Eastern Orthodoxy in Bulgaria
The Eastern Orthodox Church in Bulgaria has deep roots, extending back to the 5th and 7th centuries when the Slavs and the Bulgars, respectively, adopted Byzantine Christianity in the period of the First Bulgarian Empire (681-1018). Prior to this official conversion, Christianity had spread to the region during Roman and early Byzantine times.
Source: Wikipedia — Eastern Orthodoxy in Bulgaria (CC BY-SA 4.0)