Imagine, sang John Lennon, a world with no religion. Imagine no suicide bombers, no 9/11, no 7/7, no Crusades, no witch-hunts, no Gunpowder Plot, no Kashmir dispute, no Indo/Pakistan partition, no Israel/Palestine wars, no Serb/Croat/Muslim massacres, no Northern Ireland 'troubles'. Imagine no Taliban blowing up ancient statues, lashing women for showing an inch of skin, or publicly beheading blasphemers and apostates. Imagine no persecutions of the Jews - no Jews to persecute indeed, for without religion they would long ago have intermarried with the surrounding populations.

Of course today's religious killings and persecutions are not motivated by theological disputes. IRA gunmen don't kill Protestants (or vice versa) over disagreements about transubstantiation. The motive is more likely to be tribal vengeance. It was one of 'them' killed one of 'us'. 'They' drove 'our' great grandfathers out of our ancestral lands. The grievances are economic and political, not religious, and the vendettas stretch back a long way.

But although the tribal disagreements themselves have nothing to do with religion, the fact that there are two tribes at all has everything to do with religion. There are, no doubt, tribal distinctions of genetic or linguistic origin, but in Northern Ireland what else is there but religion? The same applies to Indo-Pakistan, Serbo-Croatia, and various regions of Indonesia and Africa. Religion is the world's most divisive label of group identity and hostility. If a social engineer set out to devise a system for perpetuating today's most vicious enmities, he could not come up with a better formula than sectarian education. Faith schools that taught all religions comparatively might do some good. But the whole point of faith schools is that the children of 'our' tribe must be taught 'their own' religion. Since the children of the other tribe are simultaneously being taught the rival religion with, of course, the rival version of the vendetta-riven history, the prognosis is all too predictable.

What can it mean to speak of a child's 'own' religion? Imagine a world in which it was normal to speak of a Keynesian child, a Hayekian child, or a Marxist child. Or imagine a proposal to pour government money into separate primary schools for Labour children, Tory children, LibDem children and Monster Raving Loony children? Everyone agrees that small children are too young to know whether they are Keynesian or Monetarist, Labour or Tory, too young to bear the burden of such labels. Why, then, is our entire society happy to slap a label like Catholic or Protestant, Muslim or Jew, on a tiny child? Isn't that, when you think about it, a kind of mental child abuse?

I once made that very point in a broadcast debate with a Roman Catholic spokeswoman. I've forgotten her name but she was probably some kind of agony aunt, and a stalwart of Thought for the Day. When I said that a primary school child was too young to know whether it was a Catholic child or a Protestant child, she bristled: "Just come and talk to some of the children in our local Catholic school! I can assure you they know very well that they are Catholic children." Well yes, I believe it only too well. The Jesuit boast - "Give me the child for his first seven years, and I'll give you the man" - is no less sinister for being familiar (in various versions) to the point of clich?.

But, you may ask, what if religion is true? (What if my particular religion is true, you should rather say, for mutually contradictory beliefs can't all be true.) Surely sectarian indoctrination would not be child abuse if it saved the child's immortal soul? Despite its smug presumptuousness, I can see how you might take that view if you sincerely believed you had the God-given truth. Let me, then, be ambitious if not presumptuous, and try to persuade you that you do not have the truth. Your confidence in your God is simply wrong!

Why do you believe in your God? Because he talks to you inside your head? That is surely not a reliable argument. The Yorkshire Ripper's murders were ordered by the perceived voice of Jesus inside his head. The human brain is a consummate hallucinator, and hallucinations are not good grounds for beliefs about the real world. Perhaps you believe in God because life would be intolerable without him. That's an even weaker argument. Maybe life just is intolerable. Tough! All sorts of things are intolerable, but it doesn't make them untrue. It may be intolerable that you are starving, but you won't make a stone edible by believing - no matter how passionately and sincerely - that it is made of cheese.

By far the favourite reason for believing in God is the argument from improbability. Eyes and skeletons, hearts and nerve cells are too improbable to have come about by chance. Man-made machines are improbable too, and they are designed by engineers for a purpose. Surely any fool can see that kidneys and wings, ears and blood corpuscles must also be designed for a purpose, by a master Engineer? Well, maybe any fool can see it, but let's stop playing the fool and grow up. It is 146 years since Charles Darwin gave us what is arguably the cleverest idea ever to occur to a human mind. He demonstrated a working process whereby natural forces, with no design whatsoever, can by slow, gradual degrees generate an elegant illusion of design, to almost limitless levels of complexity.

I have written books on the subject and I obviously can't repeat the whole argument in a short article. Let me give just two guidelines to understanding. First, the commonest fallacy about natural selection is that it is a theory of chance. If natural selection really were a chance process, it is entirely obvious that it could not explain the illusion of design. But natural selection, properly understood, is the antithesis of chance. Second, it is often said that natural selection makes God unnecessary, but leaves his existence an entirely open possibility. I think we can go further. The argument from improbability, which traditionally is deployed in God's favour, turns out to be, when you think it through, the strongest argument against him.

The beauty of Darwinian evolution is that it explains the very improbable, by gradual degrees. It starts from primeval simplicity (relatively easy to understand) and works up, by plausibly small steps, to complex entities which, by any non-gradual process, would be too improbable for serious contemplation. Design is a real alternative, but only if the designer is himself the product of an escalatory process such as evolution by natural selection, either on this planet or another. There may be alien life forms so advanced that we would worship them as gods. But they too must be ultimately explained by gradual escalation. Gods that exist ab initio are ruled out by the Argument from Improbability, even more surely than are spontaneously erupting eyes or elbow joints.

Religious faith is not only a major force for evil in the world. Its very foundations are undermined and denied by scientific logic. Imagine a world where nobody is afraid to follow such thoughts wherever they may lead.