Constantine's conversion, which was a decisive turning point in Roman history, has been a subject of endless fascination. Both Eusebius in the Life of Constantine, which was completed shortly after the emperor's death, and Lactantius in On the Deaths of the Persecutors, gave accounts of a dramatic dream or vision which supposedly determined Constantine's religious allegiance to Christianity. Lactantius, writing around 314, reports the experience that came to the emperor on the eve of the decisive battle with Maxentius at the Milvian bridge two years earlier:
The anniversary of Maxentius' accession, 27 October, was near and his first five years were drawing to a close. Constantine was directed in a dream to mark the heavenly sign of God on the shields of his soldiers and thus to join battle. He did as he was ordered and with the cross-shaped letter X, with its top bent over, he marked Christ on the shields. (On the Deaths of the Persecutors 44.3–6)
Eusebius has a much more elaborate version. The context is the moment at which Constantine sought divine help some time before the decisive battle. The emperor reflected that those who had placed their trust in many gods had failed to dislodge the tyrant Maxentius, and accordingly looked for support for his father's talisman, “the God who transcends the universe, the saviour and guardian of his empire.”1 This, then, is a classic narrative of transition from polytheism to monotheistic Christianity, which continues:
This God he began to invoke in prayer, beseeching and imploring him to show who he was, and to stretch out his right hand to assist him in his plans. As he made these prayers and earnest supplications there appeared to the emperor a most remarkable divine sign. If someone else had reported it, it would perhaps not be easy to accept; but since the victorious emperor himself told the story to the present writer a long while after, when I was privileged with his acquaintance and company, and confirmed it with oaths, who would hesitate to believe the account, especially when the time which followed provided evidence for the truth of what he said. About the time of the midday sun, when the day was just turning, he said he saw with his own eyes, up in the sky and resting over the sun, a cross-shaped trophy formed from light, and a text attached to it which said, “by this conquer.” Amazement at the spectacle seized both him and the whole company of soldiers which was then accompanying him on a campaign he was conducting somewhere, and witnessed a miracle.
He was, he said, wondering to himself what the manifestation might mean; then, while he meditated, and thought long and hard, night overtook him. Thereupon, as he slept, the Christ of God appeared to him with the sign which had appeared in the sky, and urged him to make a copy of the sign which had appeared in the sky, and to use this as a protection against the attacks of the enemy. (Eusebius, VC I.28, trans. Cameron and Hall)
It has been questioned whether the very different versions of Lactantius and Eusebius can refer to the same event, but it seems best to combine them. Lactantius, although writing soon after the episode, had no access to the emperor's personal and detailed account. He accordingly offered a simple version of the dream before the battle which led Constantine to place a Christian symbol on his soldiers' shields. Eusebius also has the dream, which revealed God's will to the emperor, but before that a circumstantial account of what appears to be a miraculous event. The details were provided by the emperor in person, speaking under oath. It is evident that this cannot have been a private interview with Eusebius, who in any case was not an intimate associate of Constantine, but must have been relayed to a Christian audience in some formal setting.2
Scholars have naturally exercised themselves over the question of what, if anything, Constantine actually saw and interpreted as a heavenly vision. Eusebius implies that it occurred an unspecified time before the battle. In a highly convincing study Peter Weiss argues that an earlier account of the same vision can be found in the panegyric addressed by an anonymous Gallic orator to Constantine in 310, shortly after the death of Maximianus, and that this vision can be identified as a spectacular solar phenomenon.3 The passage in question runs:
You learned that all the storms had abated, and all the tranquillity which you had left behind had returned. Fortune herself had so ordained matters that the successful outcome of your actions admonished you to make an offering of what you had vowed to the immortal gods at the very place where you had turned off the road to the most beautiful temple in the whole world, or rather to the very presence of the god, as you had seen him. For, Constantine, you saw, I believe, your Apollo in the company of Victory offering you laurel crowns, each of which bears the prophecy of thirty years rule. (Pan. Lat. VII  12)
Weiss interprets the descriptions of Eusebius and of the panegyricist of 310 as corresponding to the appearance of a double solar halo, an optical effect caused by the refracted light from slanting sun hitting ice crystals in the upper atmosphere and producing a semicircular halo with intense crosses of light directly above and on either side of the sun. The appearance of such haloes, coronae in Latin, was readily interpreted as a victory signal, and this was the conventional meaning which the orator of 310 attached to the vision. From a pagan perspective, the divinity responsible was of course Apollo, the sun god himself, and the vision occurred close to the important sanctuary of Apollo Grannus, which lay on the main route across Gaul leading north to the imperial residence at Trier. The psychological effect of such a dramatic experience is not to be doubted. The nineteenth-century Alpinist Edward Whymper saw a comparably awe-inspiring halo with three crosses in the sky as he made his way down from the first successful ascent of the Matterhorn in 1865. Whymper recalled that “the ghostly apparitions of light hung motionless; it was a strange and awesome sight, unique to me, and indescribably imposing at such a moment.”
The experience lived with Constantine and two years later its meaning was reinterpreted during the build-up to the battle with Maxentius. The three (or rather six) crosses of light were now understood in the Christian sense, and the victory which they heralded, in the form of the corona itself, was a sign from the Christian God, not Apollo. Eusebius takes evident relish in describing the role of the emperor's Christian advisors, among them certainly Ossius of Cordoba, who expounded their view that the crosses in the sky were a manifestation of Christ and a symbol of his triumph over death.4 In the telling or retelling of this indelible scene, Eusebius translated the symbolic victory sign into the verbal message, “by this conquer.”5
It is right to hail the vision as a decisive moment in late Roman history. Neither solar haloes nor critical battles were unparalleled events, but the coincidence of the two, and above all the religious and psychological response that they triggered in Constantine transformed a remarkable event into an epochal one. The vision itself was not the sole cause of Constantine's conversion. For the orator of 310 it was a manifestation of the sun god's power, and we may assume that this interpretation was perfectly acceptable to Constantine at the time. However, we cannot say that it was the vision which caused Constantine's conversion in 312, since by then the emperor, and his Christian advisors, were already looking for the Christian meaning that was confirmed to him by his dream. The most important difference was contextual. Whereas in 310 the vision had seemed only casually appropriate to Constantine's good fortune, coming after Maximianus' death and the evaporation of the minor threat of an Alemannic raiding party, in 312 it was treated as talismanic, a cosmic sign sent by God as an omen for the decisive engagement with Maxentius. Crucially the battle was won, and Constantine's belief in the talisman was confirmed. The close affinity between Christianity, which Constantine experienced and endorsed in 312, and sun worship is readily apparent from the reconstruction of his vision. The nature of the event will have reinforced his conviction that belief in a supreme solar deity and belief in God were virtually convergent. The imagery favored by the emperor after 312 reflects this (see Plates 8.1 and 8.2). The great symbol of the conversion, the labarum, the new standard painted on the shields of the emperor's soldiers and then recreated in gold and jewelry, took its name not from Christian vocabulary but from the Celtic word used to denote the solar apparition.
It was constructed to the following design. A tall pole plated with gold had a traverse bar forming the shape of a cross. Up at the extreme top a wreath woven of precious stones and gold had been fastened. On it two letters, intimating by its first characters the name “Christ,” formed the monogram of the Saviour's title, rho being intersected in the middle by chi. These letters the emperor also used to wear upon his helmet in later times. (Eusebius, VC I.31.1, trans. Cameron and Hall)
Plate 8.1 Bronze coin of Constantine (obverse) depicting the Labarum above a serpent, symbolizing Constantine's victory over Licinius, inscribed SPES PUBLICA (Hope of the State) on the reverse, AD 327/8 (Private Collection Munich. Photo Nicolai Kästner, Staatliche Münzsammlung, München)
Plate 8.2 Silver medallion from Ticinum of AD 315, depicting Constantine with Christogram on his helmet (obverse) and the emperor addressing his troops with the legend SALUS REIPUBLICAE (Salvation of the State) on the reverse (Photo Nicolai Kästner, Staatliche Münzsammlung, München)
The design deliberately fused the solar image of a six-pointed star with the almost identical Chi-Rho form of a christogram. The sun was the manifestation of Christ's power. As early as 315 the Chi-Rho symbol adorned Constantine's helmet as displayed on the silver medallion of Ticinum. His portraits, as Eusebius commented, show him with his gaze tilted up to the sky (Eusebius, VC 4.15; Plate 2.2). A new law was passed in 321 establishing the dies solis as the day of rest (CTh. 11.8.1).6
Constantine himself displayed his new religious convictions in different ways for different audiences, and met with a varied response. The panegyric address delivered in Constantine's presence in 313 referred to an unspecified divine will, mens divina, as the source of the emperor's victory:
What god was it, what so present majesty, that exhorted you – when almost all your staff officers and generals were not merely silently muttering but openly fearful – so that you, contrary to human advice, contrary to warnings of the haruspices, you yourself, on your own initiative, felt that the moment had come to liberate the city? For a certainty you share something with the divine mind, Constantine, something secret: that mind delegates care for us to lesser gods and deigns to reveal itself to you alone. (Pan. Lat. IX  2.4f.)
In 315 the dedication by the Roman people and Senate of the Arch of Constantine also avoided an open allusion to Christ, preferring the neutral formula instinctu divinitatis:
To the Emperor Caesar Flavius Constantinus Maximus Pius Felix Augustus, the Roman Senate and People dedicated this arch, decorated with his victories, because, by the prompting of Divinity (instinctu Divinitatis) by the greatness of his mind, he with his arm, at one moment by a just victory avenged the State both on the tyrant and on all his party. To the liberator of the city. To the establisher of peace. (CIL VI 1139, AD 315)7
However, in a letter addressed to the bishops of Africa, written in this period, Constantine provided a more personal and pointed view of his own conversion, and the consequences which it had for his own role:
The eternal and religious incomprehensible piety of our God by no means permits the state of man to stray too long in darkness, nor does it allow the hostile wills of certain men to be so strong that it does not grant an opportunity of conversion to the rule of righteousness by opening anew with its most glorious lights a path to salvation. I know this from many examples; I extrapolate the same conclusion from my own case. For there were in me at first things which seemed alien to righteousness, and I did not think that a power on high saw any of the things which I carried out inside the secret places of my heart. What fortune ought these things to have earned me? Surely one overflowing with every evil. But almighty God, sitting in the watch-tower of heaven, bestowed on me what I did not deserve: assuredly, what he has granted to me his servant by his heavenly benevolence cannot at present be described or enumerated. (Optatus, app. 5, letter of Constantine to the bishops in 314)
As far as we can judge, Constantine's own journey to Christianity had not been a long one. It seems clear that his father was an adherent of solar monotheism, a form of religious expression that was not only acceptable to many intellectuals but was also rooted in popular patterns of belief both in the western and in the eastern provinces, even though it was not a component of traditional civic paganism. As a member of the tetrarchy Constantine's father Constantius had probably been particularly associated with the divinity Sol Invictus, the unconquered Sun, although there are hints, including the fact that one of his daughters took the name Anastasia (“Resurrection”), that he was already inclined to Christianity. If Lactantius is to be believed, Constantine showed favor to the Christians in 306, by rescinding the persecution edicts as soon as he became Augustus (Lactantius, On the deaths of the persecutors 24.9; Inst. Div. I.1.13). From 312 Constantine was a Christian in the full sense of the word, believing that the supreme saving divinity should be identified with Christ. But he also accepted that the Christian God could be presented, without theological baggage, simply as the highest god, summus deus, a conception which was acceptable to a broad swathe of his subjects, especially in the eastern provinces.8Another significant figure from the early fourth century who made a step similar to Constantine's was the father of the Cappadocian bishop Gregory of Nazianzus. He had been a worshipper of the highest god, theos hypsistos. A party of bishops traveling to the Council of Nicaea in 325 is said to have converted him to Christianity.9
Much was changed by Constantine's conversion, above all Christianity itself. A cult that had been illegal became more favored than any other, and by 325 had evolved into the religion of the Roman state. Christian clergy were rewarded with privileges, and their leaders, the bishops, already highly effective community leaders who had brought their followers through a challenging and dangerous decade of persecution, were now entrusted with legal powers. More importantly they had the ear of the emperor himself. Pagan cults were restricted and undermined, without for the most part being suppressed. The emperor's own actions prompted a great flood of conversions. Eusebius, in his capacity of bishop, received a letter from Constantine requesting fifty ornamental leather-bound copies of the scriptures, which were to be prepared in the scriptoria of his city Caesarea and sent for the use of the swelling congregations of Constantinople (Eusebius, VC 4.36). By drawing attention to the size and zeal of their Christian populations, cities in the East successfully claimed privileges and benefits from the emperor.10
These events also transformed Christian society. Before 300 Christians had always been in a minority. They had emphasized their distinctiveness by strict adherence to the values that defined them. However, this stance inevitably raised questions about how Christians should behave in the secular world, and such issues preoccupied local church leaders at a series of church councils of the early fourth century, held at Arles in Gaul in 314, at Elvira in Spain soon afterwards, at Ankara in Galatia between 313 and 316 (see p. 301), and at Neocaesarea in Pontus before 319. One issue was whether Christians should hold public office. In southern Spain the canons of the Council of Elvira ordained that any Christians who served as municipal priests and conducted sacrifices, having been baptized into the church should be excluded from communion, more especially if they had been involved in sponsoring gladiatorial shows or forms of lewd public entertainment (Elvira can. 2, 3). Ideally, at least, Christians should have as little to do with the secular world as possible. Clergy were not allowed to engage in trade or sell their wares outside the province in person, although they were not prevented from using agents on their behalf (Elvira can. 18). Usury was anathematized (Elvira can. 19). Strict morality and social discipline was imposed. Women who left their husbands without cause should never be received into communion, and baptized women could not remarry even after leaving an adulterous baptized husband (Elvira can. 8, 9). Bishops, priests, and deacons who had sexual relations with their wives were to be removed from the clergy (Elvira can. 33).
The canons of the councils of Arles and Ancyra combine stringency with realism in dealing with issues raised by the conduct of clergy and lay Christians. They thus provide an illuminating entry point into the social context of provincial Christian communities in this transitional period.
Strict religious ideals were impracticable. As Christian communities emerged from repression it became obvious that there was a large gulf between the conduct laid down by canon law and that of the mass of new recruits whose lives belonged firmly in the unredeemed secular world. As Henry Chadwick memorably put it in a discussion of the dilemma facing the church after the earlier persecution of Trajan Decius, the rapid growth in Christian numbers “highlighted the conflict between the primitive conception of the Church as a society of saints, and the now growing view…that it was a school for sinners.”11
Christians very rapidly came to prominence as officeholders. There is a considerable debate about the proportions of pagans and Christians among high-ranking Christian officials in the fourth century, and the rate at which Roman senators converted from paganism. T. D. Barnes in a series of studies has challenged the majority view that Christians only came to outnumber pagans at these levels of the Roman hierarchy in the 370s or 380s, claiming that Constantine and Constantius both favored Christians to the extent that in both their reigns more than half the holders of senior government posts (praefecti praetorio, the praefecti urbis of Rome and Constantinople, the proconsuls of Africa and Asia, the counts of Oriens, and the magistri militum) were Christians.12Whatever view is taken of this discussion, it is clear that some of these offices were held by Christians from the moment of Constantine's conversion, and that such men from the first will have taken a very secular view of their Christian obligations. More importantly, as Christians and pagans had to work together in such contexts, there must have been a consensus between them, that their religious affiliations did not materially affect either their ability to hold office or to cooperate with one another.
The same must have been true of army service. It could be said that the army was made Christian by painting the sign of Christ on his soldiers' shields. Simply by fighting on behalf of Christian emperors, Roman soldiers adopted and advanced the Christian version of Roman ruling ideology, and armies became deeply permeated by Christianity during the fourth century. Plainly, merely belonging to a military unit of a Christian state in itself will have had little impact on the personal religious convictions of soldiers, who maintained many features of pagan traditionalism and were also well known for promoting and spreading eastern cults such as Mithraism and the worship of Jupiter Dolichenus in the western frontier provinces.13 We should conclude that in state service, and probably in large parts of society at large, religious affiliation was a neutral factor, which did not impinge on or inhibit the way in which ordinary people led their lives.
The necessary compromises made by lay men and women encouraged the growth of more radical forms of Christian conduct. The rite of baptism was the crucial symbol which set apart the compromised Christians of everyday society from an elect brother- and sisterhood which aspired to the ideals of the ancient church. Constantine, as we have seen, underwent the ritual on his deathbed, precluding the possibility that he might sin before he was resurrected into another life (Eusebius, VC 4.61–62; see p. 72).