Slavic town of Preslav (now known as Veliki Preslav). The council conferred the throne on Boris’s third son, Simeon, and Boris retired permanently to monastic life.