Constantine the Great - Founder of Constantinople

Nothing that Constantine the Great did shows his ability more clearly than his seizing upon the site of old Byzantium for the location for his new capital. The place was admirably sited for an imperial residence, being over against Asia which the Persians were threatening, and in easy touch with the Danube, where the Northern Barbarians were always swarming. Note that Constantinople was from the outset a Christian city; as contrasted with old Rome, where paganism still kept a firm grip, at least on much of the population, for nearly a century.

Sozomen (d. c. 450 CE), Ecclesiastical History, II.3:

The Emperor [Constantine] always intent on the advancement of religion erected splendid Christian temples to God in every place --- especially in great cities such as Nicomedia in Bithynia, Antioch on the Orontes, and Byzantium. He greatly improved this latter city, and made it equal to Rome in power and influence; for when he had settled his empire as he was minded, and had freed himself from foreign foes, he resolved on founding a city which should be called by his own name, and should equal in fame even Rome. With this intent he went to the plain at the foot of Troy on the Hellespont. . . and here he laid out the plan of a large and beautiful city, and built gates on a high spot of ground, whence they are still visible from the sea to sailors. But when he had proceeded thus far, God appeared to him by night and bade him seek another site for his city.

Led by the divine hand, he came to Byzantium in Thrace, beyond Chalcedon in Bithynia, and here he desired to build his city, and render it worthy of the name of Constantine. In obedience to the command of God, he therefore enlarged the city formerly called Byzantium, and surrounded it with high walls; likewise he built splendid dwelling houses; and being aware that the former population was not enough for so great a city, he peopled it with men of rank and their families, whom he summoned from Rome and from other countries. He imposed special taxes to cover the expenses of building and adorning the city, and of supplying the inhabitants with food. He erected all the needed edifices for a great capital---a hippodrome, fountains, porticoes and other beautiful adornments. He named it Constantinople and New Rome --- and established it as the Roman capital for all the inhabitants of the North, the South, the East, and the shores of the Mediterranean, from the cities on the Danube and from Epidamnus and the Ionian Gulf to Cyrene and Libya.

He created another Senate which he endowed with the same honors and privileges as that of Rome, and he strove to render the city of his name equal in every way to Rome in Italy; nor were his wishes in vain, for by the favor of God, it became the most populous and wealthy of cities. As this city became the capital of the Empire during the period of religious prosperity, it was not polluted by altars, Grecian temples, nor pagan sacrifices. Constantine also honored this new city of Christ by adorning it with many and splendid houses of prayer, in which the Deity vouchsafed to bless the efforts of the Emperor by giving sensible manifestations of his presence.

Source:
From: William Stearns Davis, ed., Readings in Ancient History: Illustrative Extracts from the Sources, 2 Vols. (Boston: Allyn and Bacon, 1912-13), Vol. II: Rome and the West, 295-296