On October 28 in the year 312, Constantine (full name: Flavius Valerius Constantinus – by any standard Latin names are incredibly awesome) ensured his rise to emperorship by defeating his rival Maxentius at the Battle of Milvian Bridge, just outside Rome. Much ink has been spilled in examining his ensuing rule. Did Constantine really see a sign from God before the battle? Was his conversion sincere or a calculated political move? Did he pervert Christianity and rule over both the secular and theological? Did he make Christianity the state religion or simply decriminalize it? Did he actually wage war on the pagans as depicted in The Da Vinci Code?
According to the historian Eusebius, the day before Constantine commenced his attack on Maxentius, "[A]bout 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."
Historically, Eusebius' account has been the most popular depiction of what Constantine witnessed and experienced. Here's what we know:
Constantine was deeply religious prior to his Christian conversion. We know his parents had similar inclinations to religion and are likely the source of this devotion. We also know Constantine's father (for simplicity, referred to here as Constantius), while pagan, nonetheless held Christian sympathies and was never one to enforce persecution with any degree of gusto (however, he was responsible for destruction of some churches in the Western empire after the decree of persecution in 303). Some scholars have gone so far as to claim Constantius' family was secretly Christian, although this is highly unlikely and doesn't fit with events if this were indeed accurate. Now, it is probable that Constantine's mother, Helena, was a Christian (she allowed the local bishop to use her house as a church, for instance, which would be inexplicable were she not Christian), but, Constantine took after his father: pagan, with Christian sympathies.
Eusebius draws this familial background to the forefront, as Constantine is concerned about Maxentius' incantations and deployment of magic. Because of his spiritual nature, Constantine wondered which god he could turn to in order to ward off Maxentius' supernatural attacks. Reflecting on his father's Christian sympathies, vigorous life, and peaceful death (palace intrigue being what it was in the Empire, peaceful deaths were infrequent for someone of Constantius' station), Constantine decided this god of the Christians must indeed be a powerful one. According to Eusebius when Constantine "called on [the Christian 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... 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 another person."
Unsure of what he witnessed, Eusebius records that after his vision, “while [Constantine] 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.”
Is this account credible and historically accurate? What, if anything, happened on the eve of the Battle of Milvian Bridge?
We know that Constantine saw something. He swore an oath that what he recounted to Eusebius was truthful and accurate. Even if one argues swearing an oath means nothing, or that Eusebius either lied or distorted what Constantine told him, there are two problems. One, Constantine told the same story throughout his life, without any discrepancies, to more than Eusebius alone. Second, there's the thorny issue of Constantine's army; this was a public vision. No single individual, no matter how powerful and influential, could refute the testimony of an entire army. Thus, if Constantine lied, it stands to reason his soldiers would have spoken against him. An army is greater than an individual, even an emperor, so to refute Emperor Constantine, as an army, carried little risk. Yet, not a single soldier disputed Constantine's account. There are also no indications of bribery; besides, how could an entire army be bribed? Logic and reason, then, both demonstrate that something did indeed occur.
Remember too that Constantine was deeply spiritual and religious and to change gods whimsically goes against this devout nature. His soldiers shared that nature. To summarize Peter Leithart, it must be recognized that at this time, battle standards were venerated as holy objects and credited with talismanic powers. Typically these standards reflected a particular deity. To change standards as Constantine ordered, on the eve of battle, was unthinkable without some powerful justification. In the ancient world armies won by divine intervention, and the victory of an army was a victory by that army's god - it would be folly to abandon the gods at the very moment of engagement.
It is also historical fact that after 312 Constantine increasingly moved from paganism toward Christianity, and fervently so. This fervor is unfathomable if it was nothing more than political strategy. True, Constantine was who he was: a man of his historical time, with all the pitfalls of that period. Yet, only the most ardent cynic can examine the historical record and question Constantine's piety to Christianity.
The situation, then, was thus: prior to 312, Constantine's coinage and military standard honored pagan gods, particularly Sol. After 312 he adopted a Christian standard and military insignia and put Christian symbols on his coins, gradually replacing pagan signs. We know he was a spiritual individual, we know he and his army witnessed something, we know he was victorious at the Battle of Milvian Bridge, we know that after 312 he wrote and spoke at great length on Jesus, Christianity, and the saints. We know he called the Council of Nicaea (more on that in a bit).... What happened?
Constantine and his army witnessed a natural phenomenon known as a sun halo. In contradiction of Eusebius, this vision was perhaps seen two years prior to Milvian Bridge, “somewhere in Gaul,” according to Peter Weiss, who best describes what they saw: "Constantine, with his army, unexpectedly witnessed a complex halo-phenomenon. [They] saw a double-ring halo, each ring with three mock suns arranged in cross-formation around the sun, tangent arcs or points of intersection with the circle, presumably with a more or less distinct light-cross in the middle. [They] saw it in the spring and in the afternoon, which is when the phenomenon mainly occurs." 
Does this mean Constantine and/or Eusebius lied? No, it doesn’t. Again, no one disputed Constantine. Weiss suggests the vision occurred in 310 and remained ever-present in Constantine’s mind, while the dream indicated by Eusebius took place in 312, before he faced Maxentius, and upon consultation with Christians in his entourage, acknowledged the two as signs from God. As presented in Leithart’s Defending Constantine, Weiss persuasively argues this explains Constantine’s aforementioned devotion to the “invincible sun god Sol invictus” before his Christian conversion…. I present three scenarios for consideration:
Peter Weiss is correct. Constantine and his army had a vision in 310 in Gaul and Constantine had a dream in 312 at Milvian Bridge. Constantine initially took the vision to be from Sol, yet reconsidered after his dream and the advice of Christian confidants.
Peter Weiss is correct. Constantine and his army had a vision in 310 in Gaul and Constantine had a dream in 312 at Milvian Bridge. Unlike Option A, given his parental lineage, Constantine understood the vision as a sign from the Christian god but was afraid to openly avow him (Galerius’s tepid “Edict of Toleration” was still a year away). After his dream, Constantine took this to mean the moment had come to openly profess Christian faith.
Peter Weiss is partially correct. Constantine and his army had two visions, one in 310 in Gaul and one in 312 at Milvian Bridge; the dream details remain the same as Options A and B.
Regardless which of these three options is nearest to history, the fact remains a momentous change came over Constantine before he engaged Maxentius; he clearly had an intensely profound experience (or experiences), corroborated by his army, that he emphatically took as a sign from the Christian god. Ultimately, that’s what matters, not so much the particulars.
Was Constantine's conversion genuine then? I think a case, or the skeleton of one anyway, has been made already. However, two more points deserve mention. Rodney Stark has demonstrated that by 312 the total Christian population couldn't have been any higher than 8,904,032, or 14.8% of the entire Empire's population. That's all. Converting to a religion of which, at most, 14.8% of your constituents adhere is far from politically advantageous. Ask any politician if he or she would consider 14.8% of the population a credible power base. Next, the fact Constantine wasn't baptized until the end of his life is frequently cited as proof of a "political" conversion. What this argument fails to grasp is baptism was typically done as close to death as possible, for centuries in fact. The doctrine of Purgatory was not formally developed until the twelfth century. As such, since Christians profess "one baptism for the forgiveness of sins," and always have (hence the Donatist Controversy), if one was baptized and sinned after, that individual was out of luck. Consequently, until the twelfth century it was common practice to have one's baptism as close to death as possible, as the odds of sinning after the fact were minimal (there aren't too many ways someone can sin as they lay dying in bed).
With it understood Constantine’s conversion to Christianity was sincere, we now turn our attention to his rule and consider a few areas of clarification.
First, it is frequently asserted Constantine made Christianity the “state religion”; he simply decriminalized it. The “Edict of Milan” is unambiguous – nowhere does it even hint at enforcing a state religion. 
Second, Constantine did not meddle in Church affairs and declare Jesus divine, as Dan Brown claims. In ancient Rome religious harmony in the Empire was seen as necessary for its survival; religion was intricately bound with all aspects of life, including government (Roman emperors were traditionally viewed as actual deities), it wasn't something hidden or attended to once a week on Sunday. Ancient Romans would find modern claims of sundering religion from daily life to be bizarre. As there was no organized Christian theology, various sects springing up, and Christians fighting with Christians as a consequence (specifically the Arian Controversy), Constantine feared that Christians would both divide the Empire and perhaps destroy Christianity itself. Ergo, he called upon all the bishops and influential church figures to gather, officially come to a universal understanding as to what it meant to be "Christian," and end the discord and vitriol. Constantine called this council, not in what we today might identify with something akin to papal authority, but instead in his capacity as emperor seeking to prevent his empire from breaking apart, as well as a Christian who had the influence to create a scenario that could dampen the animosity Christians were generating among one another. 
As for the Council of Nicaea itself, people typically assume Constantine “called the shots” by making theological pronouncements and strong-arming those in attendance. The sole surviving records are from the bishops in attendance, and not a single one claimed any intimidation on the part of Constantine. Some would argue the absence of any reference to wrongdoing on the part of Constantine by the bishops could mean they were cowed.... However, these bishops were incapable of being cowed by anyone, even an emperor. Most of the bishops at Nicaea were survivors of some of the worst persecution a human can experience and survive, and they still bore the scars attesting to that unyielding faith; if they would not bow down to a pagan after physical, emotional, and psychological torture, what could Constantine have possibly threatened that would have been worse?
Where it has historically become tricky is with Catholic theology. Constantine was apparently responsible for offeringhomoúsios, the Greek version of the Latin consubstantialis (“consubstantial”), to the Council, but scholars credit the notion to his theological advisor, Hosius of Cordova, not to Constantine himself. While it was initially considered with caution, the majority of bishops soon realized the term was advantageous, both theologically and “politically.”
As I alluded earlier, one of the primary impetuses for the Council of Nicaea was the Arian Controversy, in which the gifted and brilliant Egyptian priest Arius denied Jesus was of the same substance as God the Father, and instead was merely God’s first creation, thus pronouncing Jesus to be finite. To the consternation of those gathered at Nicaea, Arius was able to irrefutably counter any opposition to his thinking. It soon became apparent to the wiser, more theologically-inclined bishops a new approach was needed. Constantine’s suggestion, literally, was a godsend, and provided a means of at last gaining the upper hand against him. Because no orthodox Christian had ever suggested what is known as “extrabiblical thinking,” Arius had no ready defense. Additionally, “consubstantial,” though not yet orthodox, nevertheless was not an unknown term; it was “widely, if unofficially, used in the Latin West.” Pragmatically, the bishops acknowledged rejecting this term would impair church unity and undermine the entire purpose of the council.
Does this mean Constantine is responsible for Catholicism and its theology and dogma? While some have obviously proffered this, such thinking commits the cardinal academic sin of eisegesis (reading into the evidence) as opposed to performing exegesis (allowing the evidence to speak on its own merits). Almost all scholars accept, while homoúsios camefrom him, Constantine had it suggested to him by a theologian. Even if he wanted to impose theology, how could he? If one is to argue Constantine originated and impelled theology, they need to find evidence in the historical record, and none exists: all surviving accounts by the bishops state Constantine operated merely as a facilitator, neither taking part in discussions nor voting on resolutions.
Finally, it's often claimed Constantine waged a war against pagans. I have no idea where people come up with this, as there is nothing in the historical record demonstrating Constantine did such. In fact, the record reveals the opposite: while it was eminently clear which side of the pagan/Christian divide he himself sided with, Constantine was the very definition of a religiously tolerant ruler. Not only did he condemn and punish Christian persecution of pagans when it (rarely) occurred, many in his government, even those closest to him, were openly pagan. Hardly the actions of a ruler waging war on paganism.
What can we take away with, then, of the (in)famous Constantine? Popular understanding of him is highly inaccurate. Constantine was without doubt a man of his time and thus no saint, by any stretch of the imagination, but he was a deeply religious individual who made a sincere conversion to Christianity, decriminalized it, thus allowing for the subsequent flourishing of the faith, free from constraints for the first time in history. As Pope Pius X correctly commented, Constantine's rule saw that “the liberty bought at the price of the blood of the Divine Redeemer and His Martyrs” was not in vain.
 Eusebius, Life of Constantine: http://www.fordham.edu/halsall/source/conv-const.asp
 This scenario does seem entirely plausible and would readily explain historical facts, including Constantius' sympathetic attitude toward Christians and Helena's prolific activity, both before and after her son's conversion, which led to her recognition by the Catholic Church as a saint.
 For example, this explains why Stalin purged the army before any other institution, including the Party itself, as well as why seemingly all domestic assassination plots against Hitler had their origins in the German army.
 Peter Leithart, Defending Constantine: The Twilight of an Empire and the Dawn of Christendom, pp. 72-74
 As found in Leithart, p. 78.
 Ibid., pp. 77-78.
 Rodney Stark, Cities of God: The Real Story of How Christianity Became an Urban Movement and Conquered Rome, p. 67.
 Some see evidence of a political motive for Constantine's conversion nonetheless, as Christianity was primarily an urban religion and by 300 Christians were nearing the majority in the city of Rome. Additionally, Eusebius suggests that Christians were openly favoring Constantine over Maxentius on the eve of Milvian Bridge, further supporting a political motive. I think, perhaps, these individuals are under appreciating Constantine. While this does demonstrate political advantage in one city, as the data indicates, Christians were still far and away a minority in the Empire and no one living then could have realistically predicted they would become the majority. Hence, I'm not sure, given my own reading of the historical record, that Constantine would be so short-sided as to convert to a particular religion for the sake of one city, however important said city was. It's worth repeating: in the fourth century a soldier simply didn't switch from one god to another on the fly, especially before battle.
 A few individuals, most notably Origen (died c. 254), were laying the foundation for the formal theology developed later.
 For the full text of the “Edict of Milan” see http://www.fordham.edu/halsall/source/edict-milan.asp
 Admittedly, this had nothing to do with piety so much as staying on good terms with arbitrary, capricious gods.
 Consider Constantine’s “Letter to Alexander and Arius” as indicative of Constantine’s piety, commitment to preventing schism, and promotion of fellowship and harmony, and how the absence of such could affect the Empire.http://www.fourthcentury.com/urkunde-17/
 It cannot be stressed enough: without religious harmony, Rome would fall, or so was believed. Later this would be amended to: “abandonment of the pagan gods in favor of Christianity is making Rome fall,” to which Augustine eloquently rebutted, Rome was falling because Rome wasn’t Christian enough.
 One particularly interesting attendee was a scrawny, cantankerous curmudgeon by the name of Nicholas - you know him as Santa Claus - who punched Arius in the face for his soon-to-be heretical views on Jesus' nature, whereupon his brother bishops threw him in Nicaea’s prison to cool off.
 Joseph F. Kelly, The Ecumenical Councils of the Catholic Church: A History, pp. 21-22.
 The term doesn’t appear in the Bible, which meant the bishops would have to go outside of Scripture to discern how to best express Christian faith – the first time in history this had been done in the name of orthodoxy.
 To put this in a different context: the American people have the “extraconstitutional” power to abolish the government should it become tyrannical and establish a new one. Such authority is not explicitly granted in the Constitution, yet it is implied because the Constitution is written with the understanding humanity has God-given rights that supersede government.
 Kelly, p. 23.
 Constantine went out of his way to kiss the wounds inflicted upon the bishops during previous persecutions – not behavior of an individual interested in compelling submission and obedience.
 At least 18% of Constantine’s appointments to consul and prefect positions were pagan, with the possibility of an additional 26% whose religious inclinations are more difficult to determine, as found in Stark, Cities of God, p. 191. I doubt much, if any, of this “unknown” 26% was Christian given they had no cause to be circumspect about their faith.
 Constantine’s “Letter to the Eastern Provincials,” found in Life of Constantine, is also instructive: “For the general good of the world and of all mankind I desire that you people be at peace and stay free from strife. Let those in error, as well as the believers, gladly receive the benefit of peace and quiet. For this sweetness of fellowship will be effective for correcting them and bringing them to the right way. May none molest another; may each retain what his soul desires, and practise (sic) it. But persons of good sense ought to be convinced that those alone will live a holy and pure life, whom you call to rely on your holy laws. Those who hold themselves back, let them keep if they wish, their sanctuaries of falsehood. To us belongs the shining house of your truth, which you have given in accordance with nature. This we pray also for them, that by the means of the general concord they too may enjoy what they desire.” http://books.google.com/books?id=KchhO8KEy3cC&pg=PA113&lpg=PA113&dq=let+those+in+error,+as+well+as+believers,+gladly+receive+the+benefits+of+peace+and+quiet&source=bl&ots=INk2NeST9f&sig=G8RiTX-ZRNREjemdWSto8vL98Ig&hl=en&sa=X&ei=7SOOUv21FrOqsATm5oGoCA&ved