Despite all the military setbacks and natural disasters, the skilful diplomacy of Boris I prevented any territorial losses and kept the realm intact. In this complex international situation Christianity had become attractive as a religion by the mid 9th-century because it provided better opportunities for forging reliable alliances and diplomatic ties.

Taking this into account, as well as a variety of internal factors, Boris I converted to Christianity in 864, assuming the title Knyaz (Prince). Taking advantage of the struggle between the Papacy in Rome and the Ecumenical Patriarchate of Constantinople, Boris I brilliantly manoeuvred to assert the independence of the newly established Bulgarian Church. To check the possibility of Byzantine interference in the internal matters of Bulgaria, he sponsored the disciples of the brothers Cyril and Methodius to create literature in Old Bulgarian language.

Boris I dealt ruthlessly with the opposition to the Christianisation of Bulgaria, crushing a revolt of the nobility in 866 and overthrowing his own son Vladimir (r. 889–893) after he attempted to restore the traditional religion. In 893 he convened the Council of Preslav where it was decided that the capital of Bulgaria was to be moved from Pliska to Preslav, the Byzantine clergy was to be banished from the country and replaced with Bulgarian clerics, and Old Bulgarian language was to replace the Greek in liturgy. Bulgaria was to become the principal threat to the stability and security of the Byzantine Empire in the 10th century.