In 312 CE, Constantine had a vision of the cross as the emblem that would lead him to military victory: “In hoc signo, vinces!” (“With this sign, you will win!”) The story may be fake history: it is told only much later, in contradictory forms, by two authors with their own Christian axes to grind, and Constantine was probably never as fully and exclusively committed to the faith as the later hagiographers suggest. But officially, he did become the first Roman emperor to convert to Christianity. A small Jewish cult that had mostly been ignored by the ruling Roman elite had acquired the attention of the most powerful man in the world. A few decades later, in 380 CE, Theodosius made Christianity the sole authorised religion of the Roman empire.

Since Christianity was the historical winner against Roman polytheism, non-specialists often have a distorted impression of how the new religion took cultural hold. There is a series of Wikipedia articles about the “persecutions of the Catholic Church”, including an extensive account of Christian martyrdoms under Rome; but there is no corresponding series about the often violent suppression of the earlier Roman religion by the Church. As Catherine Nixey points out in her vivid and important new book, the idea of the widespread persecution of Christians is a product of the Church’s marketing and recruitment techniques.

Like the later suicide bombers, Christians were often eager for a public death. As Nixey notes, “This was a glory that was open to all, regardless of rank, education, wealth or sex.” But the execution of Christians for religious nonconformity was extremely rare, since most Roman rulers were smart enough to realise that there is nothing to be gained from making religious extremists into heroes. In 111 CE, Emperor Trajan insisted in a letter to Pliny, the governor of Bithynia, that he should punish only the most recalcitrant rebels; anyone willing to offer prayers to “our gods” could be pardoned, “however suspect his past conduct may be”.

Christians could be seen as a threat to the Romans’ ancestral traditions as well as their structures of political power, since they refused to worship in the ways favoured by the majority of their neighbours. But Romans such as Trajan and Pliny had no interest in conducting a searching investigation of their motives. “They must not be hunted,” insisted Trajan – who had more important things to do than worry about the minds of a few oddballs.

As long as they could perform a minimum of the normal rituals, they were unobjectionable. Traditional Roman religion was a capacious ragbag of beliefs and practices, which could accommodate the worship of many different deities – Romanised versions of the old Greek Olympians (such as Zeus/Jupiter, or Ares/Mars) alongside multicultural additions (the Egyptian Osiris), as well as more utilitarian entities (Fortuna) and deified emperors. Religious correctness was in many ways a matter of practice or etiquette rather than belief (orthopraxy, not orthodoxy).

In Nixey’s narrative, the new dominion of Christianity brought about the end of this largely tolerant society, since everybody was now forced to conform to the faith and its social, sexual, cultural and familial practices. Some prominent adherents to the old ways begged the authorities to tolerate religious difference in the newly Christianised empire. Nixey cites the orator Symmachus, who pleaded with the emperor Gratian to allow the Altar of Victory to remain in the Senate House of Rome in 382 CE. “Each person has their own custom, their own religious rite,” Symmachus argued, and asked, “What does it matter what wisdom a person uses to seek for the truth?” But his pleas were ignored. The altar was ripped out and, in 408 CE, a new law came into effect that all altars and images to the old gods must be destroyed.

The language of “persecution” and “martyrdom” has been claimed by only one side. Yet there were at least a handful of non-Christians who were persecuted and martyred for their refusal to adopt the new religion. Unlike Trajan and other polytheist authorities, the Christians did not offer their opponents an opportunity to escape punishment with a quick prayer to the correct God. Instead, they probed their homes and even their minds in search of secret sins against the one true deity.

Nixey tells the story of the sainted Egyptian monk Shenoute, who led a group of his fellow Christians to batter down the door of a citizen’s house and barge in to discover his forbidden statues of the old pagan gods. Breaking and entering was, Shenoute insisted, entirely justifiable, since: “There is no crime for those who have Christ.”

Violations of what we would now call human rights and civil liberties were allowed for the sake of religious conformity. In Alexandria in 415 CE, the philosopher and teacher Hypatia was mobbed, stoned, flayed, ripped to pieces and burned by a gang of Christians, who accused her of witchcraft. Classical learning, literature and philosophy were now all suspect. Being pious in the new faith meant not only participating in public religious practice but also a moulding of hearts, minds, art, architecture and reading matter to fit the new “reality”.

Nixey emphasises above all the aesthetic and cultural violence of the shift from Roman paganism to Christianity. She writes somewhat predictably of the turn away from the relatively “body positive” world of antiquity, in which privileged elite men could wine and dine on imported luxury goods and enjoy a wide range of sexual activities with objectified women and boys, to the asceticism of late antiquity, in which the most pious monks and hermits deprived themselves of food, sex and washing, and often became obsessed with all three.

Nixey says nothing about whether any of the slaves, prostitutes, workmen and wives who catered to the pleasures of the Roman rich man might have been glad to move to a world where self-restraint took on new value. Not eating, not washing and not having sex are, like dying in agony, forms of heroism that are open to people from any walk of life; in that sense, Christianity provided certain advantages and correctives to the hierarchies and inequalities of the later Roman empire. It may have felt to some people like a way to drain the swamp.

Nixey’s story is more shocking when she describes the widespread destruction of antiquities. The vandalism evoked in this book – such as the demolition of the temple of Athena at Palmyra, one of the most impressive buildings in the world – is disturbingly reminiscent of the destruction of cultural heritage sites by Islamic State, although Nixey does not make the comparison. Radical Christian terrorism has a long history. As the Roman Epicurean poet Lucretius wrote in the first century BCE, “Tantum religio potuit suadere malorum.” (“Religion has persuaded people to so much evil.”)

Why did the Roman empire become Christian? What caused this violence, pain and upheaval, when polytheism had apparently been working perfectly well for many centuries? Constantine, an ambitious, militaristic and ruthless emperor who had his wife and eldest son put to death in order to instil terror and consolidate his power, may not have been motivated only by religious piety. For those clutching at power over a fractious and unwieldy empire, the imposition of the new religion may have seemed like a welcome opportunity to brand diverse populations with their own cultural authority, united under one emperor and one God.

On the other hand, the move to a more private, more personal form of religion had a different psychological appeal for a population with little political or economic power and little reason to feel grateful to the old gods of Rome or the power structures that they were used to support. Religious extremism and religious obsession often serve as useful distractions and comforts for a disenfranchised people hoping to deny their own unimportance.

Nixey’s central aim is not to explain or analyse the adoption of Christianity as the state religion of the late Roman empire. She offers almost no discussion of the political, demographic, social, psychological and economic causes that might have led rulers and their subjects towards this vast and violent change, although she does hint at the sheer loutish thrill of knocking down big, expensive buildings and defacing sublime works of highbrow art.

Nixey is a funny, lively, readable guide through this dark world of religious oppression. She wisely insists at the start of her book that this account of cultural violence should not be read as an attack on those who are “impelled by their Christian faith to do many, many good things”. It is instead a reminder that “monotheism” (or, one could say, religion in general and Christianity in particular) can be used for “terrible ends”. The book is also an essential reminder, in the age of Brexit and Donald Trump, that intolerance, ignorance and hostility to cultural diversity are sadly nothing new.

Emily Wilson’s translation of “The Odyssey” is published by WW Norton in December

The Darkening Age: The Christian Destruction of the Classical World

Catherine Nixey

Macmillan, 305pp, £20