Who was Constantine?

Constantine made Christianity the main religion of Rome, and created Constantinople, which became the most powerful city in the world.

Emperor Constantine (ca A.D. 280– 337) reigned over a major transition in the Roman Empire—and much more. His acceptance of Christianity and his establishment of an eastern capital city, which would later bear his name, mark his rule as a significant pivot point between ancient history and the Middle Ages.

The Roman Empire that Constantine was born into was one of chaos and anarchy. Civil wars, invasions, and disease were rending the empire so badly that the era is regarded as the Crisis of the Third Century. Emperor Diocletian tried to bring order by distributing power to a four-ruler tetrarchy that would govern the four quarters of the empire. Constantine’s father, Constantius I, was one of the rulers.

Ancient Rome 101 Spanning over a thousand years, ancient Rome was a civilization of constant evolution. This great empire flourished through innovation and incorporation of the diverse cultures they conquered, such as the adoption of Latin and gladiatorial combat. Learn about the rise and fall of this ancient civilization and how its influence still endures today.

Diocletian’s plan fell apart. After the death of his father in A.D. 306, Constantine was declared emperor by his father’s soldiers. He spent the next 18 years battling the three other Roman rulers—his rivals—to become the sole emperor.

The Battle of Milvian Bridge outside Rome in A.D. 312 was a watershed moment for Constantine. He defeated one rival, his brother-in-law Maxentius, and gained the mantle of western Roman emperor. But of far greater import was a revelation he experienced before the battle.

According to Constantine’s biographer Eusebius, Constantine and his forces saw a cross of light in the sky, along with the Greek words for “In this sign conquer.” That night, Constantine had a dream in which Christ reinforced the message. The emperor marked the Christian symbol of the cross on his soldiers’ shields. When he triumphed at Milvian Bridge, he attributed the victory to the god of the Christians. Modern scholars still debate the tale and whether Constantine’s conversion was sincere or a political maneuver. Regardless, in A.D. 313 Constantine met with Licinius, the eastern emperor, and together they issued the Edict of Milan. The edict granted “to the Christians and others full authority to observe that religion which each preferred.”

A change of scene

Constantine assumed sole control over the empire in A.D. 324. Rome, however, was losing its luster for him. Tensions remained high between the city’s pagans and the Christian emperor. Moreover, from a military standpoint, Constantine realized it would be easier to fend off threats from the east and to protect valuable territory—and granaries—in Egypt if he moved his capital to a more defensible eastern location. He left Rome for good to build an imperial city that would glorify both his power and his faith.

Constantinople (modern-day Istanbul), his capital, was dedicated in A.D. 330. Previously known as Byzantium, it had been under Roman control for well over a century, but Constantine rebuilt and expanded it on a monumental scale. He tripled the size of the existing city and offered full citizenship and free bread to encourage men of rank to move there with their families. A large palace and imposing legislative halls established the city’s gravitas as the new capital. Churches began to punctuate the skyline; Christians were welcomed, and other faiths were generally tolerated.

The ascendant Constantinople soon eclipsed Rome. The western empire gradually crumbled until Rome’s fall in A.D. 476. Yet Constantine’s capital—and the Christian foundation he laid there for the empire—continued to thrive for nearly a thousand years.

This text is an excerpt from the National Geographic special issue The Most Influential Figures of Ancient History.