Just before the virus shut down the world, we took a trip to Germany, so my children could listen to my fascinating lectures about kingship in the early Middle Ages. On our way there, we stopped at Reims cathedral to have a look around. As we approached, one of my daughters with great excitement asked if those were statues of Apollo and Zeus et al, and when I told her they were Christian saints the crushing disappointment was palpable. Oh, it’s the boring religion again. Church and hymns and things like that.

And, of course, she was right. I felt the same at her age, forced to endure the endless, boring ordeal of church; even the words “Mass has ended, go in peace…” still fires up a little dopamine rush in my head. Christianity lacks the glamour of the old gods, but then the world of the old gods was a brutal place and we often forget how strange our society now is.

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Reims Cathedral played an important part in a story that began two thousand years ago one Friday, when the sun went down in an eastern province of the Roman Empire and the world was never the same again. A rabbi who had gathered a following in the troubled province of Judea had been condemned to death by crucifixion and suffered that agonising punishment reserved for the worst criminals.

On the Sunday, Jesus of Nazareth’s body had disappeared; soon his followers had, bizarrely, reported seeing him in the flesh and the cult grew and spread; official persecution, starting with the martyrdom of the Apostle Stephen, only made the group stronger.

Yet what might have remained one of many sects during this great period of religious fervour changed course two or three years after the death of Jesus when one of the men tasked with crushing the group, a Hellenised Jew called Saul, was struck blind while on the road to Damascus, and heard the voice: “Saul, Saul, why do you persecute me?”

These dramatic events would transform the world like no other. Saul, renamed Paul, used his position as a Roman citizen and a Greek-speaker to spread the faith across the eastern Mediterranean.

The impact of the faith, and of the Roman Catholic Church in particular, would revolutionise the way humans think, and how they related to each other, in ways that evolutionary biologists and psychologists are only now beginning to explain.

Like with many things, it is only now that we are losing Christianity that we are starting to appreciate its importance. This century began with the New Atheism, an overtly anti-religious movement spurred by the events of September 11; an array of books in the 2000s made the argument that “religion poisons everything”, in the words of Christopher Hitchens. They won, which was largely why New Atheism disappeared. As America moves past the 400th anniversary of its founding by English Calvinists, its religious exceptionalism has finally ended. Atheism is making headway everywhere.

Yet paradoxically, as the sea of faith has retreated, more public intellectuals now take it seriously. In the field of political philosophy Larry Siedentop’s Inventing the Individual repopularised the 19thcentury French idea that Christianity was the wellspring of individualism, and that contrary to the belief that Enlightenment liberalism was a revolt against the medieval Church, it was a product of it.

Christianity’s revolutionary transformation of western society was then brilliantly laid out in Tom Holland’s Dominion. Holland told the story of how the new religion had upended our worldview, tamed the sexual prowess of powerful men, sanctified the weak and given the poor kudos and pity. The abolition of slavery, the emancipation of women, even gay marriage and the current political radicalism emanating from US campuses, all were products of the Christian revolution.

In the field of evolutionary psychology, Jonathan Haidt’s The Righteous Mind explored the positive role that religion has on social bonding; Ara Norenzayan’s Big Gods looked at how deities became more moral and more universal as societies scaled up.

Joseph Henrich, the evolutionary biologist, is also interested in the way Christianity has affected and changed the western mind. When his The Weirdest People in the World came out last year, America was still in the grip of one of its periodic moments of moral convulsions. Tens of thousands of people were frenziedly protesting against the evil of racism, with some seen washing the feet of people they viewed as sanctified by race. It was an undeniably Christian event, even if its practitioners weren’t conscious of it; blessed are the meek, the first shall be last. George Floyd, another criminal put to death by the authorities, was transformed into a Christ-like figure on murals, along with the various taboos and blasphemies attached.

Over the centuries Christianity has become so dominant that we assume lots of things to be human nature when, as Henrich points out, they are very unusual in the greater scheme of human nature. Universal human rights are a western concept and to most societies completely unintelligible. “We think nepotism is wrong, and fetishize abstract principles over context, practicality, relationships and expediency,” he writes. People from countries ruled by the medieval Catholic Church, and countries founded by their offspring, are also “less inclined to distinguish in-groups from out-groups, more willing to help immigrants, and less firmly wedded to tradition and custom”.

At least since the mid-14th century, there has been an idea that medieval Christianity was a wrong turn for the West, that without monotheism polytheistic Greek-speakers might have sailed the stars, as Carl Sagan imagined. Yet as thrilling as ancient history is, and as remarkable as its achievements were, Roman society was very alien to us in a way that the medieval wasn’t.

A Roman father had total power of life and death over his children, and female infanticide was not unusual. Romans would happily watch an innocent man being torn to pieces by a wild animal for entertainment; slavery was ubiquitous and it was assumed that a man would sexually abuse his female slave; charitable giving outside the family was rare, and those who reached rock bottom in the imperial city would be left to literally die in the gutter.

This was Tom Holland’s realisation: that, although as a classicist he was fascinated by the Greeks and Romans, he couldn’t help but feel a revulsion about their worldview, one which almost completely lacked any pity for the weak. That was because he was a child of the great revolution that began on that Friday in Judea.

Like Holland, I previously imagined Christianity as being a fun-sucking succession of prohibitions, lacking the glamour and vitality of the ancient gods. I pictured ancient Valhalla as a sort of never-ending stag do, while the Christian Heaven appeared as an eerily quiet place where you sat having tea with some great-aunt who disapproved of everything. The Viking religion seemed like it was specifically designed by adolescent males, while Christianity was thought up by elderly women.

And there is a reason for that. Christ appeared that Sunday to his women followers, and, paradoxically for a religion that excluded women from senior roles, it was very female-dominated. Research from early Christian communities in Athens and Rome suggests that women may have outnumbered men by 5 or 6 to 1. The appeal of the new religion was that it domesticated men.

Men can be very idiotic creatures; our bodies start producing huge amounts of testosterone around the age of 14, which gives us the compulsive urge to fornicate or fight. Our brains don’t fully develop until 24 or 25 (and some of us don’t really grow up even after that). In that window between sexual and mental maturity the gender gap in every range of idiotic behaviour goes off the scale — car crashes, drink-related accidents, hospitalisation from violence.

Unlike the totalitarian quasi-religions of the modern era — including the one currently developing in America — Christianity had no naïve illusions about human nature. We are fallen creatures; young men in particular are barbarians who need to be tamed.

As Henrich notes, in bird species male testosterone rises as mating season begins and they prepare to fight other males. The world is cruel, and in both birds and mammals, males with higher testosterone levels tend to have more mates and offspring. Polygyny, the practice of a male having more than one mate, is also associated with higher testosterone, while monogamy comes with lower male testosterone — although in both cases the causal arrow goes both ways.

So while low-testosterone men are more likely to be monogamous, the state of monogamy lowers their testosterone. So does childcare. As men are drawn into family life their testosterone levels plummet faster than those of their single contemporaries; that’s why many middle-aged men become obsessed with exercise in order to naturally up their T-levels.

Divorce acts as a form of de facto polygyny if powerful men are able to produce offspring with multiple women. It was standard practice in Ancient Rome but Jesus of Nazareth forbade it, seeing it as desertion. His followers opposed infanticide, which was largely initiated by men against females; they also prohibited men from having sex with anyone but their wives. For women this had huge benefits; for low-status males it was an even bigger boon, producing a sort of sexual socialism, or at least sexual social democracy.

From the late second century, Rome was struck by a series of epidemics that fatally weakened the empire. The Plague of Cyprian in the 250s also had a huge cultural impact; all of a sudden Christianity, a minority religion practised by maybe 5% of the population, was everywhere. Traditionalists would have been stunned by the sudden arrival of Christian processions, of younger relatives adopting this strange new faith, worshipping a common criminal. It would have been popular among women in particular.

The western empire was already in ruins by the time that the fatal blow came with the Justinian Plague in the 6th century. That, combined with a war between the Goths and Byzantines and a freakish period of cold weather, left Rome a ruin of no more than 20,000 people and, like in many former Roman cities, the power vacuum had left the running of the city to the local bishop, who had long since adopted the title of Papa, Pope.

Catholic Christianity was not destined to conquer Europe; it might not even have triumphed over rival interpretations of the faith, such as Arianism. Its eventual dominance owed a lot to its adoption by one of the fiercest of the western tribes, the Franks. In 496 the Frankish leader Clovis, in the midst of a losing battle with a rival German tribe, the Alemanni, cried out to the Christian God who his wife followed; the battle turned in his favour, and in return Clovis was baptised in Reims Cathedral, along with his followers. And this was the fascinating tale I explained to my young children, who were obviously enthralled.

Clovis’s wife Clotilde was from Burgundy, and was already Catholic, but her husband her previously resisted her demands for him to adopt her religion. Now he was persuaded, and it began a close relationship for “the first daughter of the Church”, as France would become; Charles X would become the last of Clovis’s descendants to be crowned at Reims cathedral, in 1825.

Clovis’s conversion also set about a series of events that would transform Europe; in 800AD the Frankish king Charles came to the aide of the Pope in his conflict with yet another German tribe, the Lombards. When he arrived in Rome in triumph, the Pope placed a crown on his head and Charles the Great — Charlemagne — was named Emperor of the West. Victorian academic Sir James Bryce wrote of this event that “From that moment modern history begins”, but Charlemagne’s empire was less interesting for the usual violence than for the sex, or at least the lack of it.

As well as gathering around him a group of scholars who arguably rescued western Europe from the Dark Ages, Charlemagne’s greatest legacy was in enacting the Catholic Church’s marriage laws. No man was to have more than one wife, no one was to marry a relative. This would have a revolutionary impact on western society, setting us apart from 95% of global cultures. Without cousin marriage, clans grew weaker as people married out; they saw themselves less as members of a patrilinear line and more as wider members of society. Church rules on consent were also vital too; marriage was a sacred institution, and no one could now enter into it unwillingly.

Soon the almost unthinkable developed, the idea of marrying for love, a process known as the “Romeo and Juliet Revolution”. Adult sons and daughters were no longer subject to the patriarch but were individuals in their own rights; increasingly they would set up their own separate homes upon marriage. They were individuals.

These marriage rules were a way of taming men, in particular high-status men, and they brought huge benefits. Polygamous societies are racked by violence and turmoil, because large numbers of males are left without mates and inevitably cause trouble. The Vikings are the classic historical example, Scandinavia exporting its excess men to terrorise the British Isles, France and what is now Russia.

St Vladimir, who brought the Viking Rus into the faith, supposedly had 500 concubines before accepting Christianity and becoming an improbable saint. The Rus — the name probably means “rowers” — were Vikings who had sailed down the rivers leading to the Black Sea, creating a state that would eventually evolve into the world’s largest. The tenth century Arab chronicler Ibn Fadlan had spent some time with them, calling them “Allah’s filthiest creatures”, an adventure which ten centuries later would become one of the biggest box office bombs in history.

Ibn Fadlan recalled how at the funeral of one Rus warrior a slave woman was sacrificed, having first been drugged and forced to have sex with the dead man’s friends, passed around like meat before her murder. It was once thought that a lot of similar tales about the Vikings were Christian propaganda, but multiple examples of human sacrifice have now been uncovered in graves, including in the British Isles.

The Rus had been a nuisance in Constantinople — no more than 50 were allowed in at the same time, like a cornershop next to a troubled comprehensive — and it was left to the Byzantine princess Anna Porphyrogenita to convert them by marriage to Vladimir, a prospect which naturally horrified her.

Her barbarian husband had initially been attracted to Islam, which permitted polygamy — and had the future rulers of Russia been Muslim history would have been very different. It was said that he changed his mind when his ambassadors walked into the Hagia Sophia and felt they were in heaven; the alternative, and perhaps more plausible, explanation is that Vladimir then heard about Islam’s prohibition on alcohol, and lamented that the Rus could never live without drink (“Ivan, call those other guys back in”).

Some Vikings took longer to abandon the old ways; King Canute, whose grandfather Harold Bluetooth had adopted Christianity, still continued the old Danish tradition of a handfast, or second wife. But he was the last officially polygamous ruler of England.

Today we barely notice how unnatural our norms are in a world where traditionally powerful men behaved with the brutality of Caesar or the libidinousness of St Vladimir. That all changed one Friday on a hillside in Judea. I sometimes wonder, though, what we will lose as church attendance continues to slide and, like Romans of the 3rd century, we see our gods dying around us.

America has seen one of the most rapid de-Christianisations in the past two decades and the results so far are not good. What is left is not the rationalist paradise some naïve public philosophers were claiming at the start of the 21st century, but a sort of distilled Christianity, which without the supernatural elements is far less rational: and so what results is endless moral panics, a world seen in stark black and white between good and evil, competitive sanctimony and the sentimental glorification of victimhood in which everyone wants the glory of being on the Cross.