The Battle of Milvian Bridge which was fought between Roman Emperors Constantine I and Maxentius on October 28, 312 was important because Constantine won and it resulted in the end of Tetrarchy, a system by which four emperors ruled the Roman empire. It was also important because on one night of the battle, Constantine claimed that he had a vision from God
It is commonly stated that on the evening of October 27, with the armies preparing for battle, Constantine had a vision which lead him to fight under the protection of the Christian God. The details of that vision, however, differ between the sources reporting it. It is believed that the sign of the cross appeared and Constantine heard “In this sign, you shall conquer” in Greek. [Vision of Constantine]
Constantine, besides being the first Christian Roman Emperor also by the Edict of Milan made professing Christianity not a crime.
There is a scene in The Betrayal: The Lost Life of Jesus: A Novel by Kathleen O’Neal Gear and W. Michael Gear where one of the protagonists mentions this battle. The monk Cyrus, who once was a soldier with Constantine, tells Kalay the washer woman that Constantine was sitting in his tent all day drinking wine looking for a way to motivate the troops. Constantine knew he had to come up with a myth; either a cross of light or the letters chi-rho (the first two letters in the Greek spelling of the word Christ)
While that was fiction, the truth is not far away. Eusebius of Caesarea spoke to Constantine and wrote in Life of Constantine
when Constantine “was praying with fervent entreaty, a most marvelous sign appeared to him from heaven” (Eusebius, Life of Constantine,1.28). The famous sign in the sky was a cross of light, with theinscription, “Conquer by this”. Eusebius goes on: “At this sight hehimself was struck with amazement, and his whole army also, which followed him on this expedition, and witnessed the miracle.”[The Resurrection of Jesus as Mass Hallucination]
Another Christian writer Lactantius who was a contemporary of Constantine had a slightly different version.
Lactantius’ early account places the vision of the cross in Constantine’s dream, and on the night before. So, Constantine’s vision is not shared by his army and it is a nighttime dream rather than a vision. [The Resurrection of Jesus as Mass Hallucination]
Thus in Constantine’s time itself one version of the story had the sign as a dream; another as a sign seen by the whole army. Thus a myth was created.