khan of Bulgaria (852–889), whose long reign witnessed the conversion of the Bulgarians (Bulgarian Orthodox Church) to Christianity, the founding of an autocephalous Bulgarian church, and the advent of Slavonic literature and establishment of the first centres of Slav-Bulgarian scholarship and education. Boris' active domestic and foreign diplomacy was of great importance in the formation of a united Bulgarian ethnic community, and it left lasting traces on Bulgaria's subsequent development.

When Boris inherited the throne from his father, Bulgaria's territorial, military, and political potential had made it one of the largest states in Europe. Bulgaria's approximate frontiers were the Dnieper River in the northeast, the Carpathian Mountains in the north, the Tisa (Tisza) River in the northwest, the Adriatic Sea in the west, and the Tomorr (Tomor), Belasica, Pirin, Rhodope, and Strandzha mountains in the south. Many Slavic tribes lived within the boundaries of the state, together with the proto-Bulgarians, a tribe of Turkic origin that had settled in the Balkan Peninsula at the end of the 7th century. In view of the religious, ethnic, and language difficulties between the Slavs and the Bulgars, the introduction of a common and compulsory religion for all subjects was one of the principal preconditions for the formation of a united Bulgaria. Pagan Bulgaria needed to join the “family of Christian states,” but the existence of two competing centres of Christianity—Rome and Constantinople—made it difficult for Boris to make his choice. Boris originally intended to accept Roman Christianity, but an unsuccessful war with the Byzantines forced him to adopt the Orthodox faith of Constantinople (864). Boris (at his baptism he took the Christian name Michael), his family, and the nobles who supported his policy were baptized one night in secret by a Byzantine bishop and priests who had been sent to Pliska, the Bulgarian capital. There was serious opposition by both the nobility and the common people to Boris' attempt to enforce mass baptism. A pagan rebellion broke out, and Boris retaliated by executing 52 boyars, together with their families.

Negotiations took place between Boris and Photius, patriarch of Constantinople, on the status of the Bulgarian diocese but did not lead to the result expected by the Bulgarians. The Byzantines demanded that the Bulgarian church organization should be entirely subjected to Constantinople. Dissatisfied, Boris renewed his diplomatic contacts with the West. In 866 he sent embassies to Pope Nicholas I (Nicholas I, Saint) (858–867) and to King Ludwig of Germany. The Pope immediately responded by sending a mission to Bulgaria. The Roman clergy's stay (866–870) soon became a sore point in the acute rivalry between Rome and Constantinople. But since Pope Nicholas I and his successor, Adrian II, proved dilatory on the question of church organization in Bulgaria (they hesitated over the creation of an independent Bulgarian archbishopric), Boris again reopened negotiations with Constantinople (Constantinople, Council of). The Bulgarian church question was finally solved at the eighth ecumenical council in Constantinople in 869–870. Bulgaria was formally placed under the nominal ecclesiastical jurisdiction of the patriarch of Constantinople but received an independent archbishopric. The attempts of the popes to bring the Bulgarian ruler back into the Roman church by pleas and promises of concessions continued until 882 but produced no results.

Boris was quite active in inculcating the Christian faith among the Bulgarian people, in organizing the Bulgarian church as an independent institution, and in building churches throughout the country. In 886 he gave asylum to Clement, Nahum, and Angelarius, the disciples of Cyril and Methodius, missionaries to the Slavs, who had been driven out of Moravia. With Boris' active assistance and material support, these disciples founded centres of Slavic learning at Pliska, Preslav, and Ohrid. As a result of the intensive work of the Slav scholars, the Slavic (Slavic languages) language replaced Greek in church services and in literary life and became the country's official language.

In 889 Boris I abdicated and became a monk, but he retained the right to take an active part in the government of the state. Boris' eldest son and heir, Vladimir (889–893), abandoned his father's policy and became the instrument of a pagan reaction and a leader of the opponents of Slavic letters and literature. Boris then returned to active politics. With the aid of loyal boyars and the army, Boris drove his son from the throne. Vladimir was blinded, unfitting him for rule, and was replaced by Boris' third son, who ruled as Symeon the Great (Simeon I) (893–927). Boris afterward retired to his monastery, making generous grants to the Bulgarian Church and patronizing Slav scholarship. He was canonized by the Orthodox Church, and his feast is celebrated on May 15 (new style; May 2, old style).