In their attempt to argue that effective and binding codes can be developed without a deity, atheists often mistake inferior codes - "common decency" - for absolute moral systems.

The Golden Rule, or doing as you would be done by, is such a code. But the fact that men can arrive at the Golden Rule without religion does not mean that man can arrive at the Christian moral code without religion.

Christianity requires much more, and above all does not expect to see charity returned. To love thy neighbour as thyself is a far greater and more complicated obligation, requiring a positive effort to seek the good of others, often in secret, sometimes at great cost and always without reward. Its most powerful expression is summed up in the words, "Great love hath no man than this, that a man lay down his life for his friends."

It is striking that in his dismissal of a need for absolute theistic morality my late brother Christopher states that "the order to 'love thy neighbour as thyself' is too extreme and too strenuous to be obeyed." Humans, he says, "are not so constituted as to care for others as much as themselves."

This is demonstrably untrue, and can be shown to be untrue, first through the unshakeable devotion of mothers to their children; through thousands of examples of doctors and nurses risking (and undergoing) infection and death in the course of caring for others; in the uncounted cases of husbands caring for sick, incontinent and demented wives (and vice versa) at their lives' end; through the heartrending deeds of courage on the battlefield, of men actually laying down their lives for others.

We all know these things happen. If we are honest, they make us uncomfortable because we are not sure that we could do such things, though we know them to be right and admirable.

In a society where the absolute code has been jettisoned, and we have all become adept at making excuses for shirking such duties, selflessness of this kind will become less common, nursing less dedicated, wives more inclined to leave their babbling husbands in care homes to be looked after impersonally by paid strangers and perhaps encouraged gently down the slope of death, soldiers readier to save themselves while their comrades lie in pain within reach of the enemy.

And there will always be a worldly relativist on hand (as there already is at every marriage break-up and every abortion clinic, and increasingly by the bedside of the old and sick) to say that this was only sensible, to urge that we do the easy thing, and to say that it is right to do so.

Christianity is without doubt difficult and taxing, and all of us must fail to emulate the perfection of Christ himself, but we are far better for trying than for not trying, and we know that there is forgiveness available for honest failure.

My brother's suggestion that we are urged to be superhuman "on pain of death and torture" reveals a misunderstanding both of the nature of the commandments and of the extent of forgiveness. There is also some excuse-making involved.

The difficult is being described as superhuman. Yes, there is fear in the Christian constitution, as there must be in any system of law and justice. I should be dismayed if deliberate unrepentant wickedness did not lead to retribution of some kind. But there is far more love offered for those who honestly seek to follow the law, and unbounded forgiveness for all who seek it - even those who have most vigorously defamed the faith and then embrace it just before the darkness falls.

And that is why, while it is perfectly possible for convinced atheists to do absolutely good deeds at great cost to themselves, not least because God so very much wishes them to, it is rather more likely that believing Christians will do such things. And when it comes to millions of small and tedious good deeds which are needed for a society to function with charity, honesty and kindness, a shortage of believing Christians will lead to that society's decay.

We can live at a low level of co-operation by mutual consideration. But as soon as we move beyond subsistence and the smallest units, problems arise which cannot be resolved by mutual decency. Some men grow richer, some are stronger, some acquire weapons.

Power comes into being at a very early stage in human society. So do competition for scarce resources, and greed, and wars with other groups. Mutual benefit ceases to offer any kind of guide to behaviour. Who is to say, in a city ruled by a single powerful and ruthless family from an impregnable fortress, that the strongest man is not also always right?

In fact, the godless principle that the strongest is always right has been openly declared as recently as the twentieth century in Mussolini's Italy, and operated in practice in Hitler's Germany, Stalin's Soviet Union and many other states.

In wars, men are repeatedly asked to undertake acts of selfless courage which they will not themselves survive. Men are expected to be responsible for the women who bear their children, for as long as they live. Women in return are expected to be faithful to those men. For economies to develop, men must be trusted to guard valuables which are not their own.

Again and again, for civilisation to exist and advance, human creatures are required to do things which they would not do "naturally" as mammals. Marriage is unnatural. Building for the future is unnatural. The practice of medicine is unnatural. The deferment of immediate gratification for a greater reward is unnatural. Charity is unnatural. Education is unnatural. Literacy is unnatural, as is the passing on of lore and history from one generation to another.

Christian societies as a whole are "unnatural," requiring a host of actions which cannot be based on self-interest, however enlightened, or even on mutual obligation.

Meanwhile, the more civilised a society is, the more power is available within it. Power cannot be destroyed, only divided and distributed. It may shatter into an anarchic war of all against all. Or it may solidify into a tyranny. Or it may be resolved into a free society governed by universally acknowledged laws.

But on what basis can this be done? What agency can be used to place law above force? A law that does not stand above brute force, and have some sort of power that can overcome brute force, will not survive for long. How are inconvenient obligations, those of the banker and the messenger and the merchant, to be made binding? How are the young to made to accept the authority of parents and teachers, once they are physically strong enough to ignore them, but too inexperienced in life to know the value of peace and learning?

The answer, from a very early stage, was that such contracts were made binding by solemn oaths sworn in the name of Almighty God and, as Abraham Lincoln used to say of his Presidential Oath, "registered in heaven." These oaths called into every contract an external power, one whose awful vengeance no man could escape if he defied it, and which he would be utterly ashamed to break.

As Sir Thomas More explains in Robert Bolt's play A Man for All Seasons, "When a man takes an oath ... he's holding his own self in his own hands. Like water. And if he opens his fingers then - he needn't hope to find himself again."

In their utter reverence for oaths, men of More's era were in my view as superior to us as the builders of Chartres Cathedral were to the builders of vast urban housing projects. Our ancestors' undisturbed faith gave them a far closer, healthier relation to the truth - and so to beauty - than we have.

Without a belief in God and the soul, where is the oath? Without the oath, where is the obligation? Where is the law that even Kings must obey? Where is Magna Carta, Habeas Corpus or the Bill of Rights, all of which arose out of attempts to rule by lawless tyranny? Where is the lifelong fidelity of husband and wife? Where is the safety of the innocent child growing in the womb? Where, in the end, is the safety of any of us from those currently bigger and stronger than we are?

And how striking it is that such oaths were used to make men better, not worse, that the higher power, the magnetic north of moral truth, found an invariable answer in the urgings of conscience. These things are far higher than the mutuality and "human solidarity" on which the atheist must rely for morality - because he specifically denies the existence of any other origin for it.

This is not, alas, an argument for or against the existence of God, though it might just be an argument for the existence of good, with humankind left wondering how to discover what is good and what good is. It simply states the price that must sooner or later be paid for presuming that God does not exist and then removing him from human affairs. It also sets out the important benefit that can be obtained by placing God at the heart of a society.

I should have thought that those who are serious about their unbelief would be relieved by this logic, and glad to concede it. If they know, or are reasonably certain, that there is no ultimate authority and no judgement, they are instantly quite extraordinarily free. If they have a desire to become all-powerful, they are immensely free, since without God, law has no origin except power, and the more powerful they are, the more free they are.

But this freedom is as available to monsters and power-seekers as it is to advanced intellectuals dwelling in comfortable suburbs. And that leads to the state of affairs correctly summed up by the German philosopher Martin Heidegger, who in 1933 proclaimed from among the lovely towers and groves of the university at Freiburg-im-Breisgau, "The Fuhrer, and he alone, is the present and future law of Germany." Alas, he was absolutely right, and Adolf Hitler himself had to be destroyed before that law could be cancelled.

If atheists or anti-theists have the good fortune to live in a society still governed by religious belief, or even its twilit afterglow, they are free from absolute moral bonds, while those around them are not. This is a tremendous liberation for anyone who is even slightly selfish. And what clever person is not imaginatively and cunningly selfish?

Oddly enough, very few atheists are as delighted by this prospect as they ought to be. At least they are not delighted openly, or in public. Could this be because they really do not grasp this astonishingly simple point, based as it is on their own insistence that the only conceivable external source of law and morality does not exist? Why create such a difficulty for themselves at all?

Might it be because they fear that, by admitting their delight at the non-existence of good and evil, they are revealing something of their motives for their belief? Could it be that the last thing on earth they wish to acknowledge is that they have motives for their belief, since by doing so they would open up their flanks to attack?

Peter Hitchens is a columnist and reporter for the Mail on Sunday. In 2010 he was awarded the Orwell Prize for journalism. This article draws on his most recent book, The Rage against God.