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.