In 212 AD, the Constitutio Antoniniana of Emperor Caracallas granted Roman citizenship to all free citizens of the empire. The word Romios ("Romans") became the common name for the Greeks of the Eastern Roman Empire.
The number of native speakers of Greek is estimated to be around 600, making up a good third of the eastern Roman population, i.e. about 10 to 15 million people. The core area of the language was in the ancient settlement areas of the Greeks, mainly on the southern Balkan peninsula and in the western part of Asia Minor. The number of those who knew how to communicate in Greek, however, was probably far greater. Also the eastern Roman cities were still strongly influenced by Greek.
The invasions of the Longobards and Arabs in the 7th century led to the loss of most of the provinces, including the Italian and Asian ones, with the exception of Anatolia. The remaining territories were predominantly Greek, which meant that the population of the empire now saw itself as a more cohesive unit, ultimately developing a conscious identity.
Greek was the sole administrative and state language of the Eastern Roman or Byzantine Empire from the 7th century onwards. The Eastern Roman Empire became a kind of "Holy (=Orthodox) Roman Empire of the Greek Nation" (Ralf G. Jahn).