In 1074, a monk from the Benedictine Abbey of Cluny, in Burgundy, crossed the frontier of Christendom and entered Muslim Spain. Arriving in the infidel capital of Córdoba, he offered to walk through fire if only his audience would agree to abandon superstition and embrace the Christian faith. The Muslim king and his courtiers contemptuously ducked his challenge. Indeed, such was their scorn that they refused even to martyr him. The indignant monk was left with little choice but to "shake the dust from his feet, turn on his heels, and set off back home".

This was the de facto limit of multicultural dialogue in the Middle Ages. Just as Muslims tended to regard the civilisation of Latin Christendom with an invincible lack of interest, so did most medieval Christians find it impossible to conceive of Islam as a faith distinct and separate from their own. The conviction of the monk from Cluny that a spot of fire-walking might be sufficient to convert the Saracens reflected his deep-rooted presumption that their religion was no more self-sufficient and autonomous than a Christian heresy. Give them a little lecture, throw in a miracle, and they were bound to see the error of their ways.

Ever since Erasmus, the medieval mindset has been generating snorts of derision. No period in history is more subject to what E P Thompson termed "the enormous condescension of posterity" than the Middle Ages. The very term serves to condemn it: for the notion of a "Middle Age" derives from the determination of Renaissance humanists to cast themselves as the heirs of classical civilisation, and to dismiss everything in between as mere barbarism and backwardness. Such a conceit, if recent letters to the New Statesman are anything to go by, is still strongly maintained today. "The modern secular movement", it is argued, "which was interrupted by the age of faith in the Middle Ages", is part of a continuum which reaches back to "the early humanists" of the ancient past. As a classicist, I can only applaud such enthusiasm for the achievements of antiquity; as a historian, however, I do have some reservations.

Secularists scrabble around, hunting for any heritage so long as it is not Christian. They protest too much

Look again, for instance, at that Cluniac monk. Is he really so different from a member of the Humanist Association? In many ways, yes, of course - but in the manner, perhaps, that a theropod dinosaur is different from an ostrich. Evolution notwithstanding, the line of descent is clear enough. Monk and humanist alike are convinced that their respective belief systems embody the only sane way of interpreting the universe; both feel that it is a moral imperative to encourage everyone else to agree with them; both lay claim to a universalism that is in fact culturally highly specific. No less than the medieval Christian Church, Europe's post-Christian elite operates secure in the conviction that it has attained an enviably superior state of enlightenment, one that aspires to enfold within its embrace all other possible ways of seeing the world, and to neutralise all rival claims. Seeing secularism in that light, the tolerance that it extends to Islam is merely the mirror image of the Cluniacs' militant disdain: an expression of the complacency to which all powerful civilisations are, by their nature, prone.

To argue this is not, as a second correspondent complained, to slap atheist faces with a wet fish just for the sake of it. Rather, it is to make the point - permissible in a historian of the ancient and medieval worlds, surely? - that the origins of much that seems most modern to us can in fact be traced back to the distant past. Neutrality between different religions, as it is practised in Europe today, can never itself be culturally neutral, for the simple reason that it depends on a philosophy that is ultimately Christian in character. That the world can be divided into church and state, and that these twin realms should exist distinct from each other: here are presumptions with which many Muslims, for instance, would disagree profoundly. Certainly, there is nothing in the Quran equivalent to the New Testament injunction to render unto Caesar what is Caesar's. Muhammad, unlike Jesus, had neither the slightest hesitation in formulating a fiscal policy nor in laying claim to political authority. For those who imagine that the western model of the multicultural state can emasculate Islam as readily as it has de-fanged Christianity, this should be a detail of more than merely theological or antiquarian interest.

Yet many secularists are still determined to regard all religions as being essentially the same and to deny the glaring fact of their own descent from a specific religious tradition. Hence their scrabbling around for Greek, Roman, even Indian and Chinese progenitors - any heritage, it would seem, just so long as it is not Christian. They protest too much. To recognise the Christian roots of modern-day secularism is no more to accept the doctrinal truth or otherwise of Christianity than an acknowledgement of our cultural debt to ancient Greece is an obligation to set about worshipping Zeus. So much seems to me self-evident - and leads me to wonder whether there might not be, in the reluctance of so many secularists to trace their ethical and sociological presumptions back to Christian origins, something of Bishop Wilberforce's horror at the notion that he might be descended from an ape.

The western tradition of self-examination, of self-questioning, of self-doubt is indeed a precious one; but we can hardly afford to shrink from applying our own standards to ourselves. "Everything must be examined, everything must be shaken up, without exception and without circumspection." So wrote Diderot, that über-philosophe. If the question of what a supposedly post-Christian Europe owes to its Christian past is one that makes many enthusiasts for the Enlightenment uncomfortable, all the more reason, I would argue, for staring it in the face.

Tom Holland's "Millennium: the End of the World and the Forging of Christendom" is published by Little, Brown (£25). http://www.newstatesman.com/ideas/2008/10/europe-christian-e...