The Roman empire became Christian during the fourth century CE. At the century’s start, Christians were – at most – a substantial minority of the population. By its end, Christians (or nominal Christians) indisputably constituted a majority in the empire. Tellingly, at the beginning of the century, the imperial government launched the only sustained and concerted effort to suppress Christianity in ancient history – and yet by the century’s end, the emperors themselves were Christians, Christianity enjoyed exclusive support from the state and was, in principle, the only religion the state permitted.

Apart from the small and ethnically circumscribed exception of the Jews, the ancient world had never known an exclusivist faith, so the rapid success of early Christianity is a historical anomaly. Moreover, because some form of Christianity is a foundational part of so many peoples’ lives and identities, the Christianisation of the Roman empire feels perennially relevant – something that is ‘about us’ in a way a lot of ancient history simply is not. Of course, this apparent relevance also obscures as much as it reveals, especially just how strange Rome’s Christianisation really was.

That a world religion should have emerged from an oriental cult in a tiny and peculiar corner of Roman Palestine is nothing short of extraordinary. Jesus of Nazareth was a Jew, though an eccentric one, and here the concern is not what the historical Jesus did or did not believe. We know that he was executed for disturbing the Roman peace during the reign of the emperor Tiberius, and that some of his followers then decided that Jesus was not merely another regular prophet, common in the region. Rather, he was the son of the one true god, and he had died to bring salvation to those who would follow him.

Jesus’s disciples began to preach the virtues of their wonderworker. Quite a few people believed them, including Saul of Tarsus, who took the message on the road, changing his name to Paul as a token of his conversion. Paul ignored the hardscrabble villages of the Galilee region, looking instead to the cities full of Greeks and Greek-speaking Jews all around the eastern Mediterranean littoral. He travelled to the Levant, Asia Minor and mainland Greece, where he delivered his famous address to the Corinthians.

Some scholars now believe that Paul might have gone to Spain, not just talked about wanting to go. What matters is not whether Paul went there, or if he really was executed at Rome during the reign of the emperor Nero, but rather the person of Paul himself. When he was arrested as a threat to public order, his Jewish enemies having complained to the Romans, Paul needed only two words to change the balance of power – cives sum, ‘I am a citizen’ – a Roman citizen. The fact that he was a Roman citizen meant that, unlike Jesus, he could neither be handed over to the Jewish authorities for judgment nor summarily executed by an angry Roman governor. A Roman citizen could appeal to the emperor’s justice, and that is what Paul did.

Paul was a Christian, perhaps indeed the first Christian, but he was also a Roman. That was new. Even if the occasional Jew gained Roman citizenship, Jews weren’t Romans. As a religion, Judaism was ethnic, which gave Jews some privileged exemptions unavailable to any other Roman subjects, but it also meant they were perpetually aliens. In contrast, Christianity was not ethnic. Although Christian leaders were intent on separating themselves physically and ideologically from the Jewish communities out of which they’d grown, they also accepted newcomers to their congregations without regard for ethnic origin or social class. In the socially stratified world of antiquity, the egalitarianism of Christianity was unusual and, to many, appealing.

The promise of salvation, vouchsafed in the miracles of Jesus and/or his divine father also drew in followers. Miracles and the immanence of the supernatural abounded in the Roman world. Powerful miracles were powerfully persuasive. Stories circulated about the Christian god (or the son of god – theology was a work in progress for a very long time), far more stories than today’s canon acknowledges. It used to be said that women, slaves and the working classes took to Christianity first but, in fact, the miracle stories and the promises of salvation attracted a wide cross-section of society. Christianity offered eternal life in exchange for belief – no complex initiation rituals, no hieratic pyramid of occult revelation.

While theologians have always been able to render Christianity subtle to the point of incomprehensibility, to many it has always appeared breathtakingly simple: ‘Believe exclusively in the Christian god, who is the one and only god, and you will find eternal life.’ On earth, Christianity offered community, and it offered support – dining, celebrating, working and playing together, people who would bury you if you died. In a cosmopolitan Roman empire, where cities sucked in expendable labour from the countryside, and where artisans and craftsmen had to travel a very long way from home, that kind of community could not be taken for granted or created casually. Christians would and did look after one another, sometimes exclusively so. Stricter Christians didn’t mix with non-Christians. More importantly, they didn’t worship other gods along with their one god. Much of ancient civic life – the holidays and public festivities which were many people’s only opportunity to eat any quantity of meat – was wrapped up in sacrifice to the various deities of a flexible and syncretic Greco-Roman pantheon. Good Christians were expected to shun these celebrations, the festivals and ceremonies their fellow townsfolk kept at the centre of their social lives. That made Christians very strange.

Technically, for a time, Christianity was illegal (its god had been nailed to a cross like a common bandit after all)

The Jews had kept themselves separate for as long as anyone could remember, but Greeks and Romans were used to that. Jewish communities were concentrated, nowhere large, and they were exempt from mandatory participation in a public cult. Around the Mediterranean, people could look at Jews with a sort of tolerant, if uncomprehending, disdain. But Greeks and Romans sitting out the traditional cult of their own cities made no sense. Were these monotheist Christians pretty much the same as atheists, refusing to give the divine its due? What exactly did they get up to in their exclusive meetings? What was this business about eating their lord’s body? Were they cannibals? Probably it was all just another eccentric. The world of ancient Rome, after all, was one in which initiates of one cult bathed in the spurting blood of a freshly slaughtered bull. Those of another passed the night in temples awaiting divine revelation and sleeping with the sacred priestesses.

Of course, the eccentricity of neighbours begins to look more sinister when life gets difficult and livelihoods grow tenuous. A Christian exclusivity that was also status-blind could look suspicious – so there were occasional pogroms, though surprisingly few: the pornographic violence of martyrologies, the tormented saints of a million works of Catholic art, were the loving harvest of later centuries, not any ancient reality. Like all empires, the Roman state hated disorder more than anything, and violence that disturbed the public peace was not encouraged. Technically, for a time, Christianity was illegal (its god had been nailed to a cross like a common bandit after all). But a ‘don’t ask, don’t tell’ policy was easier on everyone, not least the emperors. As the letters of the emperor Trajan make crystal clear, Christians were not to be sought out or persecuted unless they made themselves a conspicuous nuisance, at which point they had no one but themselves to blame for their fates.

By the third century, Christian communities had grown. One would have been hard-pressed to find even a modest town without a Christian household or three. From a fringe movement, Christianity had become a central fact of urban life. Yet the religion’s normalisation made it suddenly vulnerable in the middle of the third century, when – thanks to dynastic instability, epidemic disease and military incompetence ­– imperial government went into a potentially terminal decline.

The last dynasty to have any real claim to legitimacy was that of Septimius Severus (who reigned 193-211). Its last scion was murdered in a mutiny in 235. For 50 years thereafter, no emperor could make any lasting claim to the throne. Combined with devastating military failure on the empire’s eastern front with Persia, and a plague (probably an Ebola-like haemorrhagic fever) that cut densely packed urban populations to ribbons, it seemed to many that the divine order of the universe had come undone.

The emperor Decius, with a shaky claim to a throne he’d won in an officers’ putsch, thought it prudent to assure himself of divine favour. In 249, he ordered every inhabitant of the empire to sacrifice to the gods of the state, and to prove it by producing the same sort of certificate that local magistrates issued to document the payment of annual taxes. Decius might not have actually meant to target Christians specifically, but his edict could not help but have that effect. Forbidden to worship any god but their own, many Christians refused to sacrifice. For their obduracy, some were executed. When Decius was killed on the battlefield in 251, Christians rejoiced that their god had protected them.

Imperial fortunes did not improve. A decade after Decius’s death, the emperor Valerian renewed religious persecution, this time targeting Christians explicitly. Many wondered why Valerian singled them out: the Roman senate went so far as to query whether the emperor really meant what he appeared to mean with his edict. He did. More martyrdoms followed, but then, in 260, Valerian was taken prisoner on the battlefield by the Persian king, going on to die in captivity. His son and successor Gallienus immediately ended persecution and restored the legal rights of Christian churches. That legal measure demonstrates something significant. Churches had become prosperous, socially integrated corporate entities, able to possess and dispose of property. Christianity was no longer a clandestine and minority religion.

the policing of what did and did not constitute true belief has always preoccupied Christian theologians and been a central dynamic in Christian politics

The years between 260 and 300 offered little reprieve to those who wanted to become emperor and govern, but they did amount to the first golden age for Roman Christians. Although it is likely that we’ll never have sufficient evidence to tell just how many Christians there were at any one time, or just how fast the religion spread, we can say for certain that Christian numbers grew dramatically. By the 290s, there were Christians in the senate, at court, and even in the families of emperors.

The middle and late third century also witnessed the first dramatic outpouring of Christian theological works. Some of these theological works focus on detailing heresies – wrong beliefs – of which there was already a rich variety. Because Christianity centred so much on beliefs rather than ritual behaviours, the policing of what did and did not constitute true and acceptable belief has always preoccupied Christian theologians and been a central dynamic in Christian politics.

The rulings (‘canons’) of the first council of Christian leaders to survive provide more insight into the Christianity of this period. Held in the obscure Andalusian town of Elvira, the council shows us a world in which the gathered church leaders found it necessary to legislate against a large number of mundane activities that they determined were prejudicial to Christian wellbeing. The council decided, for instance, to forbid the holding of certain kinds of public office (such as the office of duumvir, effectively the local mayor, as the role might require inflicting punishment or abusing other Christians). What this tells us is that Christians were integrated into the fabric of social and political life, serving in public office, and so forth. Clearly, both Christians and non-Christians found that integration quite normal – Christians had come a long way since the days of the last persecution.

Then, ironically, within just a couple of years of Elvira, the imperial government launched the most virulent anti-Christian persecution in the history of the ancient world. The causes were multiple. As Christianity’s appeal spread among the more educated sort of Greek and Roman, non-Christian intellectuals began to find the upstart religion more threatening. Though the third century saw a trend towards monotheism among intellectuals, the philosophical and theosophical varieties embraced by Neoplatonists and other philosophers were clearly incompatible with Christian exclusivity. So these pagans crafted sophisticated anti-Christian arguments, and their criticisms gained ground among the political class. Then, rivalry over an imperial succession provided the occasion for anti-Christian polemic to gain new political life.

Towards the end of the third century, an emperor named Diocletian (r. 284-305) had finally proved able to stabilise imperial government after 50 years of regime change and violence. In 293, he established a college of four emperors, all senior generals unrelated to one another except by marriage. The idea was to ensure that one emperor would always be on hand to deal with any outbreak of violence and to prevent rebellion or civil war. Diocletian intended for himself and his senior colleague to retire, after which their junior partners would bring two new emperors into the imperial college to replace them. The goal was to ensure a handover of power at a convenient and peaceful moment so that the framework of government would remain undisturbed. But Diocletian’s intentions were thwarted by rivalries, in which Christianity played an important role.

That is where things foundered: only two of Diocletian’s emperors had adult sons, and everyone expected them to join the college of four emperors when the two senior emperors retired. But the childless emperor Galerius was a ferocious anti-Christian, while his colleague Constantius – who had a son – was known to be sympathetic to Christians. In fact, Constantius even had Christians among his family and household, and that fact gave Galerius an opening to revise the succession plans in his own favour. By targeting Christians for renewed persecution, Galerius would damage Constantius and exclude his son from the succession. He could enhance his own power, and also gratify his hatred of Christianity.

Galerius convinced Diocletian that Christians were to blame for a series of calamities, including a mysterious fire in the palace and the silencing of famous oracles. Thus, in the year 303, the emperors began what we call the Great Persecution. The campaign against the Christians was bitterly violent in Africa and the eastern Mediterranean, more benign in the lands that Constantius controlled in the West. But it produced many heroic martyrdoms and appalling suffering among Christian communities, and left scars that would linger for centuries. The Great Persecution ultimately failed to expunge Christianity from the face of the earth. Christians were simply too numerous, and many were too stubborn to be turned away from their beliefs. Even Galerius, the most committed of persecutors, came to accept the failure of his plans, and in 311 issued an edict of toleration. By 313, persecution had ceased.

In the meantime, in 306, Constantius’s son Constantine had succeeded his father in the imperial college. Within five years, Constantine had made himself master of the western Roman empire and openly embraced Christianity. Always sympathetic to Christians, he claimed to have had a divine vision that helped lead his troops, flying Christian symbols on their standards, to victory in civil war in 312. The most reductionist reading of the evidence would say that, in 310, Constantine saw a solar halo, a rare but well-documented celestial phenomenon, in the south of France and in the company of his army, but Constantine’s account of events changed over the years and we can’t be sure. We can say with greater certainty that for several years he wavered between Christian and non-Christian interpretations of the sign. He eventually decided, to the delight of the Christian leaders in his entourage, that he had been sent a sign by the Christian God. He became a Christian, as a matter of belief and perhaps policy too.

We will never know for sure what Constantine’s true motives were in converting to Christianity. What is certain, however, is that from the moment he had sole power in the West, he ruled as a Christian. He restored Christian property seized during the Great Persecution and enacted legislation that favoured Christians. When he became sole ruler of the empire in 324, he extended similarly pro-Christian policies to the eastern empire, where he not only favoured Christians, but actively discriminated against non-Christians, restricting their ability to worship or fund their temples.

Patronage, factionalism, political advantage, social cliquishness can all play a role in the formation of intellectual positions and in continuing attachments to them

Even more momentously, though, Constantine intervened personally in conflicts among Christians over questions of discipline and right belief. In North Africa, Egypt and other parts of the Greek East, problems arose over such things as how to treat Christians who had cooperated with the authorities during persecution (the traditores, ‘handers-over’ of Christian holy books), or the correct relationship between God the Father and God the Son. Such disputes mattered, not least because Christians who believed the wrong thing would forfeit eternal life – or worse, ensure their own eternal damnation. Right belief, by contrast, opened the path to eternal salvation.

By placing the authority of the Roman state and the imperial office to police and enforce right belief, Constantine created a model that would have a long and ambiguous history. Councils of bishops, ostensibly informed by the Holy Spirit, would henceforth define what was orthodox. Those who chose to believe otherwise would find themselves branded heretics, and excluded from the communion of orthodox Christians. Bishops and theologians would find an almost limitless number of problems to debate – over the relationship of God the Father and God the Son, over the divine nature of Jesus, over what that meant for the status of his mother, and so on. Each solution opened up a whole new set of problems.

As most people know from their own experience, intellectual differences can harden into intractable convictions for all sorts of non-intellectual reasons. Patronage, factionalism, political advantage, social cliquishness can all play a role in the formation of intellectual positions and in continuing attachments to them. From the fourth century onwards, Roman history is filled with bitter religious conflicts, state persecution of heretics, and the perpetual alienation of communities whose Christian beliefs pitted them against official orthodoxy. Since the time of Constantine, in fact, Western history has been plagued by the impossibility of policing belief rather than practice. After all, how do you decide what someone really believes, or does not believe?

That problem would not have come to have its historic, and tragic, consequences had Constantine’s conversion not rapidly brought much of the imperial population with him. As social advancement came to depend on being a Christian, and as the civic calendar of non-Christian beliefs was increasingly dismantled, the majority of urban Romans actively thought of themselves as Christians by the end of the fourth century. Rejecting Christianity now stood as the marked and unusual choice that embracing it had been 200 years before. How Christianity went on to become not just a state religion, but the central fact of political life, and how Christian institutions of the Middle Ages both maintained and distorted the legacy of the ancient world, is another, different story.