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Constantine the Great (Constantine I)

Roman emperor (306–337) who consolidated imperial power, promoted Christianity, and refounded Byzantium as Constantinople, shaping late antiquity and the later Byzantine world.

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: April 27, 2026

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Overview

Constantine I (born 27 February 272, died 22 May 337) was Roman emperor from 306 until his death. He ruled longer than any emperor since Augustus and established a dynasty that dominated the Roman Empire for several decades. Constantine is commonly remembered as the first ruler of the Roman imperial system to favor and adopt Christian faith in state policy, although the nature and timing of his personal beliefs remain studied and debated.

Early life and rise to power

Born into the Constantinian family, Constantine was the son of Constantius, a military commander who later became emperor. By the early fourth century the Roman world was governed through a complex system of co-emperors and rivals. Constantine claimed imperial authority after his father’s death and fought a series of civil wars with other claimants. A decisive moment in his rise came at the Battle of the Milvian Bridge, near the Tiber River, where contemporary and later sources relate that he attributed his victory to a religious sign.

Conversion and religious policy

According to several ancient accounts, before the battle Constantine saw a vision of a heavenly sign, often described as a cross, accompanied by the phrase in hoc signo vinces. After this event he shifted from worship of the traditional Apollo and other Roman gods toward support for Christianity, while still engaging with existing religious institutions. In 313 he and his co-ruler issued measures that granted Christians legal toleration and restored confiscated property. He later convened leaders to settle church disputes, notably presiding over decisions that shaped early Christian doctrine.

Government, reforms, and culture

Constantine reorganized the imperial administration and military. He separated civil and military authority in many provinces, reorganized territorial divisions, and supported measures to stabilize the currency by introducing a durable gold coin and tightening fiscal practice. His reign encouraged urban building, Christian patronage, and a visible imperial cult that blended new and traditional elements. He pursued policies that strengthened the central government and professionalized the army, while also relying on personal dynastic rule.

Founding of a new capital

One of Constantine’s most lasting acts was the expansion and refounding of the ancient city of Byzantium as a new imperial capital. Renamed Constantinople (literally "City of Constantine" in Greek), the metropolis was developed as a Christianized, fortified center of government on the Bosporus. Today the site is part of Istanbul in modern Turkey, and Constantinople would become the cornerstone of the later Eastern Roman, or Byzantine, state.

Legacy and notable facts

Constantine left a mixed legacy: he promoted Christian institutions and reshaped the political map of the empire, yet he maintained traditional aspects of imperial power and ritual. His reign marked a turning point from a predominantly pagan state toward one in which Christianity had privileged legal and social status. Historians note his role in founding administrative patterns and an imperial capital that influenced politics and culture for centuries. Debates continue about how personally devout he was and how much his policies were driven by conviction versus political calculation.

Major actions: military victories, legal toleration for Christians, administrative reforms, and founding of Constantinople.
Dynastic outcome: members of his family continued to shape imperial rule into the mid-4th century.
Contested interpretations: the precise nature of his conversion from the old deity-centered religion to the Christian faith centered on Jesus.

For further reading on key episodes and primary sources, consult editions and commentaries that examine Constantine’s laws, coinage, and council records in context. The emperor remains a central figure for understanding the transition from classical antiquity to the medieval Christian world.

Head of the Colossal Statue of Constantine the Great, Capitoline Museums, Rome

The Roman Empire at the time of Constantine

The Imperium Romanum had entered a period of crisis in the 3rd century (Imperial Crisis of the 3rd century), in which internal political instability and pressure on the borders increased. Along the Rhine and the Danube, various Germanic tribes or new gentile large-scale groups such as the Franks, the Alamanni and the Goths caused unrest. On several occasions, groups of "barbarians" advanced into Roman territory and plundered Roman towns, which had previously been largely spared attacks for almost two centuries. In the east, the Sāsānid Empire had emerged in 224/226 and became Rome's most dangerous rival (see Roman-Persian Wars). Within the empire, numerous usurpers and attempted usurpations relied primarily on the large army units that now legitimized imperial power (soldier-emperors), so that endless civil wars shook the empire. Although not all spheres of life and provinces were hard hit by the crisis and it was by no means uninterrupted, it proved to be a severe test of endurance for the empire.

Emperors such as Aurelian therefore introduced reforms from the 270s onwards, but it was not until Diocletian came to power in 284 that the empire was placed on a new foundation. He carried out far-reaching reforms and fundamentally reshaped the empire. Among other things, Diocletian introduced a new tax system (Capitatio-Iugatio) and reorganized the army by dividing it into Comitatenses as a mobile field army and Limitanei as border troops. The crisis was finally overcome, and the empire entered late antiquity. In response to the simultaneous military strains on the various frontiers and the constant usurpations of ambitious generals, a multi-emperorship was introduced, the tetrarchy, in which Diocletian functioned as senior Augustus with three co-emperors subordinate to him. This system was based on appointment of successors rather than dynastic succession and served primarily to prevent usurpations. In Diocletian's last years of rule there was a persecution of Christians. In 305 Diocletian voluntarily resigned and forced his co-emperor Maximian to follow suit, so that the previous sub-emperors Constantius I (replacing Maximian in the west) and Galerius (replacing Diocletian in the east) now succeeded him as senior emperors (Augusti). Nevertheless, contrary to Diocletian's intention, the dynastic principle soon reasserted itself (see Dissolution of the Roman Tetrarchy). A bloody civil war broke out for years, at the end of which Constantine was the sole ruler of the empire.

Questions and Answers

Q: Who was Constantine I?

A: Constantine I was a Roman emperor from 306 until he died in 337 AD. He was the longest reigning emperor since Augustus, the first emperor, and the first ruler of the Roman Empire to be a Christian.

Q: What did Constantine do with Byzantium?

A: Constantine made Byzantium into a new, larger city called Constantinople (now Istanbul, Turkey). The city's name means "City of Constantine" in Greek.

Q: Who were members of the Constantinian dynasty?

A: Members of the Constantinian dynasty were descendants of Emperor Constantius I, including his son Constantine I. They controlled the empire until 364.

Q: What happened at the Battle of Milvian Bridge?

A: At the Battle of Milvian Bridge between Maxentius and Constantine for control over Rome, Constantine saw a cross in the sky with words that said "in this sign you shall conquer". This led him to change his deity from Apollo to Jesus and win the battle or war.

Q: Where is Constantinople located today?

A: Constantinople is now known as Istanbul, Turkey.

Q: What does 'in hoc signo vinces' mean? A:'In hoc signo vinces' is Latin for 'in this sign you shall conquer'. It refers to when Constantine saw a cross in the sky during his battle against Maxentius at Milvian Bridge which led him to victory.

Q: How long did members of the Constantinian dynasty rule for? A: Members of the Constantinian dynasty ruled for six years after Emperor Constantin declared himself emperor until 364 AD.