Karen Armstrong is one of the handful of wise and supremely intelligent commentators on religion who has become distressed by the tone of recent discussions of the subject. Her targets are religious fundamentalism on the one hand and militant atheism on the other: in other words, al-Qaida as well as Richard Dawkins. In plain language, and nowhere more eloquently than in this new book, Armstrong accuses both factions of misunderstanding the nature of God and, interestingly, of doing so in similar ways.

Both atheists and fundamentalists take God to be an essentially human sort of figure, a giant Father in the sky who watches over us, punishes the guilty, intervenes directly in our affairs and is entirely comprehensible to our minds. "We regularly ask God to bless our nation, save our queen, cure our sickness or give us a fine day for a picnic." Fundamentalists commit, in Armstrong's view, the grave error of presuming to know God's mind and also of enlisting God on their side against their enemies. Unsurprisingly, militant atheists observe this reductive vision of God and in turn slam religion as a child-like description of the world that cannot compare with the subtlety and practical powers of science.

Armstrong's new book is shaped as a response to these two distortions. She wishes to remind us of the mystery of God. Her sympathy is with the great Jewish, Christian and Muslim theologians who have denied that any human attempt to put the divine into words will be accurate. We are simply too limited to be able to know God; our apprehension must hence be suffused with an awareness of our provisional and potentially faulty natures. She writes: "He is not good, divine, powerful or intelligent in any way that we can understand. We could not even say that God 'exists', because our concept of existence is too limited."

Much of Armstrong's book is spent pointing out the deep-seated needs that religions have traditionally addressed. She begins in the caverns of Lascaux in the Dordogne and argues that the early religious rites to which the famous animal pictures belonged were connected with our ancestors' wish to atone for the overbearing guilt that came from having to butcher other living creatures for survival. Like art, religion has been a way of containing feelings that might otherwise tear individuals and societies apart. Armstrong leans heavily on the distinction first made by the ancient Greeks between the realms of mythos and logos. Logos is "a pragmatic mode of thought that enables people to function effectively in the world"; it is what we rely on when organising society or planning a journey. However, logos has its limitations: "It cannot assuage human grief or find ultimate meaning in life's struggles." For this, there is the realm of mythos or myth, to which religion and art belong. Religion offers us moments of what Armstrong calls, using another Greek term, ekstasis, a stepping outside of the norm for the sake of release and consolation.

Aside from helping us to deal with our feelings of fear, aggression and guilt, religion is also defended by Armstrong as a source of compassion. She recounts the story of a man who once came to see the great Rabbi Hillel and asked if he might undertake a rigorous course of study of the Torah with him. Hillel waved him away: "What is hateful to yourself, do not to your fellow man. That is the whole of the Torah and the remainder is but commentary. Go learn it." Armstrong traces the emphasis on compassion across the major religions. There are fascinating discussions here of the Buddhist state of "anatta", or no self, a desirable condition that can be reached only through extensive meditation. It can lead us, for brief periods, to look at the world as though we were not ourselves participants in it, and therefore free us from our noxious impulses to pass judgment and to presume that we know why other people have acted the way they have.

The concluding part of Armstrong's book traces the growth of modern atheism and attributes it largely to religions' failure to argue for what is most compelling about them. Fatally, religions tried to defend themselves against science by arguing that they knew the truth better than the geologists, rather than presenting themselves (as one feels Armstrong would have wished) as the guardians of mystery and therapeutic manoeuvres of the mind.

A melancholy tone sometimes makes itself felt in this book, as one senses the author defending religion from its own worst impulses. As a former nun, Armstrong's perspective is unique in its combination of sympathy and lack of sentimentality towards religion's wilder and more immature claims. She joins Richard Holloway and Charles Taylor as one of the most intelligent contemporary defenders of religion, making a case that scrupulously avoids reliance on the supranatural. As in so much of the rest of her hugely impressive body of work, Karen Armstrong invites us on a journey through religion that helps us to rescue what remains wise from so much that to many in Britain today no longer seems true.

 Alain de Botton's most recent book is The Pleasures and Sorrows of Work (Hamish Hamilton).

This is an eloquent and interesting book, although you do not quite get what it says on the tin. Karen Armstrong takes the reader through a history of religious practice in many different cultures, arguing that in the good old days and purest forms they all come to much the same thing. They use devices of ritual, mystery, drama, dance and meditation in order to enable us better to cope with the vale of tears in which we find ourselves. Religion is therefore properly a matter of a practice, and may be compared with art or music. These are similarly difficult to create, and even to appreciate. But nobody who has managed either would doubt that something valuable has happened in the process. We come out of the art gallery or concert hall enriched and braced, elevated and tranquil, and may even fancy ourselves better people, though the change may or may not be noticed by those around us.

This is religion as it should be, and, according to Armstrong, as it once was in all the world's best traditions. However, there is a serpent in this paradise, as in others. Or rather, several serpents, but the worst is the folly of intellectualising the practice. This makes it into a matter of belief, argument, and ultimately dogma. It debases religion into a matter of belief in a certain number of propositions, so that if you can recite those sincerely you are an adept, and if you can't you fail. This is Armstrong's principal target. With the scientific triumphs of the 17th century, religion stopped being a practice and started to become a theory - in particular the theory of the divine architect. This is a perversion of anything valuable in religious practice, Armstrong writes, and it is only this perverted view that arouses the scorn of modern "militant" atheists. So Dawkins, Dennett, Hitchens and Harris have chosen a straw man as a target. Real religion is serenely immune to their discovery that it is silly to talk of a divine architect.

So what should the religious adept actually say by way of expressing his or her faith? Nothing. This is the "apophatic" tradition, in which nothing about God can be put into words. Armstrong firmly recommends silence, having written at least 15 books on the topic. Words such as "God" have to be seen as symbols, not names, but any word falls short of describing what it symbolises, and will always be inadequate, contradictory, metaphorical or allegorical. The mystery at the heart of religious practice is ineffable, unapproachable by reason and by language. Silence is its truest expression. The right kind of silence, of course, not that of the pothead or inebriate. The religious state is exactly that of Alice after hearing the nonsense poem "Jabberwocky": "Somehow it seems to fill my head with ideas - only I don't exactly know what they are." If Alice puts on a dog collar, she will be at one with the tradition.

Armstrong is not presenting a case for God in the sense most people in our idolatrous world would think of it. The ordinary man or woman in the pew or on the prayer mat probably thinks of God as a kind of large version of themselves with mysterious powers and a rather nasty temper. That is the vice of theory again, and as long as they think like that, ordinary folk are not truly religious, whatever they profess. By contrast, Armstrong promises that her kinds of practice will make us better, wiser, more forgiving, loving, courageous, selfless, hopeful and just. Who can be against that?

The odd thing is that the book presupposes that such desirable improvements are the same thing as an increase in understanding - only a kind of understanding that has no describable content. It is beyond words, yet is nevertheless to be described in terms of awareness and truth. But why should we accept that? Imagine that I come out of the art gallery or other trance with a beatific smile on my face. I have enjoyed myself, and feel better. Perhaps I give a coin to the beggar I ignored on the way in. Even if I do so, there is no reason to describe the improvement in terms of my having understood anything. If I feel more generous, well and good, but the proof of that pudding is not my beatific smile but how I behave. As Wittgenstein, whose views on religion Armstrong thoroughly endorses, also said, an inner process stands in need of outward criteria. You can feel good without being good, and be good without stretching your understanding beyond words. Her experience of "Jabberwocky" may have improved Alice.

Silence is just that. It is a kind of lowest common denominator of the human mind. The machine is idling. Which direction it then goes after a period of idling is a highly unpredictable matter. As David Hume put it, in human nature there is "some particle of the dove, mixed in with the wolf and the serpent". So we can expect that some directions will be better and others worse. And that is what, alas, we always find, with or without the song and dance.

 Simon Blackburn's Truth: A Guide for the Perplexed is published by Penguin.