The joint declaration issued in Turkey last week by Pope Benedict and Ecumenical Patriarch Bartholomew I reaffirming their desire to restore full communion of what the pope described as the sister churches of Rome and Constantinople means that attention will now turn to a joint commission to resolve outstanding theological differences, which has resumed meetings after a long hiatus.

One effect of the renewal of the clash of civilizations in our time has been to remind us that clashes within civilizations are no less ancient. Islam has been divided between Sunni and Shiite ever since the heirs of Mohammed fell out. If anybody needed reminding just how bitter that division remains after fourteen centuries, they need only look at Iraq.

Christians have been divided, too, for most of their history. We in the West tend to think mainly of the Reformation as the parting of the ways between Rome and the rest. We forget that the thousand-year schism between Catholic and Orthodox Christians formally ended only in 1965, when Pope Paul VI and Patriarch Athenagoras lifted the excommunications that their predecessors had pronounced in 1054.

Until then, the Eastern and Western branches of Christianity were anathema to one another, using the word for once in its strict theological sense. Even today, many members of the Orthodox hierarchy are suspicious of the Vatican, which they accuse of proselytizing in traditional Orthodox heartlands. The patriarchs of Moscow have always blocked proposals for a papal visit to Russia.

The fundamental conflict between Rome and Constantinople concerns authority. Catholics believe that Christ entrusts the pope, as the successor of St. Peter, with “the power of the keys” over doctrine and discipline throughout the Catholic — meaning “universal” — church. This unique papal “magisterium” is denied by Orthodox Christians, who recognize only the early ecumenical councils as authoritative on matters of doctrine and who insist on the autonomy of each church in matters of discipline.

Profound as these divisions are, they are not insuperable. The pope told the patriarch that “we hope to overcome … our differences of opinion” about the universality of Roman authority. But this pope, who has been the most influential Catholic theologian of the last 40 years, is far too intelligent not to know how variously the primacy of the papacy over the other patriarchs may be interpreted. He is ready to concede considerable latitude to all who share the apostolic succession — in other words, those whose priesthood descends directly from Jesus. Anglican priests who convert to Catholicism may remain married; no pope would demand that Orthodox priests, who have always been free to marry, should become celibate.

Although the Catholics and Orthodox are not in full communion, they do recognize each other’s priests, religious orders, and sacraments. Both emphasize a spiritual and institutional continuity that goes back to the apostles.

This is not the case with Protestants, who differ fundamentally from both Catholics and Orthodox in their understanding of such basic matters as whether and in what way Christ is really present in the Eucharist. Headline disputes over whether priests can be female or homosexual are merely symptoms of deeper differences between the Protestant and the Catholic-Orthodox ideas of what constitutes a church.

When the Archbishop of Canterbury, Rowan Williams, visits the pope, as he did last month, the meeting is always billed as a meeting of the “spiritual leaders” of the Anglican Communion and the Catholic Church. In reality, the Archbishop of Canterbury has no authority to speak on behalf of Anglican or Episcopalian bishops around the world and no power to discipline them if they ignore him, as they frequently do.

The pope warned the archbishop that the decision of some Anglican provinces to ordain women and homosexuals not only as priests but as bishops, as well as to bless gay marriages, had led to internal as well as external “strains.” Many Anglicans care more about their liberal agenda than about ecumenical unity, but conservative dissenters will find Rome receptive.

Since the Second Vatican Council in the early 1960s, much more effort has been invested in the ecumenical dialogue between Catholics and Protestants than between Catholics and Orthodox, yet with much less result. A schism can be healed, but a reformation is irreversible. And there are no short cuts to full Christian unity.

Yet Benedict is proposing to reorientate a civilization that has lost its way. Under the guise of secularization, the West has fallen victim to a kind of amnesia. It must recover its memory of the hallowed past — Greeks call it anamnesis — if it is to survive. Western civilization is inseparable from the 2,000-year history of Christianity, which in turn rests on the foundation of Judaic morality and Greek rationality. The West is capable of reappropriating this Judeo-Christian classical inheritance, but without an identity worth defending it is vulnerable.

The arch enemy, Benedict believes, is within: the relativism and nihilism that have hollowed out Western civilization in the name of secularization. He distinguishes between reason, which is at the service of humanity, and relativism, which exercises a dictatorship over the human spirit. He is particularly struck by the occlusion of Greek rationality, the “Logos” or “the Word” of which St. John speaks in his Gospel, by a pseudorationalism that denies religious freedom.

The violently hostile reaction to his Regensburg speech, not only from Islamist demagogues, but even from some of the most senior scholars in the Islamic world, indicated that dialogue would be difficult. Indeed, the pope was informed as soon as he arrived in Turkey that a theological debate with Islamic scholars was out of the question. They would not discuss the issue that he had raised in Regensburg — whether the conception of Allah as above and beyond rational understanding might lie behind Islam’s propensity to violence. Instead, Prime Minister Erdogan tried to drag the pope into a purely political debate, about whether Turkey should be allowed to join the European Union. Mr. Erdogan even claimed that the pope had indicated his support for Turkish membership, though the Vatican immediately denied this. Some commentators took Mr. Erdogan’s counter-intuitive assertion at face value, either welcoming it as a volte face or denouncing it as craven appeasement. The truth was that Mr. Erdogan had deliberately misinterpreted a diplomatic expression of sympathy for Turkey’s efforts at Westernization as a specific pledge of support.

Benedict cannot but choose his words carefully in relation to Islam. Tens of millions of Christians live under Islamic rule, and he dare not make their already precarious situation worse. But he is determined to regain freedom of conscience and worship for them, and indeed to ensure that all who are threatened by Islamist terror or persecution are not abandoned by the foreign-policy “realists” now in the ascendant in the West. This is a platform on which all Christians can unite. But ending the “scandal” of disunity requires leadership and a sense of urgency. Benedict is the oldest man to be elected pope in centuries. He does not have much time. The pope is the best hope that someone may persuade the West to stop digging its own grave.