 Thracians , one of the three primary ancestral groups of modern Bulgarians , began appearing in the region during the Iron Age . In the late 6th century BC , the Persians conquered most of present @-@ day Bulgaria. and kept it until 479 BC . With influence from the Persians , the bulk of the Thracian tribes were united in the Odrysian kingdom in the 470s BC by king Teres , but were later subjugated by Alexander the Great and by the Romans in 46 AD . After the division of the Roman Empire in the 5th century the area fell under Byzantine control . By this time , Christianity had already spread in the region . A small Gothic community in Nicopolis ad Istrum produced the first Germanic language book in the 4th century , the Wulfila Bible . The first Christian monastery in Europe was established around the same time by Saint Athanasius in central Bulgaria . From the 6th century the easternmost South Slavs gradually settled in the region , assimilating the Hellenised or Romanised Thracians . 
