How did a persecuted sect become the empire’s religion?

Pliny the Younger, governing Bithynia, tells Trajan he has never attended a trial of Christians and does not know what is normally punished or how far it goes. He describes asking the accused three times whether they were Christians and executing those who persisted, on the grounds that the obstinacy itself deserved punishment.

Two things are visible in the letter. There was no settled imperial procedure, which means this was not yet a priority. And there were enough of them in one province to create a procedural problem, including people who said they had left the movement twenty years earlier.

Lactantius preserves the letter Licinius had published at Nicomedia in June 313, setting out what he and Constantine had settled at Milan. It states that "the Christians and all others should have liberty to follow that mode of religion which to each of them appeared best", and orders that nobody be denied leave to attach themselves to the rites of the Christians "or to whatever other religion his mind directed him".

It is a general toleration, not an establishment, though much of it goes on restoring to the Christians the meeting places and church property taken from them. Nor was it the first: in 311 Galerius, one of the emperors who had persecuted them since 303, had already allowed them "again to be Christians", provided they did not offend against good order.

Twelve years after Milan, Constantine summoned bishops from all quarters to Nicaea, gave some the use of the public post and supplied others with horses to get them there, and had the council settle a doctrinal dispute. Whatever his personal convictions were, the relationship had inverted: the state was now using the church’s structures and underwriting its decisions.

Formal establishment came later still. In 380 an edict of Gratian, Valentinian and Theodosius ordered every people under their rule into one version of the faith, branded the rest heretics, and threatened them with punishment. The decisive shift is this middle period, where an association that had been illegal a few years earlier became an instrument of government.

No census counted Christians. The figures come from lists of communities that happen to be named in surviving texts, from what a house meeting could hold, and from growth curves fitted between a guess at the start and a benchmark for 300. The usual benchmark is about a tenth of the empire, roughly six million. Harnack, who assembled the evidence for it, thought the density varied so much between provinces that a single figure was useless: close to half the population in Asia Minor, negligible in Gaul and Germany.

That uncertainty matters, because the two standard stories need different numbers. If the movement was already large by 300, imperial favour ratified a change that had happened. If it was small, imperial favour caused one.

The explanations that recur are hard to weigh against each other. A bishop of Alexandria, writing during a third-century plague, says Christians nursed the sick and buried the dead while their neighbours cast the half dead into the street, though he was describing his own people to his own people. That women and slaves were admitted on terms other cults did not offer is asserted more often than it is shown, and Hopkins reads the ancient remarks about them as a literary cliché rather than observation. Exclusivity is the firmer point. A convert to a religion that tolerated no others was subtracted from every cult at once, which compounds over generations in a way an occasional visit to a strange god does not.

Against that, persecution was real and episodic, and the same sources that report growth report apostasy under pressure. What the mix actually was is not recoverable from the evidence that survives.

He extended toleration to Christianity, funded it, convened its bishops, and was baptised in his last illness outside Nicomedia. He also accepted the office of chief priest of the old state cult, as the pagan historian Zosimus records while calling his Christianity a seduction from the path of rectitude, and went on striking the Sol Invictus coinage he had begun in 310 for about a decade, well past Milan.

Readings of this run from genuine conviction expressed in the political vocabulary available to him, to a calculation about which network could hold an empire together. The sources are mostly by Christians writing about their benefactor.

A movement whose earliest documents were written by and for a persecuted minority became, within a lifetime of its last great persecution, the religion of the state that had persecuted it. Whether something was lost in that, and what, is asked inside Christianity as well as outside it, and it is not a question evidence answers.

The providential reading is as old as the events and is on the record: Eusebius, writing the emperor’s life, presents the turn as God’s doing rather than as politics. The reading of it as a compromise is on the record too, from inside the church: Jerome, writing about 391, planned a history of the church that would show how, "after reaching the Christian Emperors", it "increased in influence and in wealth but decreased in Christian virtues".

Eusebius records, as praise, that Constantine’s liberality "was most especially exercised on behalf of the churches of God". Jerome’s regret is over the wealth that came after.