I'm an atheist, but all my best friends are religious

'I have not seen Buddha in the shower or decided to investigate my chakras with a Tantric priestess'

The subject of the debate, the invitation said, was: "Religion is the opium of the people". A good topic, and the university where the debate is scheduled to be held has a large and renowned theological department, so anyone proposing the motion can expect a robust, if courteous, reception. The Christian Union will be there, eyes bright and tracts in hand, and the trainee vicars and would-be missionaries (not to mention their teachers) will line the front row, scarves over their seat-backs. Perhaps a Jew or two or a few Muslims will be there as well.

The subject of the debate, the invitation said, was: "Religion is the opium of the people". A good topic, and the university where the debate is scheduled to be held has a large and renowned theological department, so anyone proposing the motion can expect a robust, if courteous, reception. The Christian Union will be there, eyes bright and tracts in hand, and the trainee vicars and would-be missionaries (not to mention their teachers) will line the front row, scarves over their seat-backs. Perhaps a Jew or two or a few Muslims will be there as well.

But should I be there? I long ago stopped arguing with the religious about religion. It was fun when I was younger, but it feels indulgent now. I am - have always been - an atheist, not an agnostic or a New Ager. I actively believe that there is no supernatural force that guides our destinies; and I don't feel any need for one. The marvels of the universe are - for me - more marvellous without a God than with one.

Next, I could set the religions against one another. Most - since the good old days of pantheism, when any person's God was as good as any other's - have demanded acceptance that their chosen faith (usually the one prevalent in the place where the believer was brought up) is the one true one, and that all the others are false. Since only one exclusive faith can be right, it follows that all the others are wrong. And if all the others can be wrong, then it's pretty likely that the first one is, too.

They also make the impassioned part of this speech - the historical denunciation - much simpler. Let's take the story of institutional Christianity as our main example. What a sorry tale! From the earliest days, the church was beset by schism, which was usually dealt with by declaring the losing side in a theological argument to be heretic and then excommunicating it - or worse. Monophysites, Arians, Donatists, Paulicians, all found themselves in the wilderness. Let alone the Bogomils, the Cathars and various other Manichaean sects, who were hunted to extinction by the forces of orthodoxy.

We end in the squares of Renaissance Spain, as Charles V or his representatives, accompanied by the men of the Holy Inquisition, watch the heretics of Valladolid, or Madrid or Cadiz, burn to death for their incorrect beliefs. The same cross, bearing the representation of that same Christ whose birth we are supposed to celebrate the day after tomorrow, was held aloft in those hellish plazas - and cast its shadow over the whole of free-thinking Europe. In much the same way (I could add) as the evangelical Islam of the Taliban does now in Afghanistan, or as the ayatollahs did, briefly, in Iran.

Take Catholicism (an altogether more obvious target here, among the inoffensive Anglicans). Here's a church that spreads tales of crying statues, peddles supernatural shrouds and parades the ankle-bones of St Cecilia. And, at the same time as it encourages such peasant superstition, its pontiffs and prelates live high on the hog, notorious for worldly hypocrisy. They believe that babies are evil at birth (Original Sin), that the Pope is infallible, and that celibates can instruct couples. With their teachings on abortion and birth control, they have helped to condemn the Third World (and not a small part of the First and Second Worlds as well) to poverty and despair. Case closed.

Case reopened. Twenty years ago, when I was a student, the above is what I would have argued. Two decades on, and contemplating the university's invitation, I realise that - if I accept - I will have to speak against the motion. And that's not because I have gone religious, seen Buddha in the shower or decided to investigate my chakras with a Tantric priestess. But my views on what religion can do to people have changed. To put it as simply as possible, many of the best, most truly moral people I know are religious; that requires some explanation.

In Christianity there has always been, as we know, a series of tensions between authoritarians and liberationists. In 411 a British monk called Pelagius stopped over in Rome on his way from these islands to Jerusalem. (Incidentally, if any fact makes one query the school version of the fall of the Roman Empire, supposed to have disintegrated into Dark-Ages anarchy AD410, this journey is it.) Pelagius had come to see Augustine, but that sage had nipped over to Africa to deal with the Donatists and left his mobile turned off. The two were to become not enemies, but opponents.

Pelagius did not believe in Original Sin, nor - therefore - in the transmission of sin from generation to generation by the sexual act. His view was that man had a free choice to behave in such a way as to enjoy God's grace. It was not preordained who was saved and who was damned. Nor was it enough to believe hard - one had to do good deeds, to live well. As the controversy grew, Pelagius's opponents became more vituperative, St Jerome calling him a "corpulent dog... full of Scots porridge".

Pelagius lost. But he was just an example of how, at the heart of much religious debate for thousands of years, is the question of how people can live "good" lives. Religions have been the major forum within which human beings have discussed their moral behaviour. And if we are to be impatient with the Catholic church for its stand on abortion, can we not at least agree that we do need some force that reminds us at all times of the value of human life? I may disagree with Catholics on stem-cell research and (more uncertainly) on euthanasia, but I am certain that we need to debate these questions.

For every burning of a witch or a heretic, the secular world has managed to murder 10 people for being the wrong nationality or in the wrong place. Often, those who have stood out most courageously against the great tyrannies have been religious people, whose morality transcended the political moment. In 1942, the student leaders of the White Rose movement in Germany were convicted and beheaded on the same day for handing out anti-Hitler leaflets at their university. They were motivated by their faith.

Quakers resisted the divine right of the Stuarts and became a necessary part of almost all the great progressive movements in this country after the 17th century. The pioneers of the abolition of slavery in America were churchgoers, roused by speeches from the pulpit and calls to their conscience. The secular world, meanwhile, saw slavery as a practical question. Who, then, should the slaves have looked to for their delivery?

In September, as the queues lengthened at the petrol stations, I went to Twickenham to speak to the supporters of a centre for refugees and asylum-seekers. The meeting was in a church hall, presided over by a Methodist minister, and many of the workers, helpers and friends were members of the congregation. They were the personification of conscience. And the alternative to them, all too often, is no conscience at all. Happy Christmas to them.

David.Aaronovitch@btinternet.com