I have recently been drafted in as a lecturer in comparative religion at Leo Baeck College, north London, alma mater of Reform Judaism in Europe. Having spent 20 years writing about religious ideas, I had hoped that I had transcended my own Catholic roots sufficiently to embrace all aspects of the search for the divine. But sitting every week with my group of trainee rabbis, I am repeatedly shocked by my own ignorance of their tradition, and theirs of mine.
Christianity and Judaism are, after all, two sisters with the same parents, yet their perceptions of each other are often damagingly misconceived. So what hope of any sort of appreciation of Islam in the west beyond inflammatory headlines and soundbites that equate Muhammad with fundamentalism and terrorism?
Standing out against this ignorance is the writer Karen Armstrong, who constantly reminds us of the acres of common ground between the world's major faiths. Her well-articulated plea is for knowledge and understanding, but it carries within it an appreciation of religion's appeal in a world that science and secularism have failed to make any less mysterious. In the US, books such as A History of God, The Battle for God and Muhammad: a biography of the prophet have made Armstrong famous. Since 11 September 2001, she has been much in demand as a speaker, lecturing at summer schools for US senators and congressmen and challenging the notion that the behaviour of a tiny number of extremists should allow the likes of Robert Kilroy-Silk to demonise the world's 1.2 billion Muslims.
In Britain, to our shame, she is less well known. But among Catholics in particular, she is still remembered as the "runaway nun" - the result of her 1981 book Through the Narrow Gate, a bestselling, unvarnished account of life in a convent. Her erstwhile co-believers saw the book as an act of betrayal, and this has blinded them to her subsequent achievements.
Armstrong's follow-up to Through the Narrow Gate was Beginning the World, an account of her loss of faith. It was, she admits candidly in her new book, the worst thing she ever wrote. The Spiral Staircase is her attempts to redress its lack of perspective.
The title comes from T S Eliot's six- part sequence "Ash-Wednesday". A spiral staircase goes forward without appearing to make progress, although almost imperceptibly it is reaching towards the light. Like Eliot, Armstrong is tracing the process of spiritual recovery - from anger and abandonment in her first few years outside the convent, through a series of unsuccessful career moves, leading to a recovery of a sense of the transcendent while researching her most successful book, A History of God.
I wrote the book with mounting excitement. It represented a quest and liberation for me. No wonder I had found it impossible to "believe" in God - the personalised God might work for other people but "he" had done nothing for me. I was not a chronic failure, but had simply been working with a spirituality and theology that were wrong for me. Because I had assumed that God was an objective fact, I had thought about God using the same kind of logical, discursive reflection that I employed in my secular life. Rational analysis is indispensable for mathematics, medicine or science, but useless for God.
Reading and research allowed her to rediscover the place of myth and mysticism in faith. "Theology," she writes, "is a species of poetry", something elusive that requires the same "quiet, receptive mind" you might use to "listen to a late Beethoven quartet or read a sonnet by Rilke. You have to give it your full attention, wait patiently upon it, and make an empty space for it in your mind. And finally the work declares itself to you - until it becomes part of you."
Open, accessible, writing without jargon or denominational loyalty, Armstrong manages to put into words something that most of us cannot express. And in case I am making The Spiral Staircase sound like a superior version of one of those swirly-covered Born Again books that go yellow on the Mind, Body and Spirit shelves of bookshops, I should add that she tells a good tale at her own expense.
She recounts the challenges she faced on her journey with amusement, a good ear for dialogue and an absence of self-pity or piety. Her undiagnosed temporal-lobe epilepsy not only led to her spending more time than was healthy with psychiatrists, but also nearly convinced her that she was weak and mad - a perception shared by her superiors at the convent.
At times, her honesty results in her being too tough on herself. She appears to accept the opinion of an Israeli film producer with whom she worked on a Channel 4 film about St Paul, and who told her she was ugly, clumsy and past her best. She blames herself entirely for her lack of romantic success; yet this book makes you despair of the inability of men to cope with such a clever, self-possessed woman.
Today Karen Armstrong lives a solitary life of study, reflection and writing. It is not so far removed from her early days in the convent. What she yearned for then, she has finally found - albeit in a very different form.
Peter Stanford is the author of Heaven: a traveller's guide (HarperCollins)