In October 312 A.D., armies controlled by the rival emperors Constantine I and Maxentius met in battle at the Milvian Bridge crossing of the Tiber River. The contest would decide control over the western half of the Roman Empire and ultimately over who would control the empire itself, which since the reign of the emperor Diocletian had been ruled by tetrarchy (rule of four). 

On one hand, the Battle of Milvian Bridge is famous because it ended with Constantine’s decisive victory, thus providing him with a clear path toward establishing his sole rule over the entire empire, both eastern and western halves. On the other hand, the prelude to the battle was the context for Constantine’s “conversion” to Christianity – an embrace which brought, for the first time, Christians into positions of power within Roman imperial institutions and marked the conjoining of Church structures and the Roman state. The conversation experience took place, in part, through a dream. 

Two contemporaneous records of Constantine’s recognition and embrace of the Christian God survive – that of the Church historian and theologian Eusebius and that of the theologian and Christian polemicist Lactantius. Here are their two accounts of Constantine’s pre-battle "embrace" of Christianity:  

CHAPTER XXVII. 

Being convinced, however, 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. While engaged in this enquiry, the thought occurred to him, that, of the many emperors who had preceded him, those who had rested their hopes in a multitude of gods, and served them with sacrifices and offerings, had in the first place been deceived by flattering predictions, and oracles which promised them all prosperity, and at last had met with an unhappy end, while not one of their gods had stood by to warn them of the impending wrath of heaven; while one alone who had pursued an entirely opposite course, who had condemned their error, and honored the one Supreme God during his whole life, had formal I him to be the Saviour and Protector of his empire, and the Giver of every good thing. Reflecting on this, and well weighing the fact that they who had trusted in many gods had also fallen by manifold forms of death, without leaving behind them either family or offspring, stock, name, or memorial among men: while the God of his father had given to him, on the other hand, manifestations of his power and very many tokens: and considering farther that those who had already taken arms against the tyrant, and had marched to the battle-field under the protection of a multitude of gods, had met with a dishonorable end (for one of them had shamefully retreated from the contest without a blow, and the other, being slain in the midst of his own troops, became, as it were, the mere sport of death (4) ); reviewing, I say, all these considerations, he judged it to be folly indeed to join in the idle worship of those who were no gods, and, after such convincing evidence, to err from the truth; and therefore felt it incumbent on him to honor his father's God alone. 

CHAPTER XXVIII. 

ACCORDINGLY he called on him 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 XXIX. 

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 XXX. 

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 XXXI. 

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 XXXII. 

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. 

Moreover, he made the priests of God his counselors, and deemed it incumbent on him to honor the God who had appeared to him with all devotion. And after this, being fortified by well-grounded hopes in Him, he hastened to quench the threatening fire of tyranny.