On this day (October 28) 312 A.D. : Constantine was directed in a dream to cause the heavenly sign to be delineated on the shields of his soldiers, and so to proceed to battle. He did as he had been commanded, and he marked on their shields the letter X, with a perpendicular line drawn through it and turned round thus at the top (P), being the cipher of CHRISTOS. Having this sign, his troops stood to arms.
The enemies advanced, but without their emperor, and they crossed the bridge. The armies met, and fought with the utmost exertions of valour, and firmly maintained their ground. In the meantime a sedition arose at Rome, and Maxentius was reviled as one who had abandoned all concern for the safety of the commonweal; and suddenly, while he exhibited the Circensian games on the anniversary of his reign, the people cried with one voice, “Constantine cannot be overcome!”
Dismayed at this, Maxentius burst from the assembly, and having called some senators together, ordered the Sibylline books to be searched. In them it was found that:- “On the same day the enemy of the Romans should perish.” Lactantius Account of Conversion of Constantine
He (Constantine) said that about noon, when the day was already beginning to decline, he saw with his own eyes the trophy of a cross of light in the heavens, above the sun, and bearing the inscription, CONQUER BY THIS. At this sight he himself was struck with amazement, and his whole army also, which followed him on this expedition, and witnessed the miracle. He said, moreover, that he doubted within himself what the import of this apparition could be. And while he continued to ponder and reason on its meaning, night suddenly came on; then in his sleep the Christ of God appeared to him with the same sign which he had seen in the heavens, and commanded him to make a likeness of that sign which he had seen in the heavens, and to use it as a safeguard in all engagements with his enemies. AT dawn of day he arose, and communicated the marvel to his friends: and then, calling together the workers in gold and precious stones, he sat in the midst of them, and described to them the figure of the sign he had seen, bidding them represent it in gold and precious stones. And this representation I myself have had an opportunity of seeing.
Now it was made in the following manner. A long spear, overlaid with gold, formed the figure of the cross by means of a transverse bar laid over it. On the top of the whole was fixed a wreath of gold and precious stones; and within this, the symbol of the Saviour’s name, two letters indicating the name of Christ by means of its initial characters, the letter P being intersected by X in its centre,and these letters the emperor was in the habit of wearing on his helmet at a later period. From the cross-bar of the spear was suspended a cloth, a royal piece, covered with a profuse embroidery of most brilliant precious stones; and which, being also richly interlaced with gold, presented an indescribable degree of beauty to the beholder. This banner was of a square form, and the upright staff, whose lower section was of great length, bore a golden half-length portrait of the pious emperor and his children on its upper part, beneath the trophy of the cross, and immediately above the embroidered banner. Eusebius of Caesarea’s account of the Conversion of Constantine