There are few more delightful pleasures than to return to the scenes of your youth – especially if, like me, you were lucky enough to spend three years studying at Magdalen, the most beautiful of all the Oxford colleges. Last Saturday I attended a “gaudy,” or college reunion, at Magdalen and spent a happy evening among the dreaming spires.

A few of my contemporaries had made their fortunes on Wall Street and, thanks not least to its Anglo-American alumni, the college is flourishing. The choir sang Evensong as mellifluously as it would have done half a millennium ago; the deer grazed contentedly in their riverside park; the bells rang out from their medieval skyscraper to greet us, just as they had three decades ago when we first arrived for the most daunting interview any of us would ever have. We feasted in the great hall; we toasted college and Queen. The new president may have garbled the Latin grace, but then he is a scientist. Magdalen’s wine cellar was still in good shape, to judge from an exquisite Sauternes, made in 1989, the year Hitler and Stalin ceased to overshadow our post-war generation.

And yet, all through this feast, I was troubled by a melancholy that lurked just out of sight, but never quite out of mind. Perhaps prompted by the intimations of mortality that naturally insinuate themselves on such occasions, I could not banish the thought that this ancient foundation, which had played such a notable part in English history, may not, in its present form, outlive my children. It may not even outlive me.

For Magdalen, like so many other institutions that have helped shape the culture of the West, is not only named after a Jewish woman and Christian saint, but embodies Judeo-Christian values. Those values are now under deadly assault, not only from without, but from within. The prophecy of Bernard Lewis, the greatest living scholar of the Middle East, that Europe might be a Muslim continent by the end of this century is not to be taken lightly. Unless Islam can transform itself to become an integral part of Western culture, its rapidly growing influence can only threaten everything Oxford represents. Talk of reconciliation between “Abrahamic” faiths is meaningless as long as even moderate Muslim leaders demand to live under Sharia law and all schools of Sharia demand the death penalty for apostasy or blasphemy.

The morning after the feast, I found myself listening to an American couple, both alumni, over breakfast. One of them – a professor of constitutional law in California, I gathered – embarked on a systematic denunciation of her country. Most Americans, she declared, were illiberal, bigoted, ignorant, selfish, gun-toting racists. A Mormon student had taken her to task for criticizing the US Constitution, which the student saw as a sacred text. Only such unthinking patriotism could explain how they could have twice elected a man like George W. Bush. But, the professor insisted, even the most liberal Americans were, on average, far to the right of the most illiberal Europeans or Canadians. The Brits at the table sat nodding, as each and every anti-American prejudice they might have was confirmed.

I couldn’t help wondering why the British anti-Americans bother to spread their poison, when Americans themselves are eager to do the job. Prejudice that shows itself too blatantly offends the British public. When the mayor of London, Ken Livingstone, referred to the US Ambassador this week as “a chiseling little crook,” there was unanimous condemnation of the mayor’s bad manners, even if the occasion of his insult – US diplomats’ failure to pay the congestion charge in central London – is an unnecessary cause of friction that undermines their mission.

When anti-Americanism manifests itself more subtly, however, it may elicit unanimous approbation. This week, the Archbishop of Canterbury, Rowan Williams, was interviewed in prime time on the BBC. The subject was the threat of global warming, which, as Dr. Williams explained, is “a vast moral question.” “Unless there is a real change in attitude, we have to contemplate these very unwelcome possibilities [of governmental coercion] if we want the global economy not to collapse and millions, billions of people to die.”

The archbishop offered no evidence to support this apocalyptic vision, but he gave his sermon an anti-American twist. “I don’t think it’s compatible with a Christian ethic to ignore climate change,” he said. For a Christian, President Bush had been “very slow” to recognize the “profoundly immoral” consequences of our lifestyle. The president, he implied, was not merely selfish but a hypocrite, too.

Why was the BBC so eager to hear an Oxford-trained theologian pronounce on an essentially scientific issue? The environment now occupies the place in public discourse once occupied by theology: Where you stand determines your moral status. Consumers are polluters, and polluters are sinners. Our lifestyles are killing the poor. Global warming is a fundamentalist faith, and its adherents believe in justification by faith alone. The fact that the Archbishop of Canterbury may emit more greenhouse gases on the subject than a Boeing 747 is beside the point.

How does a discussion of climatology turn into an excuse for anti-Americanism? As it happens, the archbishop was in the United States at the time, but this is not why he singled out Americans for criticism. He did so because anti-Americanism, too, has become an article of faith – the faith of the liberal intellectual. Dr. Williams was merely doing what is expected of any member of his class.

One of the few people to challenge this consensus is the prime minister. This week, Tony Blair flew to Australia to castigate anti-Americanism. Mr. Blair is not a global warming skeptic. Yet he also knows that climate change, like Islamist terrorism, cannot “be resolved or even contemplated without [the United States].” He continued: “The strain of anti-American feeling in parts of European and world politics is madness when set against the long-term interests of the world we believe in.”

This message went down well in Australia. Nobody in Europe was listening, though. The “madness” is too ubiquitous. But Mr. Blair is right. The greatest threat to our civilization is not global warming. It is not even Islamist terrorism. It is anti-Americanism. For if the West is divided against itself, it cannot stand.