The God Delusion, by Richard Dawkins

Amos Starkadder, the hellfire preacher in Stella Gibbons's parody Cold Comfort Farm, surveys the congregation at the Church of the Quivering Brethren. He thunders: "Ye're all damned!" "An expression of lively interest and satisfaction" passes over their faces, with "a general rearranging of arms and legs". We may similarly imagine Richard Dawkins's readers making themselves comfortable as they settle down to be damned for belief or for harbouring the seeds of it. Like the Brethren, they know what they are in for.

This time, the pews will be emptied of the fence-sitters and the doubt-afflicted. The God Delusion compiles arguments that Dawkins has expounded in the past, but is distinguished by its heightened concern for the position of atheists. Dawkins relates horror stories about their pariah status in the US, such as a reported remark by George Bush Snr that atheists should not be considered citizens. His invective against agnostics and "the Neville Chamberlain school of evolutionists" who seek an accommodation with religion shows that, to those inside the embattled atheist camp, such unbelievers are free-riders evading the costs of unbelief. If there's any quivering, it's with indignation.

In its mission to deny that religion has any redeeming features, The God Delusion also jeers at theologians, depicts the God of Abraham as a monster, suggests that religion is a kind of mental parasite, dismisses it as a source of morality, argues that labelling children by religion is a form of child abuse, and of course denies the existence of God. When there's as much on the agenda as this, and passions are running high, arguments are compressed and rhetoric inflated. The allusion to Chamberlain, implicitly comparing religion to the Nazi regime, is par for the course.

The charge of parasitism is of a different order. Dawkins has always insisted that what is should not be confused with what ought to be. To say that a kind of behaviour is adaptive, in the evolutionary sense, is not to say it is good. If religious behaviour arose because it was adaptive - because those practising it thereby left more offspring than those who did not - that should make no difference to our judgment of its virtue. Killing rivals might also be a way to increase numbers of offspring, but we should still now call it murder.

Nevertheless, Dawkins includes an argument that religion is not adaptive for practitioners; the benefits, he maintains, go to the ideas. The costs of obligation, denial, ritual, punishment and all the other burdens of the supernatural are borne by believers in order to provide conditions that favour the replication of the religion "memes". If this is what has happened, in every known society throughout history and, from what we can make out, deep in prehistory, religious ideas are surely the ultimate parasites. They make the pathogens behind the Spanish flu and Black Death look like Sunday amateurs.

But another, perhaps simpler, explanation for the universality and antiquity of religion is that it has conferred evolutionary benefits on its practitioners that outweigh the costs. Without more discussion, it isn't clear that Dawkins's account should be preferred to the hypothesis that religion may have been adaptive in the same way that making stone tools was.

Dawkins argues that scientists, particularly Einstein, shouldn't be seen as believers just because they drop God's name. He notes how some fudge the issue by nodding towards a generalised awe in the face of the universe, which allows them to assume a religious mantle without believing in anything supernatural.

Turning to agnosticism, he dismisses it as a principle and reaches for Bertrand Russell's teapot: an imaginary celestial object said to orbit invisibly between Earth and Mars. This move is something of a reflex among atheists: they should adopt the teapot as their symbol. Their point and Russell's was that not being able to disprove the existence of such an object does not warrant belief in it; their implicit message is that gods are also trivial human artefacts. God is thus detached from the terrible and exhilarating question of why anything should exist at all.

Instead, Dawkins recasts agnosticism as a humdrum matter of probability captured by a spectrum of opinion-poll responses. But it is possible, along with Dawkins, to be a de facto atheist who lives on the assumption that there is no God, while remaining awed by the possibility that we cannot begin to comprehend how far beyond our comprehension the question may be. Our minds are adapted to solving problems on this planet, not the problems of the universe. Dawkins reflects upon this - but his reflections lead him to speculate that, through science, there may prove to be no limits to human understanding. To infinity, and beyond!

There are limits that cannot be passed. The premature death of Douglas Adams released Dawkins to write the most affecting words he has published. They are echoed here when the pangs of loss provoke the book's most arresting moment. He breaks off to address Adams directly - "Douglas, I miss you..." - as if the dead man were still alive. The loss of his friend is so keenly felt that he is compelled to speak of it as if he believes in the supernatural, and thus to affirm that such beliefs do bring a unique solace.

It's an isolated moment. Dawkins does not admit sympathy for believers, or acknowledge the extent to which religion may constitute their sense of identity. He disregards the risk that attacking a people's religion may amount to an attack on them as a group. Some comments and quotes in this respect are reckless. The most shocking quotes, though, are all from the Bible. His greatest polemical asset is having that particular God on his side.

Marek Kohn's 'A Reason for Everything' is published by Faber & Faber