How the Church Took Over the Empire

In the year 30, or thereabouts, a Galilean teacher was executed on a Roman cross outside Jerusalem, and his movement should have died with him. It had no army, no wealth, and no political standing. Its founder had written nothing and built nothing. Its followers numbered, at most, a few hundred people, in one corner of one province of an empire that stretched from Britain to the Euphrates.¹ Rome had crushed messianic hopefuls before and would again; the sensible bet, in the year 30, was that no one outside Judea would ever hear the man’s name.

Three centuries later the emperor himself was a Christian—and, some fifty years on, an emperor who set out to reverse the whole thing concluded that he could not defeat it, only imitate it. Within a lifetime the faith of the crucified teacher was the majority religion of the Mediterranean world. That is one of the most improbable outcomes in ancient history, and it invites a simple question: how? The honest account is worth the trouble, because it is far more interesting—and far more durable—than the overstatements that usually crowd it out.

The arithmetic that defuses the miracle

Start with the surprising thing about the numbers: they do not require anything miraculous, forced, or even unusual. They require only steady, ordinary, person-to-person growth—kept up for a very long time.

The most influential attempt to model the growth is the sociologist Rodney Stark’s, in The Rise of Christianity (1996). His method is deliberately plain. Assume the church grew at a constant compound rate, anchor it to a starting figure and an endpoint historians roughly accept, and see what the exponential curve produces. Begin with about a thousand Christians in the year 40 and let the movement grow at 40 per cent per decade—a little under three and a half per cent a year—and you reach roughly 7,500 by the year 100, about 218,000 by 200, and about 6.3 million by the year 300: something over a tenth of an empire of perhaps sixty million.²

The force of the model is what it rules out. Forty per cent a decade is not a stampede; it is what you get if, over a decade, roughly two or three believers together bring in one new person. There is no need for mass conversions or coercion—neither of which existed in this period anyway. Stark drives the point home by noting that his rate is close to the documented growth of the Church of Jesus Christ of Latter-day Saints across the twentieth century.³ That comparison cuts both ways: it makes the ancient figures credible, and it concedes that an exponential curve of this shape is not unique to Christianity, nor by itself evidence of anything supernatural.

Two cautions keep this from becoming a conjuring trick. First, the table is a model, not a census, and Stark said so plainly; there was never any count of Christians in the empire, and every figure in every book is an estimate built from fragments. Second, the scholarly agreement is thinner than it can look. Keith Hopkins, often cited as independent confirmation, is not: he takes Stark’s arithmetic as his starting point.⁴ Bart Ehrman, working the problem more recently, puts the year-300 figure closer to two and a half or three million—less than half Stark’s—and warns that the rate cannot simply be extended, since running it on to the year 400 yields more Christians than the empire had inhabitants.⁵ So the honest summary is a range—roughly two and a half to six million by 300, unevenly spread and concentrated in the cities. What is not in dispute is the shape: a standing start and steep, sustained growth, achieved while the faith was illegal. That is astonishing enough without inflating it.

What the growth looked like from the outside

The best evidence that this was happening comes not from Christians, who had every reason to advertise their success, but from officials and critics who found the movement irritating, contemptible, or alarming—and who described it anyway.

Around AD 112, Pliny the Younger, governing Bithynia-Pontus, wrote to the emperor Trajan for guidance on prosecuting Christians. His letter is the earliest surviving administrative description of the church, and his alarm is that of a manager, not a theologian: the “contagion of this superstition,” he reports, has spread not only through the cities but through the villages and countryside, reaching people of every age and rank and both sexes, so that the temples had stood nearly deserted before his crackdown began.⁶ A governor has an interest in magnifying the problem he is solving. But Pliny also reports many recanting under pressure—not what a man exaggerating his triumph would invent—which tells us the numbers he saw were real.

Half a century later the satirist Lucian of Samosata, who thought Christians credulous fools, described them swarming to support a jailed leader: bribing the guards to sit with him, sending delegates at collective expense from the cities of Asia, sparing no trouble and no money—which is precisely why, he sneers, any competent charlatan could grow rich among them.⁷ The contempt is what makes the testimony valuable. Lucian is not praising the behaviour but explaining how exploitable it made them—and in the process he confirms, from outside and a century and a half before Constantine, that the movement was organised, generous, and spreading.

And from squarely inside the illegal period there is a hard administrative number. Writing about 251, Cornelius, bishop of Rome, listed his church’s establishment for a colleague: forty-six presbyters, seven deacons, and more than fifteen hundred widows and persons in distress supported by the congregation.⁸ That is a relief roll, not a boast—a single illegal congregation in one city already carrying fifteen hundred dependants. Whatever else the church was by the mid-third century, it was large, structured, and pouring its resources into the poor.

Why it grew

So what, concretely, drew people in? No single cause explains three centuries across a whole empire, and one honest caveat belongs up front: the most serious dissenting historian, Ramsay MacMullen, argued that conversion in this period turned less on charity than on public demonstrations of power—healings and exorcisms a pagan could read as a straight contest between gods.⁹ His is a rival account of the mechanism—one that discounts charity in favour of displays of power—and it is noted, not answered here; but it is no help to the Constantine objection, because whatever exorcism was, it was not imperial power. Three things, then, recur across the sources.

Care for the sick, at real cost

Twice the empire was gutted by epidemics—the Antonine plague from about 165, and the plague named for Cyprian from about 249—and ancient cities had no capacity for mass nursing. Bishop Dionysius of Alexandria, writing of the second outbreak in a letter preserved by the historian Eusebius, describes Christians staying with the dying, taking their care upon themselves, and perishing in numbers as a result, while frightened neighbours abandoned their own.¹⁰ It is a Christian source describing Christian conduct, and should be weighed as such—though flight from plague was ordinary enough that pagan sources record it too, so the contrast is not invented. Stark’s much-quoted claim is that this was not merely admirable but demographically consequential: even elementary nursing can cut mortality sharply, so tended communities lost fewer of their own and won the loyalty of the strangers they had nursed.¹¹ That model rests on modern estimates applied to diseases we cannot securely identify, so it is a plausible argument rather than a proven result. But the behaviour is well attested, and it is the sort of thing that changes how a neighbour thinks.

A wider door than anyone else offered

Leaving an unwanted newborn outdoors to die or be taken was legal and unremarkable in the Greco-Roman world. Christian texts forbid it from their earliest layer: the Didache bans abortion and the killing of a newborn in the same breath as murder, and the Epistle to Diognetus singles out Christians as people who “marry, as do all… but they do not destroy their offspring.”¹² The same communities admitted, without a bar of status, the categories the surrounding cults ranked and rationed—slaves, the poor, women, and non-Greeks—so that a slave and a matron could belong to the same congregation on the same terms. This point is easy to overstate, and careful specialists such as Lynn Cohick have shown that Greco-Roman women had wider social and economic scope than the older caricature allowed.¹³ So the defensible claim is the narrower, sturdier one: Christian communities were unusually mixed, and membership was not priced by rank. To people the ancient order treated as disposable, that was an astonishing offer.

Networks, not spectacle

Stark’s most durable contribution is sociological rather than statistical: new religious movements grow overwhelmingly along existing relationships—family, household, trade, friendship—rather than through public preaching to strangers.¹⁴ The book of Acts shows exactly this, converting whole households at a stroke—Cornelius, Lydia, the Philippian jailer, Crispus—alongside the public preaching it also records. It is an unglamorous mechanism, and it is precisely what produces the steady compounding the arithmetic describes: not a wave, but a web, thickening decade by decade—the faith spreading the way trust spreads, one relationship at a time.

The Constantine objection

The standard sceptical reply deserves to be stated at its strongest, because in one respect it is right: the real cause of Christianity’s success, it says, is not the mechanisms above but a single political event. In 312 Constantine won at the Milvian Bridge and credited the Christian God; in 313 he and Licinius agreed the settlement conventionally called the Edict of Milan, ending persecution; imperial patronage followed; and in 380 Theodosius made Nicene Christianity the empire’s official religion, with the coercive power of the state behind it.¹⁵ On this telling, Christianity did not win the Roman world—it was handed it.

The concession owed here is substantial. Everything after 312 is entangled with state power, and the church’s growth after 380 with the legal suppression of rival worship. The steepest part of the curve in raw numbers is the fourth century; anyone who points to the Christian majority of AD 400 as proof of the gospel’s persuasive force is trading on a period when the state’s thumb was on the scale.

But the objection explains the wrong stretch of the curve, and three facts sit entirely outside the window the state can cover. Pliny’s letter and Lucian’s satire describe a spreading, organised movement in the second century, when no emperor favoured it. Cornelius’s relief roll is an administrative document from an illegal church in 251. And—this is decisive—the state’s own attempts to destroy the church had already failed with the full machinery of empire engaged. Decius’s universal sacrifice order of 250, Valerian’s edicts of 257 and 258, and Diocletian’s persecution from 303 were all spent, and the last was formally abandoned by Galerius in the edict of toleration of April 311—a full year before the Milvian Bridge.¹⁶ The state conceded first. By the time Constantine acted, the church already reached into the army and the palace and numbered in the millions; his patronage recognised and accelerated a movement already too big to suppress. The Constantine story does not explain the growth—it reverses cause and effect.

The witness of a hostile emperor

The sharpest testimony to what had driven that growth comes from the one man with the most reason to deny it—and its date has to be stated rather than blurred, because it is not evidence from the illegal centuries. It comes from fifty years after Constantine. In 362 the emperor Julian—raised Christian, and the last emperor to abandon the faith—set out to restore the old gods, and found he could not. Writing to a pagan priest, he diagnosed why, and the diagnosis is an unwilling tribute: it was that the Christians simply out-behaved the pagans. When the pagan priesthood neglected the poor, Julian observed, “the impious Galilaeans”—his term for Christians—noticed and devoted themselves to philanthropy; it was their care for strangers and for the poor, he complained, that had done most to advance their cause.¹⁷ His remedy was telling: he ordered his own pagan priesthood to fund and imitate the very charity he could not defeat.¹⁸

Julian is therefore not a witness to why the church grew in the second and third centuries—he is far too late for that, and it must not be pretended otherwise. But he is a witness to something worth having: an emperor holding the whole apparatus of the state, and using it against the church, judged that the thing he had to defeat was Christian charity, and that his best hope was to copy it. When your fiercest opponent, with an empire at his disposal, concludes that his surest course is to imitate you, he has told you where the movement’s real strength lay.

What the growth shows—and where the truth of it rests

Two things follow, and they should be kept distinct. The rapid, illegal, person-to-person spread of the church shows that the faith was profoundly attractive and that its people were utterly convinced—convinced enough to nurse plague victims at the cost of their lives, to take in the children a whole civilisation discarded, and to keep confessing the name when a pinch of incense would have bought their safety. The evidence for it is strong, external, and honest.

But it is not, by itself, proof that the gospel is true, and this essay does not offer it as one. Movements can spread quickly and still be mistaken; popularity is not a truth-detector. The case that Jesus actually rose from the dead rests on different evidence—above all the eyewitness testimony behind the early creed Paul hands on in 1 Corinthians 15:3–7, datable to within a very few years of the events—and that case is made at length elsewhere in this Library. What the growth does do is exactly what you would expect if that resurrection claim were true and its first witnesses sincere: a movement that behaved as though it had genuinely met the risen Christ, that could not be bought off or stamped out, and that changed the people it touched.

So the honest takeaway is not a boast but a recognition. The behaviours that carried the church across the empire—staying with the sick, rescuing the abandoned, feeding somebody else’s poor—were not clever strategies for winning an argument. They were what Jesus had told his followers to do, done at cost, by people who mostly left no names, because they believed he had been raised and would one day ask what they had done with it. A tiny, powerless, illegal movement outlasted the empire that tried to kill it—not by force, which it never had, but by love, which it could not stop giving away. That is a fact of history that still wants explaining, and a good place for a Christian to stand.