In the 4th century, a group of Goths arrived in northern Bulgaria and settled in and around Nicopolis ad Istrum. There the Gothic bishop Ulfilas translated the Bible from Greek to Gothic, creating the Gothic alphabet in the process. This was the first book written in a Germanic language, and for this reason at least one historian refers to Ulfilas as "the father of Germanic literature".^[17] The first Christian monastery in Europe was founded in 344 by Saint Athanasius near modern-day Chirpan following the Council of Serdica.^[18]

Due to the rural nature of the local population, Roman control of the region remained weak. In the 5th century, Attila's Huns attacked the territories of today's Bulgaria and pillaged many Roman settlements. By the end of the 6th century, Avars organized regular incursions into northern Bulgaria, which were a prelude to the en masse arrival of the Slavs.

During the 6th century, the traditional Greco-Roman culture was still influential, but Christian philosophy and culture were dominant and began to replace it.^[19] From the 7th century, Greek became the predominant language in the Eastern Roman Empire's administration, Church and society, replacing Latin.^[20]