Under the Ottoman system, Christians were considered an inferior class of people.Thus, Bulgarians, like other Christians, were subjected to heavy taxes and a small portion of the Bulgarian populace experienced partial or complete Islamisation.The latter gradually inflicted total linguistic replacement of Thracian, if the Thracians had not already been Romanized or Hellenized.
A symbiosis was carried out between the numerically weak Bulgars and the numerous Slavic tribes in that broad area from the Danube to the north, to the Aegean Sea to the south, and from the Adriatic Sea to the west, to the Black Sea to the east, who accepted the common ethnonym "Bulgarians".
This might have facilitated their eventual Slavonization, The major port towns in Pontic Bulgaria remained Byzantine Greek in their outlook.
The large scale population transfers and territorial expansions during the 8th and 9th century, additionally increased the number of the Slavs and Byzantine Christians within the state, making the Bulgars quite obviously a minority.
By the 4th century the major city of Serdica had predominantly Thracian populace based on epigraphic evidence, which shows prevailing Latino-Thracian given names, but thereafter the names were completely replaced by Christian ones.
The Early Slavs emerged from their original homeland in the early 6th century, and spread to most of the eastern Central Europe, Eastern Europe and the Balkans, thus forming three main branches: the West Slavs in eastern Central Europe, the East Slavs in Eastern Europe, and the South Slavs in Southeastern Europe (Balkans).