When I was a boy, my upbringing as a Christian was forever being weathered by the gale force of my enthusiasms. First, there were dinosaurs. I vividly remember my shock when, at Sunday school one day, I opened a children’s Bible and found an illustration on its first page of Adam and Eve with a brachiosaur. Six years old I may have been, but of one thing – to my regret – I was rock-solid certain: no human being had ever seen a sauropod. That the teacher seemed not to care about this error only compounded my sense of outrage and bewilderment. A faint shadow of doubt, for the first time, had been brought to darken my Christian faith.

With time, it darkened further still. My obsession with dinosaurs – glamorous, ­ferocious, extinct – evolved seamlessly into an obsession with ancient empires. When I read the Bible, the focus of my fascination was less the children of Israel or Jesus and his disciples than their adversaries: the Egyptians, the Assyrians, the Romans. In a similar manner, although I vaguely continued to believe in God, I found Him infinitely less charismatic than my favourite Olympians: Apollo, Athena, Dionysus. Rather than lay down laws and condemn other deities as demons, they preferred to enjoy themselves. And if they were vain, selfish and cruel, that only served to endow them with the allure of rock stars.

By the time I came to read Edward Gibbon and the other great writers of the Enlightenment, I was more than ready to accept their interpretation of history: that the triumph of Christianity had ushered in an “age of superstition and credulity”, and that modernity was founded on the dusting down of long-forgotten classical values. My childhood instinct to think of the biblical God as the po-faced enemy of liberty and fun was rationalised. The defeat of paganism had ushered in the reign of Nobodaddy, and of all the crusaders, inquisitors and black-hatted puritans who had served as his acolytes. Colour and excitement had been drained from the world. “Thou hast conquered, O pale Galilean,” Swinburne wrote, echoing the apocryphal lament of Julian the Apostate, the last pagan emperor of Rome. “The world has grown grey from thy breath.” Instinctively, I agreed.

So, perhaps it was no surprise that I should have continued to cherish classical antiquity as the period that most stirred and inspired me. When I came to write my first work of history, Rubicon, I chose a subject that had been particularly close to the hearts of the philosophes: the age of Cicero. The theme of my second, Persian Fire, was one that even in the 21st century was serving Hollywood, as it had served Montaigne and Byron, as an archetype of the triumph of liberty over despotism: the Persian invasions of Greece.

The years I spent writing these studies of the classical world – living intimately in the company of Leonidas and of Julius Caesar, of the hoplites who had died at Thermopylae and of the legionaries who had triumphed at Alesia – only confirmed me in my fascination: for Sparta and Rome, even when subjected to the minutest historical inquiry, did not cease to seem possessed of the qualities of an apex predator. They continued to stalk my imaginings as they had always done – like a tyrannosaur.

Yet giant carnivores, however wondrous, are by their nature terrifying. The longer I spent immersed in the study of classical antiquity, the more alien and unsettling I came to find it. The values of Leonidas, whose people had practised a peculiarly murderous form of eugenics, and trained their young to kill uppity Untermenschen by night, were nothing that I recognised as my own; nor were those of Caesar, who was reported to have killed a million Gauls and enslaved a million more. It was not just the extremes of callousness that I came to find shocking, but the lack of a sense that the poor or the weak might have any intrinsic value. As such, the founding conviction of the Enlightenment – that it owed nothing to the faith into which most of its greatest figures had been born – increasingly came to seem to me unsustainable.

“Every sensible man,” Voltaire wrote, “every honourable man, must hold the Christian sect in horror.” Rather than acknowledge that his ethical principles might owe anything to Christianity, he preferred to derive them from a range of other sources – not just classical literature, but Chinese philosophy and his own powers of reason. Yet Voltaire, in his concern for the weak and ­oppressed, was marked more enduringly by the stamp of biblical ethics than he cared to admit. His defiance of the Christian God, in a paradox that was certainly not unique to him, drew on motivations that were, in part at least, recognisably Christian.

“We preach Christ crucified,” St Paul declared, “unto the Jews a stumbling block, and unto the Greeks foolishness.” He was right. Nothing could have run more counter to the most profoundly held assumptions of Paul’s contemporaries – Jews, or Greeks, or Romans. The notion that a god might have suffered torture and death on a cross was so shocking as to appear repulsive. Familiarity with the biblical narrative of the Crucifixion has dulled our sense of just how completely novel a deity Christ was. In the ancient world, it was the role of gods who laid claim to ruling the universe to uphold its order by inflicting punishment – not to suffer it themselves.

Today, even as belief in God fades across the West, the countries that were once collectively known as Christendom continue to bear the stamp of the two-millennia-old revolution that Christianity represents. It is the principal reason why, by and large, most of us who live in post-Christian societies still take for granted that it is nobler to suffer than to inflict suffering. It is why we generally assume that every human life is of equal value. In my morals and ethics, I have learned to accept that I am not Greek or Roman at all, but thoroughly and proudly Christian.

Tom Holland’s most recent book, “Dynasty: the Rise and Fall of the House of Caesar”, is published by Abacus