Many centuries before Constantine established the city of Constantinople, the Megarians had founded a colony at the same place on the European side of the Bosporus. When they established the colony in the seventh century b.c. they were fully aware of the strategic and commercial advantage in locating the community on the border of two continents and at the entrance to two seas, the Black and the Mediterranean. Even the Greek historian Polybius saw the importance of this location, and wrote in the second century b.c. that the inhabitants of Byzantium controlled all commercial vessels entering or leaving the Black Sea, thus placing them in a very powerful position.

When Constantine became emperor at the beginning of the fourth century a.d., he immediately recognized that Rome was too far away to deal with the eastern problems of the Roman Empire. At first he had planned to locate his eastern capital at the site of ancient Troy, but he soon changed his mind for the site at Byzantium. On 11 May 330, Constantine dedicated the “New Rome” and called his new capital “Constantinople.” Historians mark this event as the beginning of the Byzantine Empire, even though this officially took place under the Roman Empire.

Constantine was responsible for an added dimension to the imperial throne. He considered himself the representative of God on earth and so brought a sacred character to his sovereign power. When he made Christianity the state religion of the Roman Empire, it was only natural he should take his role among the bishops of the church as if he were one of them. The emperor fixed this into the physical structure of the city by making the church building the center of the city, which is still to be found in many eastern towns and cities. Thus church and state began to operate in a more uniform way in Constantinople.

The fourth and fifth centuries a.d. were the period of the great heresies in the Christian Church. As these theological battles were fought, Constantinople began to emerge as the second most important religious center of power in Christianity, next to Rome. The bishop of Constantinople began to compete with the bishop of Rome for the primacy of the Christian Church. The new power which came with the New Rome provided the basis for the Greek East to consider itself equal to the Latin West. As more Germanic influence penetrated Rome and since paganism still dominated much of Roman life, the Eastern church leaders felt less and less loyal to the authority of the Roman papacy. Constantinople, on the other hand, was a capital with all the power and prestige that went with this role. The eastern city was, moreover, Greek and tended therefore to separate from Latin Rome so far as ecclesiastical authority was concerned.

After the Turks captured the city in 1453, the patriarch of Constantinople continued to keep his office in the city. The Turks tended to treat their Christian subjects with generosity. In fact, with the establishment of the Ottoman Turkish millet system (religious nations within an empire), it made possible the survival of the Greek Orthodox Church during the four centuries of Turkish rule. To this day the patriarch of Constantinople is the head of the Greek Orthodox Church, though the Greek Orthodox Church does not include the other branches of Eastern Orthodoxy.