News of his death was announced to the army at Nicomedia (Izmit, in today’s Turkey on the borders of the Sea of Marmara). On a hill outside the city, the army lifted up their swords and unanimously acclaimed their general Diocles as their new emperor. Diocles raised his sword in acknowledgement. Then he turned and thrust it into the man standing beside him, Lucius Flavius Aper, commander of the Praetorian Guard, who was, Diocles declared, guilty of murdering the former emperor. Historians now believe that it was Diocles who had been responsible for the assassination, as he probably had been of the previous emperor, Numerian’s father Carus.*

Diocles, Diocletian as he then became, would be the saviour of the empire. He would also be pagan Rome’s most savage persecutor of Christians. Did success require the persecution of this minority group? Diocletian thought it did.

Though Christians through the ages have liked to magnify the numbers of their early martyrs, pagan Rome had mostly turned a blind eye to what was becoming an increasingly popular cult. It is in the nature of imperial states to accept the differences, including religious ones, of their diverse peoples. Most empires, after a phase of expansionary conquest, find their raison d’être in tax collection, which is by its nature indifferent to most things but the bottom line.

But it was particularly easy for a pagan empire like Rome to adopt a laissez-faire attitude towards religious diversity. Pagan Rome did not care how many gods its subjects believed in. The pagan god required minimal beliefs, and little in the way of constraining behaviour, just a sacrifice now and again. It was a straight quid pro quo between worshipper and worshipped: a do ut des as the Romans said, ‘I give so that you might give.’ When Rome conquered a new piece of territory, it preferred to co-opt the gods of its conquered people and fuse them with the Roman gods rather than destroy them. In Britain, Sulis, the Celtic goddess of the thermal springs in Bath, became Sulis Minerva; in Gaul the healing god Lenus latched on to the Roman god of war Mars to become Lenus Mars; in Egypt the goddess Isis merged with Roman Venus. Rome left its peoples free to worship whom and what they pleased – just as long as they were good subjects and paid their taxes.*

But Rome did test its subjects’ loyalty: they must sacrifice to the divine empire and the emperor, just as they must pay tax. Most of Rome’s subjects had no problem with adding the divine emperor to their pantheon of gods. In the glory days of Rome, during its riotous festivals when spectators packed the circuses to watch chariot races, when gladiatorial stars fought each other in the amphitheatres and became sexual trophies for senators’ wives, the emperor simply joined the numerous gods whose images were paraded down the paved streets.

For pagans there was no such entity as a ‘false’ god, just as there was no such being as a heretic. But the empire’s Jews, the world’s first monotheists, could not sacrifice to the divine emperor. While the pagan gods could make room for another addition to their ranks, the single Jewish God is a jealous god who brooks no rivals. But as the largest minority in the empire after the Greeks, Jews had the bargaining power to extract a remarkable concession.† It was most probably engineered by Herod the Great, King of Judea, before his death in 4 bce. Rome agreed that the Jewish Temple priests (the Temple in Jerusalem was the Jews’ holiest building and place of pilgrimage, just as the Kaaba in Mecca would later be for Muslims) would not have to offer daily sacrifices to the divine emperor. Instead they would offer a daily sacrifice (two lambs and a bull) to their own God for the emperor’s well-being.¹

Periodically the Romans evicted Jews from Rome or closed down their synagogues. But it was not for religious reasons. The Jews’ belief in a single all-powerful God – laughable as it seemed to the Romans – put their loyalty to the emperor in doubt. When it came to a choice between God and emperor, it was not clear which the Jews would opt for. The same was true of the cult of Jesus, the group that had broken away from its Jewish parent and had a much more active programme of recruitment.

But the authorities usually followed the pragmatic logic of empires and extended to Christians the same tolerance they showed Jews. Christians had, in fact, far more to fear from their pagan neighbours, who would denounce Christians to the authorities for failing to sacrifice or even take matters into their own hands and beat them up, stone them or lynch them. As closely knit communities who refused to honour the Roman gods, Christians were often subjects of suspicion. The same was true for Jews, but Christians had none of the advantages of being so well established and were easy scapegoats in bad times. As Tertullian, the Early Church theologian from North Africa put it, pagan Romans ‘suppose that the Christians are the cause of every public disaster, every misfortune that happens to the people. If the Tiber overflows or the Nile doesn’t, if there is a drought or an earthquake, a famine or a pestilence, at once the cry goes up, “The Christians to the lion”.’ ²

As this book will show, the status of being tolerated is an extraordinarily fragile and precarious one. Can you trust those who are putting up with you? When they feel powerful, they can afford to be generous; once they feel threatened, their tolerance evaporates. By the third century ce, not just ordinary pagans, but the Roman authorities themselves, were feeling very threatened indeed. Their once invincible empire was disintegrating. Almost every week a revolt broke out or a frontier was threatened – in the west by Germanic tribes from across the Danube and in the east by the rival Sasanian (Persian) Empire. Smallpox was rampant. Cities emptied of people, fields were left untended. Skyrocketing prices and starvation were followed by riots and civil war as general contended with general for the ever-thinner pickings associated with the imperial throne. Between 235 and 284 ce, the year that Diocletian took the imperial throne, twenty-six emperors had come and gone, assassinated by their own troops, casualties of the plague or dying in mysterious circumstances.

The rapid increase in converts to Christianity, especially in the army, concerned Diocletian. Their numbers had been growing during the anarchy of the second and third centuries. More and more people turned away from the gods who seemed to be failing them and towards a god who offered kindness, social justice, inclusion in a community of believers and the wonderful prospect of an eternal life of blissful love.

The Christians’ version of the Jews’ monotheism was proving dangerously appealing. Yahweh’s promise that he would bring Jews back to their homeland if they worshipped only him, the jealous one, and followed his law, had preserved Jews as a united and distinct group since their Babylonian exile in the sixth century bce. But that very distinctiveness posed a constant problem for Jews when the demands of their god conflicted with the demands of Rome.

Jesus – and Paul after him – had tried to solve that problem by claiming there were two separate realms: Caesar’s and God’s. But for most Jews, their law, Halakha, which applied to every aspect of life, from sex to trading, farming to praying, made it impossible to disentangle the two realms, just as the Islamic equivalent, sharia, would make it impossible for Muslims in later centuries.

After Jesus’s death, his cult, as it evolved under Paul, did away with the law and universalized Yahweh. While Yahweh was the Jews’ God and they were his chosen people, the Christian God was a god for Jew and non-Jew alike; nor did the Christian God require his followers to observe the detailed rules of behaviour that Yahweh expected of his followers.

At the time of Jesus’s death in about 32 CE his followers numbered about one hundred. By 250 ce, that number had swelled to around 6 million. Christians made up about 10 per cent of the empire’s population of 60 million, the same proportion as Jews.* The Gospel was being preached in every corner of the conquered world. Christians were becoming increasingly visible; they no longer worshipped in the privacy of their own homes but in large churches occupying prominent positions in the major imperial cities. In the case of Nicomedia, which Diocletian was turning into the capital of the eastern part of his empire, the church was on a hill facing his palace. Aulae ecclesiae – ‘halls of the Church’ as they were called – were attracting large crowds.

Diocletian saw with alarm that an increasing number of his soldiers were turning their backs on the pagan Roman gods and all that they stood for. Instead of giving their loyalty to him, the emperor, and to Rome, Christian soldiers were giving their loyalty to the one God. In c.295 CE he demanded that every soldier should sacrifice to the Roman gods and to the emperor. Those who refused were dismissed.

In 303, he went further and embarked on a policy to eradicate Christianity altogether. He ordered the destruction of all Christian churches, banned all meetings for worship and ordered that all copies of the scriptures and Christian liturgy should be burned. The following year, he decreed that any Christian who refused to make sacrifice to the Roman gods should be executed.

It is not known how many Christians died during the ‘Great Persecution’. Enforcement was very uneven, with Gaul and Italy being less affected than the East. Eusebius, Bishop of Caesarea, the most important city in Roman Palestine (its ruins lie near modern Caesarea between Tel Aviv and Haifa), claimed there were about 20,000 Christian martyrs. But writers at the time were always loose with numbers. And it was, of course, in Eusebius’s interests to exaggerate the number of ‘these truly astounding champions of pure religion’ – many martyrs make good advertisements for the power of their beliefs. Certainly Eusebius claims to have personally witnessed the miracles that took place in the arena during the damnatio ad bestias. This was a form of execution particularly appreciated by the crowd, in which the victim was tied naked to a pole and attacked by panthers, bears, wild boars or bulls who had been starved, or goaded with red-hot irons. ‘For some time the man-eaters did not dare to touch or even approach the bodies of God’s beloved,’ Eusebius recorded in his History of the Church, ‘as they stood naked and in accordance with their instructions waved their hand to attract the animals to themselves [but] were left quite unmolested: sometimes when the beasts did start towards them they were stopped short as if by some divine power, and retreated to their starting-point … so that in view of the ineffectiveness of the first, a second and third beast were set on to one and the same martyr.’³

But the martyrs notwithstanding, even Eusebius admitted that ‘numbers of men, women and children crowded up to the idols and sacrificed’. Mass apostasies took place all over the empire.

On 27 October 312, Constantine, the junior emperor in the eastern part of the empire, was marching with his troops to do battle with his rival for control of Rome. As they looked up at the sky, a light appeared with the message ‘In this sign conquer’. When he first recounted this event, Constantine would say that the sign had been sent by his guardian deity Apollo and the goddess Victory. Several years later he made a politically advantageous alteration: what he had seen etched against the sky was a vision of the cross. Or so Constantine told Eusebius, his friend and biographer. From that time on, Constantine became the protector and friend of the Christians. Perhaps he had seen that this once scapegoated group, the ‘enemy within’, held the key to military and political victory.

The following year, in 313, freedom of religion was enshrined in imperial legislation for the first time in the history of the Roman Empire. Constantine and his co-emperor, the pagan Licinius, announced that they were granting the Christians ‘free and unrestricted opportunity of religious worship’. On top of that they ‘also conceded to other religions the right of open and free observance of their worship, that each one may have the free opportunity to worship as he pleases’.⁴ The ‘Edict of Milan’, as it has become known, was issued, as both emperors acknowledged, ‘for the sake of the peace of our times’. Tolerance was always adopted by Muslim and Christian empires alike for pragmatic reasons, not moral ones.

Although Constantine called himself the thirteenth apostle and claimed his horse’s bridle was made with one of the nails which had crucified Christ, he was never a wholehearted Christian. He did not discontinue the practice of emperor worship and even considered himself to be divine. Indeed, he could not afford to antagonize the pagans who still formed the majority of his subjects. Polytheists were free, he said, to ‘celebrate the rites of an outmoded illusion’, ...