Eusebius, Life of Constantine, I. 28-32

Chapter 28. How, While He Was Praying, God Sent Him a Vision of a Cross of Light in the Heavens at MID-Day, with an Inscription Admonishing Him to Conquer by that.

Accordingly he called on him [his father's God] with earnest prayer and supplications that he would reveal to him who he was, and stretch forth his right hand to help him in his present difficulties. And while he was thus praying with fervent entreaty, a most marvelous sign appeared to him from heaven, the account of which it might have been hard to believe had it been related by any other person. But since the victorious emperor himself long afterwards declared it to the writer of this history, when he was honored with his acquaintance and society, and confirmed his statement by an oath, who could hesitate to accredit the relation, especially since the testimony of after-time has established its truth? 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.

Chapter 29. How the Christ of God Appeared to Him in His Sleep, and Commanded
Him to Use in His Wars a Standard Made in the Form of the Cross.

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.

Chapter 30. The Making of the Standard of the Cross.

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.

Chapter 31. A Description of the Standard of the Cross, Which the Romans Now Call the Labarum.

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. The emperor constantly made use of this sign of salvation as a safeguard against every adverse and hostile power, and commanded that others similar to it should be carried at the head of all his armies.

Chapter 32. How Constantine Received Instruction, and Read the Sacred Scriptures.

These things were done shortly afterwards. But at the time above specified, being struck with amazement at the extraordinary vision, and resolving to worship no other God save Him who had appeared to him, he sent for those who were acquainted with the mysteries of His doctrines, and enquired who that God was, and what was intended by the sign of the vision he had seen. They affirmed that He was God, the only begotten Son of the one and only God: that the sign which had appeared was the symbol of immortality, and the trophy of that victory over death which He had gained in time past when sojourning on earth. They taught him also the causes of His advent, and explained to him the true account of His incarnation. Thus he was instructed in these matters, and was impressed with wonder at the divine manifestation which had been presented to his sight. Comparing, therefore, the heavenly vision with the interpretation given, he found his judgment confirmed; and, in the persuasion that the knowledge of these things had been imparted to him by Divine teaching, he determined thenceforth to devote himself to the reading of the Inspired writings.