Bulgaria’s adoption of Christianity as the formal and only state religion took place in 864-865 AD under the leadership of Khan / Knyaz Boris I Mihail (r. 852-889; 893 AD).
As a result of the successful reigns of Khan (or Kanas) Krum (r. 803-814 AD), Khan (Kanas) Omurtag (r. 814-831 AD), Khan (Kanas) Malamir (r. 831-836), and Khan (Kanas) Presian (r. 836-852 AD), by the middle of the 9th century the First Bulgarian Empire had become a huge empire spanning from the Black Sea in the east to the Adriatic Sea in the west, and from the Northern Carpathian Mountains in the north to the Aegean Sea in the south, including the entire or part of the territory of modern-day Bulgaria, Romania, Serbia, Greece, Turkey, Albania, Macedonia, Hungary, Moldova, and Ukraine. However, the major peoples inhabiting the Bulgarian Empire – the Ancient Bulgars (whose religion is known as tengriism) and the Slavs as well as the local Thracian population and others – worshipped different gods according to their own religions and mythologies. This was true even though there were entire areas in the then Bulgarian Empire which had been Christianized in earlier periods, and even though the first Khans from the House of Dulo are believed to have been Christians who were baptized by the imperial court of the Eastern Roman Empire, i.e. Byzantium: Khan (Kanas) Kubrat (r. ca. 630-ca.660) who founded the so called Old Great Bulgaria in 632 AD on the territory of much of modern-day Ukraine and Southwest Russia; Khan (Kanas) Asparuh (r. ca. 680-700) who expanded the state to the southwest technically creating modern-day Danube Bulgaria around 680 AD; and Khan (Kanas) Tervel (r. 700-718/721) who saved Europe from an Arab invasion during the siege of Constantinople in 717 AD. This led Khan Boris I to decide to unite the different ethnicities in the First Bulgarian Empire with a new common religion, and to pick Christianity (even though the adoption of Islam and Judaism were also offered to him by foreign emissaries) because Bulgaria was then the only still pagan major European power, and he wanted Bulgaria to be treated as an equal by the Byzantine Empire in the east and the successors of the Frankish Empire in the west.
While Khan Boris I initially intended to adopt the Western form of Christianity from the Pope in Rome via the Kingdom of the East Franks (East Francia in modern-day Germany) because Byzantium had been Bulgaria’s major geopolitical foe, he was forced to change his decision after an unsuccessful war with the Byzantines imposed on him the adoption of the Eastern form of Christianity as part of a peace treaty signed in 863 AD. This resulted after the First Bulgarian Empire had had to fight simultaneously Byzantium in the southeast and Great Moravia in the northwest. Thus, in 863 or 864 AD, a mission from the Patriarch of Constantinople Photios came to Pliska and converted the Bulgarian Tsar, his family and high-ranking dignitaries, who were baptized as Christians. Khan Boris I became Knyaz Boris I Mihael – taking the name of his baptist, Byzantine Emperor Michael III (r. 842-867 AD), and in 865 AD there was baptism en masse of the entire Bulgarian population. Thus, even though the subsequent years saw the first major clashes between the Pope in Rome and the Ecumenical Patriarch in Constantinople over the “Bulgarian Question”, i.e. whose diocese the large and powerful newly baptized First Bulgarian Empire should belong to, Bulgaria remained in the camp of Eastern Orthodox Christianity subsequently helping pass it on to later emerging nations such as Serbia and Russia, and thus modifying forever the history of Europe.
Bulgaria’s adoption of Christianity, however, went far from smoothly, and not only because of the clashes between the Pope in Rome and the Ecumenical Patriarch in Constantinople over whose diocese the newly converted Bulgarians should belong to. In 865, conservative Bulgar aristocrats from all 10 komitats (administrative regions) of the First Bulgarian Empire revolted against Boris, who now took the Christianized title of Knyaz (i.e. King) in order to restore the old religion, tengriism. Knyaz Boris I managed to suppress the revolt executing 52 Bulgarian boyars (heads of noble families). According to some sources, he also had their entire extended families executed. Until the end of his life, Knyaz Boris was haunted by guilt about the harshness of his measures and the moral price of his decision in 865. In his later correspondence with Pope Nicholas I, the Knyaz asked whether his actions had crossed the borders of Christian humility, for which the Pope offered forgiveness: “You have sinned rather because of zeal and lack of knowledge, than because of other vice. You receive forgiveness and grace and the benevolence of Christ, since penance has followed on your behalf.”
Knyaz Boris realized that the Christianization of Bulgaria gave Byzantium great influence over the domestic affairs of the Bulgarian Empire. Thus, juggling the differences of Rome and Constantinople, he eventually managed to get Byzantium’s Ecumenical Patriarchate as well as the Pope in Rome to recognize an independent (autocephalous) Bulgarian Archbishopric, which was created in 870 AD in an unprecedented development for Europe because independent churches had been only those founded by Apostles or Apostles’ disciples. For example, the Papacy in Rome had been challenging Constantinople’s claim of equality to Rome on the grounds that the Church of Constantinople had not been founded by an Apostle of Jesus Christ. Nonetheless, this development was also a success for Byzantium, and during the decade after 870 AD, Pope Adrian II and his successors kept trying desperately to convince Bulgaria’s Knyaz Boris to leave Constantinople’s religious sphere.
Knyaz Boris I Mihail sealed the success of his deed, the adoption of Christianity, in 886 AD when Bulgaria welcomed the disciples of St. Cyril and St. Methodius, St. Kliment Ohridski and St. Naum Preslavski, helping them to teach thousands of Bulgarian clergymen to serve in Bulgarian. Thus, Bulgaria adopted the Bulgarian script, also known as the Slavic script – first the Glagolithic and then the Bulgarian (Cyrillic) alphabet. This allowed Knyaz Boris, and his successor Tsar Simeon I the Great to declare Bulgarian (also known as Old Bulgarian or Church Slavonic) as the official language of the Bulgarian Orthodox Church during the Council of Preslav in 893 AD (which also moved Bulgaria’s capital from Pliska to Veliki Preslav (Great Preslav)). As all over Europe religious services were held in the “official” church languages Latin and Greek, this “nationalization” of the liturgy language by Bulgaria became another exceptional development in medieval Europe after the recognition of the independent Bulgarian church.