"In hoc signo vinces": by this sign you will conquer

The Roman emperor Constantine was at war with a rival when, on the night before the battle at the Milvian Bridge, in October 312, on the outskirts of Rome, he and his troops saw a cross of light in the sky bearing the command ‘In this sign, you will conquer’ (in Latin "In hoc signo vinces "). That same night, Constantine saw Christ in a dream, who explained to him the meaning of this vision: the emperor had to have a Christian symbol (historians contend that it was either a staurogram, i.e a Latin cross, or a Chi-Rho sign, i.e the superimposition of the Greek letters X and P - the first two letters of the word "Christ") sewn onto his banners, in order to lead his troops to victory.

On October 28, 312, at the Milvian Bridge, Constantine went into battle against his opponent, whose army outnumbered him two to one, and won a decisive victory. The battle marked the beginning of Constantine's conversion to Christianity, and led him to end the Tetrarchy and become the sole ruler of the Roman Empire. Constantine therefore became the first Christian Roman emperor.

Giulio Romano, Battle of the Milvian Bridge, 1520-1524, Vatican Museums / ©CC0/wikimedia

Reasons to believe:

The army of Maxentius was about twice as large as Constantine's. In addition, Maxentius had a strategic advantage as he held Rome and had stockpiles of food. The victory of October 28, 312, was therefore highly unlikely.

Even pagan chroniclers of the time acknowledged that Constantine's victory was nothing short of miraculous.

Constantine had nothing to gain by converting to Christianity, a religion whose claim to truth was highly unpopular in polytheistic and relativistic Roman society. The army, the emperor's main supporter, was largely pagan and still attached to Roman traditions and beliefs.

If he had wanted to get the Christians on his side, all Constantine had to do was put an end to the persecutions. Converting to Christianity was not necessary in itself. His conversion therefore didn't serve a political agenda.

Constantine's ultimate victory put an end to the civil war. His reign was marked by great progress in human rights: major improvements in the treatment of slaves (ban on branding their faces, ban on separating families), protection of women's rights (ban on forced marriages), etc.

Summary:

Constantine, born c. 272 as the son of the emperor Constantius Chlorus, grew up in a very religious, but rather relativistic, society: traditional Roman religion was still practised, but many no longer believed in it; the idea of a unique God (sometimes identified with the sun) was appealing, but was very different than the revealed God of Judaism and Christianity; the initiatory religions of Egypt and the Middle East were fashionable, as were the various philosophical schools.

Acclaimed emperor in 306, Constantine had to assert himself against the other candidates for power: in this complex period, there were as many as six emperors at the same time. By the end of the summer of 312, Constantine had only one opponent left: the emperor Maxentius. Maxentius had two major advantages: he held Rome, and his army outnumbered Constantine two to one. Constantine therefore had almost no chance of victory.

The bishop Eusebius of Caesarea, who later became a close adviser to the emperor, recounts how Constantine, observing within himself that the paganism of his predecessors had until then caused nothing but wars and destructive rivalries, decided to turn to the God of the Christians. This is what Eusebius tells us in his Life of Constantine (I, 27-29): "To the emperor, who was praying and asking in supplication, a marvellous sign, sent by God, appeared. It would be hard to believe if it came from someone else, but it was the prince himself who told me about it, long afterwards, when I had become close to him; and he swore it to me with a solemn oath. So how could I not believe him? Especially as the events that followed confirm the truth of this testimony. Thus, as he told me, he saw with his own eyes, in the middle of the afternoon, a luminous cross above the sun, with this inscription: "By this sign you will conquer." At this sight, he himself and all the soldiers who were following him, doing some manoeuvre, and who therefore witnessed the miracle, were filled with amazement. He himself, he told me, wondered what it all meant. During the night, while he was thinking and going over it all in his mind, Christ, the Son of God, appeared to him in his sleep with this sign that he had seen in the sky, and ordered him to make a military standard with this effigy, and to go into battle under its protection." The emperor did just that, winning an unexpected victory over his adversary on October 28, 312 - a victory that marked the beginning of a glorious and prosperous reign.

Constantine's conversion is one of the most important events in ancient history. Many historians and commentators have claimed that it was nothing more than a political calculation on the part of the emperor, but this theory does not hold: in fact, by adhering to the new religion, Constantine made himself rather unpopular both with the army, which had remained predominantly pagan, and with a portion of the Roman elite, who were practising Eastern initiatory religions. Stopping the persecutions, but not becoming a Christian himself, as his father Constantius Chlorus had done, would have been more than enough to get the Christians on his side. In fact, even the pagan chroniclers of the time acknowledged that Constantine had acted under the inspiration of a divinity: "You surely had, Constantine, a secret conversation with the divine spirit [...] who deigned to give you a revelation, to you alone " ("Panegyric of Constantine (313)", published in the collection Panégyriques latins, tome II, Éditions des Belles lettres).

Tristan Rivière

Beyond reasons to believe:

Emperor Constantine did much to elevate the status of Christianity in Rome, decriminalize Christian practice and cease Christian persecution. He also played in important in promoting the unity of the Church. In particular, he convened the first ecumenical Council of Nicaea in 325 (a meeting of bishops representing the whole of Christendom), whose theological definitions are still authoritative, not only for the Catholic Church, but also for the Orthodox Churches and most of the separate Eastern Churches.

Going further:

The Life of the Blessed Emperor Constantine by Eusebius Pamphilus, Aeterna Press (January 18, 2015)

More information:

Constantine and the Christian Empire (Roman Imperial Biographies) by Charles Matson Odahl, Routledge; 2nd edition (September 5, 2012)
Constantine the Great General: A Military Biography by Stephen English and Elizabeth James, Pen and Sword Military; Illustrated edition (May 19, 2012)