Instead, all he could talk about was a bestselling paperback. Dan Brown's The Da Vinci Code may have sold 40 million copies around the world with its bizarre plot about Christ marrying Mary Magdalene and having a baby; it may have been smeared with suntan cream, dropped in the bath and passed around friends, but that does not make it a great threat to the Church. It's a blockbuster novel, about as true to Christian beliefs as Ian Fleming's James Bond is to MI6.

Before Dr Williams became Archbishop of Canterbury three years ago, he enjoyed a reputation for being thoughtful, articulate and measured. The man from Monmouth with the big eyebrows seemed equal to any theological challenge. Yet here he is, three years later, seemingly cashing in on the publicity of a book that will be binned with next year's bonkbuster. His sermon made headlines in all the Sunday newspapers, but it was dispiriting rather than uplifting.

In a recent interview with the editor of the Guardian, the Archbishop made it clear that he doesn't want to be forced to take the moral lead on issues such as marriage or terrorism. "Leadership is, to me, a very, very murky and complicated concept," he said, describing this aspect of his role as being "comic vicar to the nation".

Dr Williams, a modest man, says he dislikes publicity, headline-grabbing tactics and pontificating. He refuses to give glib answers. That is understandable and in many ways commendable. Britain no longer wants fire and brimstone; it should welcome a leader who has integrity, is scrupulous and is humble enough to question all his assumptions.

But the Archbishop should stick to his rules. If he is not going to weigh into significant religious issues, then he shouldn't become bogged down in frivolous ones, either. At the moment, all he seems to do is squabble about whether or not the Church should allow gay bishops. When he is not doing that, he is apologising for Britain's distant past. "The Crusades were a serious betrayal of Christian beliefs," he said last year.

Now he is berating populist authors. If he wants readers to boycott The Da Vinci Code, why not the Pullman quartet or The Lion, the Witch and the Wardrobe as well? A man of his intellect should be consistent. Instead he sounds bewildered by his role as the leader of a church that is losing its way.

Pope Benedict XVI, meanwhile, who began his pontificate without huge expectations as "God's rottweiler", is proving adept at broaching a wide range of issues. His first encyclical, on love, was praised by both traditionalists and reformers. Using poetic language, he delivered a firm message in a positive way. Next, he took a risk with the Roman Catholic Church's relationship with China by making a liberal-minded Hong Kong bishop a cardinal.

His Easter message tackled terrorism, the Sudan and Iranian nuclear proliferation. He also condemned the attempt to clone a child as an "attack on the family". The Archbishop of Canterbury can't dismiss this kind of moral engagement as an attempt to play the role of comic vicar to the world.

Another religious leader, the Dalai Lama, whom I interviewed last month, also had no difficulty in engaging with modern life. He advised Westerners not to give up on marriage, and he explained that, to a practising Buddhist, homosexual sex is, "Wrong. Full stop. No way round it." He also ruled out oral sex. But he never sounded pompous or pious. He tempered a rigid religious code with compassion and humour.

The Anglican Church needs a strong leader. Fewer than 900,000 people in Britain attend Sunday services. If the Church doesn't provide it, many will of course turn to cranks and extremists to fill the gap. Even the Prime Minister is apparently only waiting to leave office to convert to Catholicism - and his wife believes in crystals and rebirthing ceremonies.

END