Constantine is the first Roman Emperor to convert to Christianity. He did so after witnessing the sight of a cross in the sky along with his entire army. However, his spiritual growth and eventual conversion did not happen at once with this one dramatic event. It began years before this while he was stationed in Gaul along the Rhine frontier.
As Constantine contemplated the imminent outbreak of war with Emperor Maxentius in the Spring of 312 AD, he was greatly worried. Maxentius had an experienced army he had lead to many victories. He controlled a series of well fortified cities in northern Italy, and had been improving the already significant fortifications of Rome for years.
He decides to invade the Italian peninsula from Gaul. This is no easy task, as he must maintain an elaborate supply chain for his army while leaving behind some troops to defend the Rhine frontier while he is gone. This left him with forces much smaller than that of his enemy. Constantine decides to cross the Alps into the Italian peninsula near Mt. Cénis.
His armies fight their way south until they begin to approach Rome. As a sign of confidence, Maxentius moves his army to meet Constantine outside the safety of the city's defenses.
There are two accounts of Constantine's conversion to Christianity. The first is by Lactantius, a tutor to Constantine's son and a good authority. He states that in Gaul, before setting out towards Rome, Constantine and his army saw a great cross in the sky. Underneath was written, "In this sign, conquer."
But the Bishop Eusebius of Antioch, who would later write a favorable biography of Constantine, tells that he and his army experienced this vision just before the battle outside of Rome began.
Both accounts tell of Constantine not fully understanding the meaning of this vision and praying for an explanation. He dreams of a common Christian symbol, the Greek symbols chi and rho, an X with an R, which looks like a long P, drawn through the middle.
The emperor explains the heavenly dream to his army and tells them to make the battle standard that is described, placing the symbol of the "Highest God" on their shields.
The Battle of Milvian Bridge
On October 28, 312 AD, the Battle of Milvian Bridge was fought outside of Rome against Maxentius. Constantine's infantry decisively win the battle. His calvary chased the remnants of the enemy's forces across the Tiber river. Maxentius himself was seen to fall from the bridge into the river as his army was retreating and drown due to his heavy armor.
Eusebius says that Constantine doesn't know which god has given him this sign in the sky, but that he was so moved by his vision of the cross that he vowed to worship no other God than the one represented to him. So he begins to seek out others who might help him to learn more about what he has seen. It is known that Bishops regularly traveled with Constantine, Maternus from Cologne, Recticius from Autun, Marinus from Arles, and Ossius from Cordoba. It is likely that these Bishops affirmed to him that Jesus was the only begotten son of God and that the cross he had seen in his vision was a symbol of Jesus' triumph over death.
Constantine devoted himself completely to God, and would from then on immerse himself in the reading of inspired writings. He made the priests of God his close advisers, and believed that it was his duty to honor the God who had appeared to him in his original vision.
As Constantine contemplated his future, the purpose of his life may have gradually fallen into place, convincing him that he would cast away the old worshiping of Roman gods and lead a life of faith as taught by the Christian God.
A brief stay in Rome
Constantine would not stay in Rome for very long. He made a ceremonial entry into the city, at the end of which he did not make any sacrifices to Roman gods. While there he ordered the restitution of property confiscated from Christians during their previous persecution. Constantine had previously met with Licinius in Milan in March 312 where they discussed the future of the empire. It was from these meetings that Licinius drafted the Edict of Milan, granting to all in the Roman Empire the freedom to worship any god they chose. This edict was passed in February 313 AD after Constantine left Rome. This represents a dramatic change in the attitudes of religious tolerance within the Roman empire.
As Constantine contemplated his future, the purpose of his life may have gradually fallen into place, convincing him that he would cast away the old worshiping of Roman gods and lead a life of faith as taught by the Christian God. When Constantine declared himself a Christian, a third of the Roman empire is thought to have been Christians.
There is no good reason to doubt these accounts of Constantine's conversion. Eusebius refers to the story of the emperor's conversion to Christianity in 336 while giving a speech in honor on him. The four other bishops who regularly traveled with him continuously wrote about his spiritual growth.
Christianity would later be made the official religion of the Roman Empire in 380 AD under Theodosius I.
Constantine the Emperor by David Potter
Lives of Famous Romans by Olivia Coolidge