Constantine-also known as Emperor Constantine I or Constantine the Great-decreed tolerance for Christians in the Edict of Milan, convened an ecumenical council to discuss Christian dogma and heresy, and constructed Christian edifices in his new capital city (Byzantium/Constantinople, now Istanbul)
Was Constantine a Christian?
The short answer is, "Yes, Constantine was Christian," or seems to have said he was, but it belies the complexity of the issue. Constantine may have been Christian since before he became emperor. For this theory, read "Constantine's Conversion: Do We Really Need It?" by T. G. Elliott; Phoenix, Vol. 41, No. 4 (Winter, 1987), pp. 420-438. He may have been Christian since 312 when he won the Battle at the Milvian Bridge, although the accompanying medallion showing him with the Sol Invictus deity a year later raises questions. The story goes that Constantine had a vision of the words "in hoc signo vinces" upon the symbol of Christianity, a cross, that led him to promise to follow the Christian religion if victory were granted.
Ancient Historians on the Conversion of Constantine
A contemporary of Constantine and a Christian, who became bishop of Caesarea in 314, Eusebius describes the series of events:
"CHAPTER XXVIII: 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 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, (1) 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: 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 XXX: 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 XXXI: 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."
Eusebius of Caesarea The Life of the Blessed Emperor Constantine
That's one account.
The fifth-century historian Zosimus writes about the pragmatic reasons for Constantine seeming to embrace the newish faith:
" Constantine under pretence of comforting her, applied a remedy worse than the disease. For causing a bath to be heated to an extraordinary degree, he shut up Fausta Constantine's wife in it, and a short time after took her out dead. Of which his conscience accusing him, as also of violating his oath, he went to the priests to be purified from his crimes. But they told him, that there was no kind of lustration that was sufficient to clear him of such enormities. A Spaniard, named Aegyptius, very familiar with the court-ladies, being at Rome, happened to fall into converse with Constantine, and assured him, that the Christian doctrine would teach him how to cleanse himself from all his offences, and that they who received it were immediately absolved from all their sins. Constantine had no sooner heard this than he easily believed what was told him, and forsaking the rites of his country, received those which Aegyptius offered him; and for the first instance of his impiety, suspected the truth of divination. For since many fortunate occurrences had been thereby predicted to him, and really had happened according to such prediction, he was afraid that others might be told something which should fall out to his misfortune ; and for that reason applied himself to the abolishing of the practice. And on a particular festival, when the army was to go up to the Capitol, he very indecently reproached the solemnity, and treading the holy ceremonies, as it were, under his feet, incurred the hatred of the senate and people."
THE HISTORY OF COUNT ZOSIMUS. London: Green and Chaplin (1814)
Constantine may not have been a Christian until his deathbed baptism. Constantine's Christian mother, St. Helena, may have converted him or he may have converted her. Most people consider Constantine a Christian from the Milvian Bridge in 312, but he wasn't baptized until a quarter century later. Today, depending on which branch and denomination of Christianity you're following, Constantine might not count as a Christian without the baptism, but it's not an event that clear in the first few centuries of Christianity when Christian dogma had yet to be fixed.
A related question is:
Why Did Constantine Wait Until He Was Dying to Be Baptized?
Here are some responses from the Ancient / Classical History forum. Please add your opinion to the forum thread.
Was the deathbed conversion of Constantine the act of a moral pragmatist?
"Constantine was enough of a Christian to wait until his deathbed to be baptized. He knew that a ruler had to do things that were against Christian teachings, so he waited until he no longer had to do such things. That may be the thing that I most respect him for."
Was Constantine a duplicitous hypocrite?
"If I believe in the Christian god, but know that I will have to do things which are against the teachings of that faith, I can be excused for doing so by postponing baptism? Yes, I'll join Alcoholics Anonymous after this crate of beer. If that isn't duplicity and subscription to double standards, then nothing is."
See: "Religion and Politics at the Council at Nicaea," by Robert M. Grant. The Journal of Religion, Vol. 55, No. 1 (Jan. 1975), pp. 1-12