The people who lived in the "Byzantine Empire" never knew nor used the word "Byzantine." They knew themselves to be Romans, nothing more and absolutely nothing less. By transferring the Imperial capital from Rome on the Tiber to the New Rome on Bosphorus, dubbed Constantinople, the Emperor Constantine I had transferred the actual identity of Rome to the new location. Long before Constantine I, the idea of "Rome" had become dissociated from the Eternal City on the Tiber. For a Roman meant a Roman citizen, wherever he lived. Before the Imperial period, in 89 BC, a Roman law had granted Roman citizenship to people throughout Italy. Afterwards, citizenship became extended to an increasing number of people in different parts of the Empire. In 212, Emperor Caracalla declared all free persons in the Empire to be Roman citizens, entitled to call themselves Roman, not merely subject to the Romans. Within a few decades, people begin to refer to the entire Empire less often [in Latin] as "Imperium Romanorum" [Domain of the Romans] and more often as "Romania" [Romanland]
In the provinces close to Constantinople, where the Greek language predominated over the Latin of Old Rome, the idea of Roman citizenship and identity appealed to a broad segment of the population. Greek speaking citizens were proud to be Romans: in Latin, "Romani," or, in Greek, "Romaioi." The word "Romaioi" became descriptive of the Greek speaking population of the Empire. The old ethnic name applied to Greeks, "Hellene", fell into disuse. In ancient times, of course, "Hellene" had meant Greek. Hellene meant Greek from the seventh century BC onward, if not earlier. Although Homer called Greeks by other names, Herodotus, Pericles, Plato and Alexander were all "Hellenes," as were Greek speaking inhabitants of the Roman Empire in the first and second centuries AD. In the fourth century AD, as the Empire became Christianized, the term "Hellene" became redefined by common convention to include people who still worshipped the old gods and studied philosophy in hopes of resisting the new faith of Christianity. Emperor Julian II [361-363], an Emperor who tried to stop the Christian tide, described himself as a "Hellene." By "Hellene," Julian signified his combination of Neo-Platonic philosophy and worship of the Olympians.
In the final years of the fourth century AD, Emperor Theodosius I [379-395] made Christianity the sole state religion after subduing the rebellion of an "Hellene" usurper, a westerner named Eugenius. After Theodosius' critical decision, fewer and fewer people were willing to call themselves "Hellenes." For centuries more, the word "Hellene" remained in bad repute, associated with outlawed religious ideas and disloyalty to the state. Greek speakers found the identity of "Romaioi" in place of "Hellene" to be a safe refuge in changing times. Greek speaking "Romaioi" inhabited the Empire until the its demise in the fifteenth century.
The Empire at Constantinople should not be called the "Byzantine Empire" at all. If it requires a special name, we might better name the Empire at Constantinople with the title of the "Romaion Empire" from the Greek "Basileia Romaion" [Empire of the Romaioi].
It is from these ideas that we begin our journey into the world of Justinian the Great.