From the 7th century onwards, Greek was the only language of administration and government in the Byzantine Empire. This stage of language is thus described as Byzantine Greek. The study of the Medieval Greek language and literature is a branch of Byzantine Studies, or Byzantinology, the study of the history and culture of the Byzantine Empire.
The beginning of Medieval Greek is occasionally dated back to as early as the 4th century, either to 330 AD, when the political centre of the Roman Empire was moved to Constantinople, or to 395 AD, the division of the Empire. However, this approach is rather arbitrary as it is more an assumption of political, as opposed to cultural and linguistic, developments. Indeed, by this time the spoken language, particularly pronunciation, had already shifted towards modern forms.
The conquests of Alexander, and the ensuing Hellenistic period, had caused Greek to spread to peoples throughout Anatolia and the Eastern Mediterranean, altering the spoken language's pronunciation and structure.
Medieval Greek is the link between this vernacular, known as Koine Greek, and the Modern Greek language. Though Byzantine Greek literature was still strongly influenced by Ancient Greek, it was also influenced by vernacular Koine Greek, which is the language of the New Testament and the liturgical language of the Greek Orthodox Church.
Constantine moved to Byzantium (renamed Constantinople) in 330. The city though a major imperial residence like other cities such as Trier, Milan, and Sirmium was not officially a capital until 359. Nonetheless the imperial court resided there and the city was the political centre of the eastern parts of the Roman Empire where Greek was the dominant language. At first, Latin remained the language of both the court and the army. It was used for official documents, but its influence waned. From the beginning of the 6th century, amendments to the law were mostly written in Greek. Furthermore, parts of the Roman Corpus Iuris Civilis were gradually translated into Greek. Under the rule of Emperor Heraclius (610–641 AD), who also assumed the Greek title Basileus (Greek: βασιλεύς, "monarch") in 629, Greek became the official language of the Eastern Roman Empire. This was in spite of the fact that the inhabitants of the empire still considered themselves Romaioi ("Romans") until its end in 1453.. This is because they saw their State as the Roman Empire continued.
Despite the absence of reliable demographic figures, it has been estimated that less than one third of the inhabitants of the Eastern Roman Empire, around eight million people, were native speakers of Greek. The number of those who were able to communicate in Greek may have been far higher. The native Greek speakers consisted of many of the inhabitants of the southern Balkan Peninsula, south of the Jireček Line, and all of the inhabitants of Asia Minor, where the native tongues (Phrygian, Lycian, Lydian, Carian etc.), except Armenian in the east, had become extinct, replaced by Greek, by the 5th century.
In any case, all cities of the Eastern Roman Empire were strongly influenced by the Greek language.