The Bulgarian Orthodox Church has its origin in the flourishing Christian communities and churches, set up in the Balkans as early as the first centuries of the Christian era.
Christianity was brought to the Bulgarian lands and the rest of the Balkans by Apostle Paul and Apostle Andrew in the 1st century AD when the first organised Christian communities were also formed. By the beginning of the 4th century, Christianity had become the dominant religion in the region and towns like Serdica (Sofia), Philipopolis (Plovdiv), Odessus (Varna) and Adrianople (Edirne) were significant centres of Christianity in the Roman Empire.
The barbaric raids and incursions in the 4th and the 5th and the settlement of Slavs and Bulgars in the 6th and the 7th century wrought considerable damage to the ecclesiastical organisation of the Christian Church in the Bulgarian lands, yet they were far from destroying it. Christianity started to pave its way from the surviving Christian communities to the surrounding Slavic mass and by the middle of the 9th century, the majority of the Bulgarian Slavs, especially those living in Thrace and Macedonia, were already Christianised. The process of conversion also enjoyed some success among the Bulgar nobility. However, it was not until the official adoption of Christianity by Tsar Boris I in 865 that conditions for the establishment of an independent Bulgarian ecclesiastical entity were created.
From the very start Boris I was aware that the cultural advancement and the re-affirmation of the sovereignty and prestige of a Christian Bulgaria could be achieved through an enlightened and zealous clergy governed by an autocephalous church. To this end, he manoeuvred between the Patriarchate of Constantinople and the Roman Pope for a period of five years until the Fourth Council of Constantinople granted in 870 AD the Bulgarians an autonomous Bulgarian archbishopric.
The archbishopric had its seat in the Bulgarian capital of Pliska and its diocese covered the whole territory of the Bulgarian state. The tug-of-war between Rome and Constantinople was also resolved by putting the Bulgarian archbishopric under the jurisdiction of the Patriarch of Constantinople from whom it obtained its first primate, its clergy and theological books.
Although the archbishopric enjoyed full internal autonomy, the goals of Boris I were scarcely fulfilled. A Greek liturgy offered by a Byzantine clergy furthered neither the cultural development of the Bulgarians, nor the consolidation of the Bulgarian state; it would have eventually resulted in the loss of both the identity of the people and the statehood of Bulgaria. Thus, the arrival of the most distinguished disciples of Saints Cyril and Methodius to Bulgaria in 886 came as a highly beneficial opportunity. Boris I entrusted the disciples with the task to instruct the future Bulgarian clergy in the Glagolitic alphabet and the Slavonic liturgy prepared by Cyril and based on the vernacular of the Bulgarian Slavs from the region of Thessaloniki. In 893, the Greek clergy was expelled from the country and the Greek language was replaced with the Slav-Bulgarian vernacular.
Info: wikipedia