PART ONE The Romanization of Christianity 1 ''By This Conquer .'' Early in the second decade of the fourth century AD, the Roman emperor Constantine was deeply embroiled in his own game of thrones. In the previous political generation, imperial power had been shared between four generals, two in the western half of the Empire and two in the east. Diocletian had come to sole power in ad 285, but by 293 had made himself head of an imperial college: the Tetrarchy (''rule of four''), comprising two senior Augusti (himself and Maximian) and two Caesars (Constantius Chlorus and Galerius). In 305, the two Augusti retired, the existing Caesars were promoted in their place, and two new junior colleagues (Severus in the west, Maximinus Daia in the east) appointed. It was supposed to be a better mechanism for handing over imperial power than dynastic succession, but quickly broke down into multiple civil wars. Constantius Chlorus died the year after his promotion, at which point his son Constantine declared himself the western Augustus in his father''s place. Maximian''s son Maxentius threw his hat into the imperial ring as well, and, to add to the confusion, Maximian himself came out of retirement.

By 312, Severus and Maximian had been eliminated, and the struggle for power in the west came down to a straight shoot-out between Constantine, who controlled Britain, Gaul and Spain, and Maxentius, ruler of Italy and North Africa. Vast as it was, the western Empire was never going to be big enough for both of them. Over the summer, Constantine gathered his armies and, Hannibal-style, forced his way over the Alps. Then God intervened. What happened next detonated the first of three massive revolutions which would, between them, turn a small, Near Eastern mystery cult into the dominant religious structure of the European landmass, from where it subsequently spread worldwide in the era of European imperialism. The story was told, just after the emperor''s death, by Constantine''s biographer Eusebius, bishop of Caesarea, who heard it from the man himself: About the time of the midday sun, when day was just turning, he [Constantine] 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 the 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 himself a copy of the sign, and to use this as protection against the attacks of the enemy. The sign was the Chi-Rho labarum, which the emperor employed on his military standards, and Constantine duly scored a stunning victory over Maxentius on 28 October 312 at the battle of the Milvian Bridge on the outskirts of Rome. A triumphant, thankful Constantine repaid his debt with rich gifts to the capital''s Christian communities, and turned the barracks of Maxentius'' elite cavalry corps into a huge church: St John''s Lateran, the first headquarters of the medieval papacy. But this was only the beginning. Victory in the west was the precursor of still greater glory. By the end of 312, a parallel struggle for post-Tetrarchic imperial power in the eastern half of the Empire had produced its own winner: Licinius (initially an appointee of Galerius, one of the original Tetrarchs). The two reigning Augusti--Constantine in the west and Licinius in the east--immediately declared undying love, alliance and endless co-operation, but there was only ever going to be one outcome. It took over a decade but, after several separate rounds of conflict, Constantine finally eliminated his eastern rival in 324.

Thanks to the vision that God had afforded him, the Christian Constantine remade the complexion of imperial politics, reuniting the entire Roman world under a single, unchallenged emperor for the first time since the mid-third century. Eusebius'' account of the crucial visionary experience has been repeated and--exquisitely (Plate 1)--illustrated countless times, but it''s deeply problematic. And I don''t just mean the fact that Constantine saw a vision in the sky. I''ve never experienced one myself, but many people, across the entirety of human history, have, and at least the subjective ''reality'' of supernatural religious experience - however you might wish to explain it--is not something to be discounted a priori. But Constantine''s story also poses problems of a much more mundane kind, because the emperor appears to have had several different supernatural experiences, not all of which were Christian, at more or less the same time, and to have told different people different things about them. Apart from the combined vision/dream story told by Eusebius, three other variants survive. Writing about twenty years before Eusebius, the rhetor Lactantius--a distinguished university-level teacher of the Latin language and literature, in which elite children of the western half of the Empire were all customarily educated (their eastern counterparts received an identical training in Greek)--who knew the emperor, and was tutor to his eldest son, Crispus, in the early 310s, reports that Constantine had a dream on the eve of the Milvian Bridge, in which he was told to ''mark the heavenly sign of God'' on his soldiers'' shields.3 The timing of the divine intervention is different in Lactantius'' account, and there is no vision in the sky.

Our second witness is another rhetor, this time from Gaul, who gave a formal speech on a ceremonial occasion to Constantine and his assembled court in 310, two years before Maxentius'' defeat. In a passage, which must have had prior imperial approval, this speaker described another heavenly vision. Having turned off the main road to visit ''the most beautiful temple in the world . You saw, I believe, Constantine, your Apollo, accompanied by Victory, offering you laurel crowns, which brought an omen of thirty years [of life, or rule]. This time we have a vision, but of a different God (Apollo as the Sun God), and no dream. Last, but not least, in 321 a third rhetor, in another official--and hence imperially endorsed--oration, referred back to Constantine''s victory over Maxentius. This speech mentions neither Divine visitation, nor any kind of cross in the sky. Instead, in the midst of battle, all of Constantine''s troops are said to have been buoyed up by a vision of a heavenly army, led by the emperor''s deified father, Constantius Chlorus, coming to his son''s assistance.

There have been many attempts to rationalize Constantine''s multiple reported religious experiences. Most influential among recent efforts, Peter Weiss argued in 1993 that the visions of Apollo reported in 310 and Eusebius'' cross in the sky referred to one and the same event, differently interpreted. Given that Apollo was, by this date, overwhelmingly represented as the Sun God, Weiss argued that what Constantine actually saw was a solar halo, which can take a kind of cross form, and that the emperor eventually came to understand this natural phenomenon as a message from the Christian God. This is obviously a possible way forward, and Lactantius'' dream could then be reconciled with Eusebius'' account--so long as you posit a delay of two years between initial vision and explanatory dream, whereas Eusebius clearly supposed that the dream occurred the following night. To my mind, however, all these reports need to be handled with a great deal more suspicion. It was standard practice for ancient rulers, who claimed to be appointed by Divine Power (as all Roman emperors, and Constantine in particular, did), to report suitable omens as confirmation of their special destiny to rule. The future emperor Claudius reportedly had the--no doubt unnerving --experience of an eagle landing on his shoulder when he first entered the forum as consul, while some mysterious force ejected anyone else who tried to sleep in the nursery of the first Roman emperor, Augustus. No historian worth his or her salt (not even Constantine) would dream of taking these earlier stories literally, and both the convenience and variety of the stories the first Christian emperor told about his personal encounter with the God of the New Testament suggest that we''re again some way from factual reportage.

The received image of a vision-powered Constantine--forced, like Paul on the road to Damascus, into a complete and sudden Christian conversion by a direct personal experience of the Almighty--becomes still more problematic when set alongside how the emperor presented his evolving religious allegiance to the Empire''s population at large, over the course of his long and brutally successful reign. The official religious self-presentation of Constantine''s regime went through four distinct phases. In his first years, Constantine styled himself a loyal adherent of the religious ideologies of Diocletian''s imperial college, the Tetrarchy, to which his father had belonged. While all Roman emperors claimed to be appointed and supported by Divine power, the specific divinity in question could change, or at least be presented in different forms. Many third-centur.