I've managed to resist Karen Armstrong for the best part of two years. When some of my friends learned that I was carrying out a series of interviews with well-known people on the subject of their religious beliefs (or lack of them) they kept telling me that she would be absolutely perfect as a subject. Not only did she know a great deal about all the religions of the world but she also had this special capacity to understand the spiritual instinct. Why didn't I find out for myself? Why didn't I read her first book Through the Narrow Gate, the story of her seven years as a Catholic nun, or its sequel The Spiral Staircase (published earlier this year and chosen as Radio 4's Book of the Week)? Then I'd know how good she was.

But I had several objections to following such advice: none of them particularly honourable. In the first place I have to admit to a longstanding aversion towards converts and ex-converts who rush into print. I can still remember how irritated I was by the fuss created in the early sixties when Monica Baldwin told the world about her twenty-seven years as a cloistered nun in I Leap over the Wall. Then there'd been the equal annoyance prompted at roughly the same time by the success of Douglas Hyde's I Believed: the Autobiography of a former British Communist in which a leading journalist told the world how he had lost his faith in Stalin and found peace in the bosom of Rome. In both cases I took the rather righteous view that people who'd made the ridiculous mistake of expecting to find salvation in a closed religious order or an authoritarian political party shouldn't be allowed to capitalise on their stupidity by selling us the life-changing story of their second conversion. After all, if they'd been so gullible the first time round what guarantee did we have that their new conversion was any more valid than their last?

I'd also gained the suspicion from my friends' tone of voice that Karen Armstrong might turn out to be one of those slightly folksy purveyors of religion who populate Radio 4's Thought for the Day slot. People like Lionel Blue who tell a couple of jokes (in his case, rather good jokes) in the hope of persuading you that religion is really nothing much more than a rather nice club that you can subscribe to by simply being good and thoughtful and sincere.

What changed my mind hearing Karen Armstrong talking on a Radio 4 programme called Midsummer Sins. She was intelligent and coherent and not the least bit cosy or vacuously spiritual. In fact she made some of the other contributors sound slightly bungling and lightweight. That was enough to send me to her books. These were also a minor revelation. Instead of the confessional soul-searching that I'd expected, there was a rigorous emotional analysis, a determination to think through her personal dilemmas about religion and its relevance to her life, rather than a slack daytime television catalogue of personal feelings. She certainly did regard her life in the rather trite terms of a personal journey towards truth but the image she selected to describe this biographical itinerary - the spiral staircase - was taken not from any glib account of spiritual progress but from the dense intellectual reflections on the nature of doubt which preoccupy TS Eliot in Ash Wednesday.

After my reading, I wasn't too surprised to find that Karen Armstrong was very business-like about the interview. She wanted to see some back copies of the New Humanist so that she would know her audience and asked me to arrive first thing in the morning so that she might have a free day afterwards in which to write. I had the slight sense that I was not so much arranging a personal chat as booking in for a professional consultation. "Yes, I'm very busy," she told me as I seated myself across the table from her in the downstairs room of her neat fashionable Islington terraced house. "I'm off to America tomorrow. I have a little book on the Buddha coming out in paperback, so I'm going to a literary festival to talk about it. Which is a nice rest from Islamic fundamentalism." Was she regarded as an expert on Islam in the States? "Well, it has turned out a bit like that. I was supposed to be flying to America to take up a post at Harvard on September 12 in 2001 and was actually packing when the terrible news came through. So, when I got to Harvard I never even had a chance to set foot in the library. I was continually on the radio and writing articles. Vanity Fair and GQ and Time wanted huge articles on fundamentalism. I had to give two talks to the American Congress, to Senate and to the UN. Suddenly people wanted to know what Islam was."

I knew that Karen had researched and written extensively about Islam but wasn't it still rather strange that she should have been selected on the basis of such book learning to be one of its chief interpreters in the West? What did she tell her audiences?

"I tell them that it is very important to understand their enemies but that it is equally important to know who they are not, so that one does not antagonise potential allies." So, she was trying to correct misconceptions? "Yes. In particular the misconception that Islam is a fundamentally violent religion that impels people to violence. In fact it was the last of the three monotheisms to develop a fundamentalist strain. Until the 20th century it was vastly more tolerant than western Christianity which slaughtered Jews, Muslims and each other with great relish. Western Christianity has been one of the most intolerant religions in the world."

I told her that I could see how this message might be useful antidote to bigotry among non-Muslim audiences, but how did it play with Muslims themselves? How did they react to someone - a woman, no less - popping up at the United Nations and acting as the official interpreter of their faith?

"Actually, when my book on Islam came out (Mohammed: A Biography of the Prophet) the Muslims were the first people to take me seriously and realise that I might be something more than a runaway nun writing wildly provocative books about religion. Publishers didn't want to touch the book. They thought that I would be joining Salman Rushdie in hiding. They thought that the Muslims would be enraged by a western woman writing about their prophet at this historic moment. But to their great credit they took the book to their hearts."

But surely they would rather have spoken up on their own behalf?

"It is very odd. When I meet them I tell them frankly that I can't go on explaining their religion to others for ever. I go to a lot of mosques and Islamic centres and insist that they must get out there and speak. But in America many of them are first generation immigrants who don't have the kind of oomph to go on the radio successfully. I don't, though, think of myself as an ambassador for Islam. What I really want to do is make a plea to my own culture. And that began a long time ago during the Rushdie affair when I noticed that some of the liberal defenders of Rushdie segued very easily from a denunciation of the Ayatollah to an out-and-out denunciation of Islam. I began to think that we had learned nothing from the 20th century because it was that sort of cultivated inaccuracy that led to the death camps."

But wasn't she being somewhat idealist about contemporary Islam? I could understand her wish to protect the religion from crude detractors who saw it as intrinsically violent, but surely she still had to face the reality of fundamentalism? There were many Muslims in the world who saw the violent overthrow of the West as a religious crusade justified by the teachings of the Koran. "You're right. There is always that literal fundamentalist tendency in all religions. Look at Christian fundamentalism in the USA. It's that fundamentalism that leads so many people to reject religion. But the tendency to read the Bible or the Koran as though they were literal texts, holy encyclopaedias where you look up information about God is an unfortunate offshoot of modernity. The Koran is constantly saying that every single one of its pronouncements is a parable, it constantly says that God only speaks in analogies. Religion is an art form. And it has always turned to art when it wants to express its truths, to architecture and music and poetry and dance. Theology should be poetry. Even if it is occasionally boring and dull it should also fill you with the same sense of wonder and the same intimations of transcendence as when you read a great poem. Like poetry, religion is an attempt to express the inexpressible."

I tell her that this sounds awfully like the sort of line we were given in my sixth form by some of the Christian Brothers who knew that they were faced by a thoroughly sceptical audience. Don't worry about all the doctrinal injunctions in the catechism, they'd tell us. They're only there for the more simple minded believers. Intellectuals can manage without such literal props. They can use their intelligence to find God. Wasn't she in some danger of perpetuating this distinction with all her talk about religion not being about definite beliefs?

"But religion was not about beliefs until the 18th century. That was the time when faith started to be equated with believing things instead of putting your trust in something. Until then religion had always been about doing things rather than believing things. The Koran, for example, is very sceptical about doctrines. It just tells you to share your wealth equally and look after the vulnerable. Judaism is also a religion about doing things. And the Buddha had absolutely no time for metaphysical ideas. He thought they were a complete bore. He had a monk who kept pestering him about whether there was a God or not and the Buddha told him he was like a man who had been shot by an arrow but refused to have any treatment until he found out the name of the person that shot him and the village he came from. And the Buddha said you will die before you get this perfectly pointless information. Religion is not about finding things out. It is about giving you peace of mind. And it's about compassion. All the world's religions came to the conclusion that compassion was the key."

But didn't her dismissal of beliefs mean throwing away nearly everything that was universally regarded as at the heart of religion, the stories about creation and resurrection and the afterlife? "We love telling stories. We are story people. We encapsulate difficult truths in stories. But these are myths. They are not meant to be historical truths. Look, in the New Testament you have five mutually exclusive accounts of the resurrection. In the Hebrew scriptures there are several different creation stories. These are stories telling you what you must do. When the Greeks told their stories about heroes fighting monsters or threading their way through labyrinths, they were saying that what you have to do is bring out the heroic potential in yourself and only then will you discover the truth of this myth. Myths tell you something important about human nature. Doctrines are something else. Doctrines are a peculiar disease of western Christianity. One of the reasons for the split between the eastern and the western churches was because the Greeks thought the westerners far too anthropomorphic and rationalistic in their notion of the divine. Take the notion of the Trinity. This was designed to remind Christians that they couldn't simply think about God as a personality. It was more complex than that. And then Christians turned this into a dove-like being floating around an old man holding a crucifix and that is the Trinity. We have created an idol out of these really absurd doctrines. Religions are doing a fabulous job putting themselves out of business."

Did she feel the same way about rituals as she did about doctrines? I gave her the example of the penances that were handed out by the priests. 'Say twelve rosaries.' That was surely a mind numbing exercise without any point whatsoever? Karen disagreed. "Yes, of course, if you are Jewish and not mixing milk and meat that can be just a soulless grim observance, but its purpose is to remind you that the world is not yours to do with as you choose. I was never very good at the rosary either, but it is one way of pushing you out of the normal. It is like Buddhist chanting or the Muslim with the ninety-nine names of Allah on beads. It is a way of losing yourself, of dethroning yourself from the centre of the world."

I told Karen that her religion now seemed to have been reduced to not much more than a husk. She'd thrown away religious doctrines and was now pronouncing religious ritual as nothing much more than a device for reducing egotism. All that seemed left was the golden rule of compassion for others and some intimations of the transcendent. That would surely allow an awful lot of atheists and agnostics to qualify as religious beings. "I would say that I am an agnostic, an agnostic in the best sense of not knowing, not speaking about things, in the way that the Buddha always refused to talk about Nirvana. If an agnostic or an atheist is living according to the golden rule of compassion towards others which is propounded in every single of the traditions that is absolutely fine. But if atheism is simply a flippant demand to get rid of all religion then that is silly because the religious enterprise is a serious one and must be taken seriously. What I want to see is not people subscribing to certain beliefs but acknowledging the absolute sacredness of life. When Hindus greet one another and bow, they are acknowledging the divinity they are encountering in the other person. The golden rule in every single religion is that you treat everyone with absolute respect and you don't exclude any creature, even a mosquito, from your radius of concern."

During all the time that we've been talking I've been noticing how carefully Karen constructs her sentences, her care with words, her capacity to alight on the perfect phrase with all the effortless delicacy of a sparrow on a washing line. I remind her of the section in The Spiral Staircase in which she describes how her friend, Sally, finally persuaded her to start writing. She laid out an exercise book and a new felt-tipped pen and said "I'm going to go out now for an hour. So get started. Just sit down and write the first two pages of your book. Just two pages, that's all! And then, when I get back, we'll have a drink to celebrate." That had been the beginning of Through the Narrow Gate and her present life as a writer. To what extent was writing now her religion?

"It is. It is. Writing and study are my prayers. What I now realise is that writing and reading is a disciplined attempt to go beyond, and that brings about a state of ecstasy. It gives me the kind of experience I was hoping to get from meditation in the convent. It is my religion now. Lionel Blue tells me that I am very Jewish in my spirituality because this is what Jews do when they study. They immerse themselves in text and get intimations of transcendence."

Karen Armstrong is a persuasive talker and writer. Very persuasive. It was only when I was half way home that I realised with some alarm that she somehow got me to nod enthusiastically at an observation by Lionel Blue.