What sort of omen Constantine received is a matter of dispute. Eusebius says that Constantine saw a vision in the sky; Lactantius says it was a dream. Both agree that the omen informed Constantine that he would conquer under the sign of Christ (Greek: en touto nika; Latin: in hoc signo vinces).
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.
Being convinced...that he needed some more powerful aid than his military forces could afford him, on account of the wicked and magical enchantments which were so diligently practiced by the tyrant, he sought Divine assistance, deeming the possession of arms and a numerous soldiery of secondary importance, but believing the co-operating power of Deity invincible and not to be shaken. He considered, therefore, on what God he might rely for protection and assistance ....[W]hile he was...praying with fervent entreaty, a most marvelous sign appeared to him from heaven...
He 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. ... 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.