How Constantine Changed the Destiny of Christianity

According to legend, in the 4th century, Emperor Constantine (also known as Constantine the Great) converted to Christianity after witnessing a miracle. In reality, his decision was more of a political calculation. Christianity provided a means to unify his empire. However, this did not prevent him from remaining a ruthless ruler until the end—ordering the assassination of his wife and son.

Statue of Constantin the Great in York, United Kingdom.
Statue of Constantin the Great in York, United Kingdom.

In June 325, in the imperial palace of Nicaea (in present-day northwestern Turkey), nearly three hundred bishops from across Christendom awaited the opening of the council convened by Emperor Constantine. The emperor entered in full ceremonial regalia, draped in the imperial purple cloak. Walking solemnly between two rows of bishops, he reached the front of the hall and formally asked for permission to sit. Taking his place on a small gold-inlaid wooden chair, he officially began the council’s proceedings.

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The sight of a Roman emperor engaging in theological debates with bishops would have been unimaginable just twenty years earlier. Back then, Christians faced the most brutal persecution in their history—their books were burned, churches destroyed, and bishops martyred. Those who refused to perform pagan sacrifices were sentenced to death.

Who Was Emperor Constantine?

Constantine the Great
Constantine the Great, Head of the Colossus of Constantine, Capitoline Museums. Image: Wikimedia

How did such a drastic change occur? And why did Constantine convert to Christianity? Seventeen centuries after his reign, this question remains debated. To understand it, one must look at the life of the emperor himself.

Constantine was born in 273 in Naissus, a garrison town on the Danube in what is now Serbia. At the time, the Roman Empire was divided, ruled by Emperor Diocletian, who governed alongside two Augusti (senior emperors), each assisted by a Caesar (junior emperor).

Constantine’s father, Constantius Chlorus—nicknamed “Chlorus” (meaning “the Pale”) due to his fair complexion—was a Caesar, overseeing the administration of Britain (modern-day UK), Spain, and part of Gaul. In 305, following the unexpected abdication of Diocletian, Constantius became Augustus. Meanwhile, young Constantine was completing his education in the East. But his father soon summoned him to join his ranks.

The following year, in 306, Constantius Chlorus died, and Constantine was proclaimed Augustus by his legions. His ultimate goal was to become sole emperor, but he faced two rivals in Italy: Maximian and his son Maxentius. A brutal war of succession ensued.

Constantine proved to be the most skillful contender. He crushed Maximian’s forces in Marseille, leaving only Maxentius as his remaining rival—who still controlled Rome, the symbolic heart of the empire.

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He Engraved the Initials of Jesus Christ on His Helmet

Constantine's vision and the Battle of the Milvian Bridge in a 9th-century Byzantine manuscript
Constantine’s vision and the Battle of the Milvian Bridge in a 9th-century Byzantine manuscript.

On October 28, 312, two armies stood face to face for a decisive battle at the Milvian Bridge, on the outskirts of Rome. According to Christian chroniclers of the time, the night before the battle, Constantine had a vision: Jesus appeared to him, showing him a chrismon—the intertwined Greek letters “Chi” (Χ) and “Rho” (Ρ), symbolizing Christianity—and told him, “In hoc signo vinces” (“By and/or in this sign, conquer“).

The next day, according to the same accounts, Constantine and his troops saw a cross appear in the sky during the battle. Regardless of the legends, what is certain is that Constantine utterly crushed his rival. Maxentius drowned in the Tiber while trying to flee. Triumphant, Constantine entered Rome, where the Senate proclaimed him Emperor of the West. However, the empire was still not fully unified, as Licinius (c. 250–325) ruled over the eastern half.

For most historians, this victory marks the moment of Constantine’s conversion. However, it is possible that his conversion had begun earlier, and he had simply concealed it to avoid jeopardizing his political ambitions. What truly matters is not the precise evolution of his personal religious beliefs—impossible to reconstruct—but his actions.

In 312, by Constantine’s will, Christianity ceased to be a marginal cult—often tolerated, sometimes banned—and became a protected religion, increasingly intertwined with imperial power. That same year, the emperor ordered the chrismon to appear on his soldiers’ shields, on military banners, and even on his own helmet—an emblem still visible on coins minted at the time.

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A year later, in 313, Constantine met with his Eastern counterpart, Licinius, in Milan. Together, they issued an edict granting “Christians, as well as all others, the freedom to follow the religion of their choice.” Confiscated Christian properties, seized during Diocletian’s great persecution, were returned. The Church was granted the right to own buildings and wealth, including land and slaves inherited through bequests.

At the same time, ever the shrewd politician, Constantine was careful not to alienate the Roman aristocracy, which remained wary of the new religion. Using imperial funds, he had five basilicas built in Rome, including Saint Peter’s Basilica in the Vatican. However, all were strategically located outside the pomerium—the sacred boundary within which temples of the traditional pagan cults were supposed to be situated.

Constantine Establishes the Commemoration of Christmas on December 25th

Constantine retained his title of Pontifex Maximus, which made him the head of the cults dedicated to the protective deities of Rome, to which he did not publicly renounce. However, under his influence, time became Christian. In the Greco-Roman world, only religious holidays, which were numerous and varied, were days off. In 321, following Christian teachings, Constantine established the seven-day week, with the last day being a day of rest for all. Likely to avoid displeasing non-Christians, he declared that this day of rest would be celebrated as the “Day of the Sun” by pagans (as reflected in the Anglo-Saxon etymology of Sunday) and as the “Day of the Lord” by Christians (“Dies Dominica,” which would become dimanche in French).

Later, Constantine would fix the commemoration of Christmas on December 25th, aligning it with both the birth of Jesus and the celebration of an ancient pagan festival marking the winter solstice.

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The Emperor Built Churches… but Ensured the Preservation of Pagan Temples

First Council of Nicaea
The Council of Nicaea, with Arius depicted as defeated by the council, lying under the feet of Emperor Constantine.

In 324, civil war erupted again between the Eastern and Western empires. Constantine defeated Licinius, becoming the sole emperor, the first to rule over the entire empire in forty years. In the East, where Constantine founded his new capital, Constantinople, Christianity was more established than in the West, but it was also divided by many theological controversies, most notably the Arian controversy. Should, as suggested by Arius, a priest from Alexandria, and his followers, God alone be eternal, with Jesus being a partly divine human? Or should both be considered eternal and divine?

Constantine was not a theologian, and he did not focus on the doctrinal issues themselves, but on the disorder these issues were causing in the churches of the East. As he positioned himself as the champion of Christianity, which he saw as a unifying force for his empire, he found it to be divided.

To resolve this, Constantine called for the First Council of Nicaea on May 20, 325, gathering various Christian factions. After two months of deliberation, the council concluded with the adoption of the Nicene Creed, a common statement of faith for all Christians. This formula definitively declared that Jesus was eternal, the Son of God, and consubstantial with God the Father.

In the later years of his reign, Constantine’s policies became increasingly favorable to Christians. While he never impeded the practice of pagan cults, the emperor publicly expressed his new faith. In 326, he refused to ascend the Capitoline Hill to hold a traditional banquet in honor of Jupiter. Around this time, animal sacrifices were prohibited, as was crucifixion or forced participation in gladiatorial games. The East saw an explosion of churches built with imperial treasury funds, particularly in Palestine, which became known as the Holy Land.

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Constantine’s mother, Helena, went on a pilgrimage to Jerusalem, where she oversaw the restoration of sacred sites. She brought back the (supposed) cross of Jesus Christ, a relic that would long bring glory to Constantinople.

To Erase His Sins, He Was Only Baptized the Day Before His Death

As the first Christian emperor, Constantine also displayed a ruthless side. In 326, he had his eldest son Crispus executed. The following year, he ordered the assassination of Fausta, his second wife. Was Crispus guilty of plotting against his father, or was he the victim of a scheme by Fausta, who sought to elevate her own three sons, all of whom would eventually become emperors? The historical sources remain silent on this matter, and the details remain shrouded in mystery.

Though Christianity was not yet the official state religion, it became, under Constantine, the religion of the political elite. The emperor primarily recruited his high officials from among the Christian ranks. The imperial family’s example led to numerous conversions, particularly among landowners, who spread Christianity throughout the countryside. As for Constantine himself, he was baptized—following a common practice of the time—only on the eve of his death, on May 22, 337, ensuring that he would not commit any further sins before his passing. Given his propensity for violence, it seems the timing was indeed optimal.