As the first decade of the third Christian millennium draws to an increasingly troubled close, the verdict of historians on its significance can already be anticipated. Two themes will predominate. The first, exemplified by the present carnage in the financial markets, will be the quickening of the west's decline relative to China and India; the second, not entirely coincidentally, will be the tensions in the relationship between the west and the Muslim world.
A grim irony, that so many of the defining crises of the 21st century should have emerged from a swirl of identities and misunderstandings that reach back ultimately to a distant, medieval past. The attacks of 11 September 2001; the presence in Iraq and Afghanistan of what Osama Bin Laden is certainly not alone in describing as "crusaders"; the rise of anti-immigrant, and specifically anti-Muslim, feeling across Europe: all have combined to foster an agonised consciousness that history might be a nightmare from which we have not, after all, woken up.
And still the resulting culture wars rumble on, heard even above the din of crashing banks. In London, the Islington offices of Gibson Square books (which also serve as the publisher's home) were firebombed by Muslim radicals; in Austria, the next government may contain a party pledged to ban the building of minarets. If the banking system is being menaced by a drying up of credit, then the prospects for multi cultural harmony in Europe appear no less threatened by a dialogue crunch. All too often, people of rival convictions are simply refusing to listen to one another. Even attempts to set up frameworks within which conversations might be held are beset with difficulties. As well they might be, for every attempt to fashion Europe's future seems to stir up any number of ghosts from its distant past.
It might have been thought timely, for instance, that 2008 was designated by the European Union as its official Year of Intercultural Dialogue. Yet the entire jamboree is proving worse than a damp squib. Among the tiny minority who are so much as conscious of its existence, there has been much resentment that the organisers should have sought to promote dialogue that was not merely "intercultural", but "interfaith" as well: as though the truest determinant of identity must ultimately be religious. So for every African or Middle Eastern leader invited to address the European Parliament, there has been a host of what one indignant Swedish Green described as "old men in dresses": an assortment of muftis, patriarchs and lamas. That such a guest list should have provoked indignation is hardly surprising. After all, the con viction that the religious and political spheres should be rigorously ring-fenced - and even more rigorously patrolled - has widespread support in Brussels. As one group of MEPs protested, in an official letter of complaint to the president of the European Parliament: "The EU is of a secular and neutral nature."
It is an opinion, ironically enough, that would not be disputed by the most unyielding and formidable religious leader that Europe has. To Pope Benedict XVI, however, the EU's claim to an identity that transcends religion, whether as an honest broker between rival faiths, or as an institution that should have nothing to do with such faiths at all, is hardly a positive. "Is it not surprising," he demanded last year, in an address given to mark the 50th anniversary of the signing of the Treaty of Rome, "that today's Europe, while hoping to be seen as a community of values, more and more seems to contest that universal and absolute values exist?"
Its militant secularism, in the opinion of the papacy, is doubly a betrayal: first, of the undoubted fact that many of the founding fathers of the European project, men such as Konrad Adenauer or Robert Schuman, were devoutly Catholic; and second, and more profoundly, of the continent's one-time identity as "Christendom". Papal mutterings about this perceived "apostasy" have been increasing in volume for some time now - but what really infuriated the Vatican was the presentation, back in 2003, of the first draft of the ill-fated European constitution. In its preamble, the authors had indulged themselves in a spot of root-tracing. Europe's debt to ancient Greece and Rome was solemnly acknowledged. So, too, were the achievements of the Enlightenment. About the Christian roots of European civilisation, however, there was nothing.
The implication was obvious: everything between Marcus Aurelius and Voltaire was to be reckoned mere backwardness and superstition. No wonder that the papacy was appalled. No wonder either that Pope Benedict, invited by the European Parliament to be its keynote Christian participant in the Year of Intercultural Dialogue, should very pointedly have refused.
So what, many secularists may be tempted to shrug: for when it comes to identifying the traditions that define Europe, there are few more venerable than that of baiting pontiffs. Nevertheless, it is hard not to agree with the Vatican that the desire of the laicist tendency in Brussels to ignore more than a millennium and a half of European history is not altogether a healthy one. As the recent referenda in France, the Netherlands and Ireland all served powerfully to demonstrate, electorates are reluctant to buy in to any vision of the future that seems not to take proper account of the past. Never is an acknowledgement of where we have come from more important than when we are attempting to plot a way ahead. If this is true on a national level, how much more so on a continental? The question of what precisely Europe owes to its Christian past may be neuralgic for many - but that is precisely why it needs to be aired, and not closed down.
As it stands, the current attitude of European secularists towards Christianity is like that of a once openly gay man who has since barricaded himself inside the closet and taken to sneering at homosexuality as something deviant. Secularism, in its western form, derives ultimately not from Greek philosophy, nor Roman law, nor even from Enlightenment anticlericalism, but rather from teachings and presumptions that are specifically Christian. Its fons et origo is to be found in the celebrated retort of Jesus to the Pharisees who had thought to catch him out by asking whether it was lawful to pay taxes to Rome: "Render unto Caesar the things that are Caesar's; and unto God the things that are God's." This admonition, far from prescribing political quiescence, was rather a reflection of Jesus's presumption that the Kingdom of Heaven was soon to be established on earth, causing Rome and all her works to melt like so much mist in the morning sun. But the centuries passed, the Kingdom of Heaven did not descend from the skies - and in due course Caesar himself ended up a Christian. The resulting upheaval, under Constantine and his successors, was seismic: the enshrining of a division between church and state, and between clergy and laity, that would have been unrecognisable to the pagans of classical antiquity.
Yet still the distinctions were less than fundamental. In particular, Caesar himself, by laying claim to the rule of the world as the lieutenant and complement of the celestial emperor, God, was a figure universally regarded as being quite as implicated in the mysterious dimensions of the heavenly as any priest. His subjects took it for granted that he had not merely a right to intrude upon the business of the Church, but a positive duty. Such a presumption, passing from Constantinople, the second Rome, to Moscow, the third, was destined to outlive the Roman empire itself. Indeed, today in Russia, where Vladimir Putin's nomination of Dmitry Medvedev as president was blessed on national television by the Patriarch, and where proselytising by non-Orthodox churches is increasingly banned by the Kremlin's surrogates out in the provinces, perhaps it has a ghostly afterlife still.
In the west, it expired long ago. One name more than any other stands for the insistence by the Church that spiritual power would not give way to temporal: Canossa. It was here in 1077, amid the bleak snows of an Apennine winter, that the emperor of the west, Henry IV, found himself obliged to beg for absolution from a rival who wore no crown, nor even a sword, but who had revealed himself nevertheless to possess a hitherto unsuspected influence and might. Pope Gregory VII's excommunication of Henry the previous year had left the king's enemies so emboldened, and his friends in such despair, that his entire kingdom had in effect been rendered ungovernable. Only a papal absolution, Henry had come to realise, would enable him to cling on to his throne - and so he had ridden through the winter to Canossa to obtain it.
After leaving the penitent to stand out in the ice and wind for three days, Gregory duly admitted him into the papal presence, and absolved him with a kiss. "The King of Rome, rather than being honoured as a universal monarch, had been treated instead as merely a human being," recorded Henry's grandson Otto of Freising, " a creature moulded out of clay."
Once, back in the heroic early days of European liberalism, this was regarded as one of the totemic episodes of history, a turning point more than fit to be ranked alongside the storming of the Bastille. Perhaps, the times being what they are, it deserves to be so again. By presuming to challenge the fabulously ancient nimbus of tradition that hedged emperors and empires about, Gregory had helped to set Europe upon a fateful course. His ambition, a truly breathtaking one, was nothing less than to transform the whole of Christendom: to divide it, from its summit to the meanest village, into two, one realm for the spiritual, one for the secular. Yet a piquant irony was to follow what may fairly be regarded as Europe's first revolution - because it was the very success of Gregory and his followers that would ultimately result in the banishment of God from western political life.
Even as the papacy set about fashioning the apparatus of a fully functioning state, complete with taxes, laws and bureaucrats, so a succession of kings was inspired to do the same; except that what the kings constructed was raised on foundations largely bled of any sacral dimension.
In due course, following the Reformation and Europe's collapse into warring Catholic and Protestant factions, it was this same inheritance that provided states with the muscle to impose upon their exhausted peoples the principle of religious toleration. The Enlightenment, a break from what had gone before although it undoubtedly was, hardly ranked as a total rupture. Just as the philosophes and their heirs could not help but draw on the ethical capital of the faith they so insistently rejected, so, too, were the parameters of the evolving liberal state shaped by presumptions that were ultimately centuries old. Gay weddings and multiculturalism: both ultimately rank as waymarks on the road from Canossa. Secularism in its contemporary form, by an irony fit to perplex both Benedict XVI and the liberal commentator Polly Toynbee, has as its truest godfather a medieval pope.
Given the continent's current state of cultural introspection, perhaps the kinship this implies between Europe's churches and her post-religious elite has the potential to serve as something more valuable than merely a historical paradox. While Christians, and Catholics especially, have often sought to buttress their beliefs by drawing succour from the traditions of their faith, secularists prefer to cast themselves as having been liberated from the moorings of religious and cultural identity altogether. No less than the devout, they, too, have their myth of origin: one that casts the Enlightenment not as the refinement of Christian presumptions that in so many ways it was, but as their utter abrogation. This, in effect, is to cast secular humanism as the offspring of a Virgin Birth: emerging ex nihilo, untainted by any trace of Christian DNA.
And yet a cultural inheritance, even when unacknowledged, cannot so easily be bucked. What is the show of relativism paraded by the European Union's Year of Intercultural Dialogue, for instance, if not a trace element of the Sermon on the Mount? "You have heard that it was said, 'You shall love your neighbour and hate your enemy.' But I say to you, 'Love your enemies . . .'" A command, like many that Jesus gave, so counter-intuitive and paradoxical as to appear wholly contrary to human nature; and yet one which, in the wake of Nazism, and the traumatic demonstration of just what depths the peoples of Europe might be led to by their capacity for hatred, has served to underpin the construction of a new and multicultural identity for Europe. In this dispensation, it is not enough for citizens merely to tolerate different cultures: they must respect them as well. So much so, that the roots of this presumption in the New Testament must be veiled, occluded, denied - precisely because for so long it has been the holiest text of Europe's dominant religion.
When the very desire to pretend we are neutral between different cultures serves only to emphasise how rooted our ideals remain within the seedbed of the Christian past, it is hard not to wonder whether the multicultural ideal might not be better served by openly acknowledging as much. Secular ethics have their own honourable traditions, of course, their own symbols, narratives and prophets - yet, for all that, they seem to lack the sheer emotional heft of an ancestral mono theism. It is hard to explain otherwise why so many people (some 70 per cent, according to the last census) should still describe themselves as Christian, even in non-churchgoing, energetically materialist Britain. Perhaps secularists should stop regarding this as a problem to be overcome, and recognise it as a resource to be drawn upon in the furtherance of a shared ideal: the establishment of a society in which those who are not our neighbours are, indeed, treated with love.
"Most people," Richard Dawkins assures us in The God Delusion, "pay lip-service to the same broad liberal consensus of ethical principles." Perhaps so - and yet the "liberal consensus" as it exists in today's Europe is no less contingent, no less the product of specific historical influences, than was the enthusiasm of the Spartans for state-sponsored infanticide or the Romans' taste for watching criminals being torn to death. A secularism that is content to trace its origins back to the classical world, but not to the Christian Church, is a secularism in profound denial. To acknowledge as much is hardly to open the back door to the Inquisition, nor even, necessarily, to imply a belief in God. Rather, it is to recognise that cultural presumptions, no less than species, are shaped by a continuous process of evolution; and that, even as they change and adapt, so also do they continue to bear witness to their origins.
The alternative - to insist that all faiths are bound to follow the same parabola as the western forms of Christianity have done - is not only to misrepresent Christianity itself as somehow normative, but also to place an insupportable burden of expectation upon those minorities in Europe who do not come from a Christian background. Just because an atheist is bound to regard all religions as being equally nonsensical does not mean that they are in fact all the same. Christianity, in its struggle to adapt to a liberal ethics of citizenship, has had a centuries-old headstart over other faiths. After all, if western secularism bears continued witness to its origins in the medieval Church, then so, too, do the churches today bear potent witness to the lasting impact of secularism.
It is not enough in modern Europe for Christians merely to acknowledge the legitimacy of a civil society in which all citizens are held to be equals. Rather, the secular state requires that such an acknowledgement be incorporated into the very fabric of their faith. For the churches themselves, it has often been a painful one to meet such a demand.
Not until 1965, after all, and the Second Vatican Council, did the Catholic Church finally concede the relevance of a system of values that approached liberalism. Nor, as the widening schism in the Anglican Communion over homosexuality and the ordination of female bishops suggests, is the agonising done with yet. How much more of a challenge, then - indeed, a wrenching dislocation - for faiths with no tradition of dialogue with post-Enlightenment secularism. How unsettling for them is the sense of secularists looking up impatiently at the clock, waiting for them to hurry up and have their own reformations, their own enlightenments.
"I am neutral between Rome and Geneva." So wrote Frederick the Great in 1740. Nowadays, European elites are obliged to affect no less strict a neutrality towards Mecca, Amritsar and Varanasi. Yet it remains precisely that: a calculated hypocrisy. Neutrality, in the dimension of culture and religion, can never itself be a neutral concept: it is too much the product of Christian presumptions and history ever to rank as that. Today's secular Europe may well pride itself on having arrived at a post-religious state of moral and intellectual superiority - but it is no less Christendom's heir for that.
Tom Holland's "Millennium: the End of the World and the Forging of Christendom" is published by Little, Brown (£25)