Dan Brown has resurrected a heresy that rattles the Church Jesus had a baby, yes Lord. Jesus had a baby, yes my Lord. It sounds pretty blasphemous, put like that, doesn't it? The only reason I dare to begin with those words is that they represent the beliefs of growing millions of otherwise sane British adults. Yup, folks, we all seem to be swallowing the new gospel. You on the Tube, madam, turning the pages with such narcosis that you miss your stop: you believe it, don't you? You, sir, sneaking your dog-eared copy off to the loo for a quick fix - you think there's probably something in it, too, hmmm? According to astonishing statistics from the Roman Catholic Church, 22 per cent of British adults have now read The Da Vinci Code by Dan Brown, and of those an amazing 60 per cent believe that, yeah, it is probably the case that Jesus indeed got married to Mary Magdalene and sired a line of descendants. By my maths, that means that there are at least six or seven million people in this country who now believe that it's true: that for two millennia the Roman Catholic Church has been engaged in a desperate struggle to conceal the existence of the Christ family, and that they are probably all over the place: behind the fish counter at Sainsbury's; creating loaves for Hovis; causing people to rise from their beds in hospital. They could be anywhere. They could be reading this paper. They could (gulp) be you. There is something in the logic of Dan Brown's book that has convinced millions that they have really uncovered the biggest, the spookiest, the most chilling conspiracy in history. Never mind the autoflagellant cowled assassins and the idiotic anagrams. This story has clearly touched something in the popular psyche, and if you need any evidence, look at the global panic that book and film seem to have induced in the Roman Catholic Church. In the Vatican, the papal portavoce has described this pot-boiler as "shameful and unfounded lies". In India, no fewer than 200 Christian organisations have succeeded in having the film blocked from release, and even here in placid little Britain the officials of Cardinal Cormac Murphy-O'Connor, the Archbishop of Westminster, have called for it to carry a "health warning". You may think that the Church is barmy to get so hot under the dog-collar, and you may think that Austen Ivereigh, the Archbishop's public affairs man, has forgotten the golden rule of his trade. Why, you may ask yourself, are they rising to the bait? And yet the more one thinks about the doctrinal message of The Da Vinci Code, the clearer it is that the Catholics are right to think this a seditious text. It is not just the sex. Among Dan Brown's assertions is that Jesus had a long, loving and matrimonial relationship with Mary Magdalene, a former prostitute. This is, of course, a vaguely embarrassing allegation to make about a man who has always been taken to be a model of chastity, but it does not seem in itself a fatal blow to Christianity. They were married, says Dan Brown; there is no suggestion of fornication; and plenty of other early Christians were married and had children. No, it is not the News of the World aspect of the book that worries the Church, or which is now filling the shelves of WH Smith with Da Vinci-ana. It is the simple possibility of Christ's reproduction that is so mesmerising; and, in discussing this idea with such awful readability, Dan Brown has reopened a controversy that the Church thought had been settled in ad325. The reason this piffle is such a howling hit is that it resurrects the great unspoken doubt in the minds of all Christians, that has existed ever since the doctrine of the Incarnation. It is about whether Christ can really be man and God at once. If you walk round the Louvre at a less frenzied pace than Tom Hanks and co, you will notice a fascinating gradual change in the depiction of the ancient gods. As the human race gains in intellectual self-confidence, the image of the divine becomes more and more anthropomorphic. Egyptian jackals, Babylonian curly-bearded cow-hoofed centaurs: they all give way to the human-shaped gods of the Greeks and the Romans until finally, at the very moment when the Romans have first declared that their emperor is a god, a Jewish heresy also announces that God has been made man in the form of Christ, and from then on there were those who couldn't get their heads round it. If he was a god, how come he died? And if he was a man, how did he rise from the dead? From the very beginning of Christianity, there were Gnostics, who contested the full divinity of Christ, and by the third century AD the chief exponent of this type of view was a Libyan Christian bishop called Arius. The Catholic Church said Christ was of the same substance as the father, coeternal. No, no, said Arius, he couldn't be of the same substance; he was just similar; he was just a chap really; not homoousios, but homoiousios. Arius spoke for everyone who has ever said that "Jesus was a really great guy and a great teacher, but I don't think he was really the biological son of God". He had many supporters, and the wrangle engulfed the Christian world until Constantine settled it rather incompetently at the Council of Nicaea in 325, and the doctrine of the Trinity was pronounced. But the controversy rumbled on for hundreds of years, until it produced its most potent successor, Islam, which regards the idea of the son of God as blasphemous. By depicting Jesus as a man who fathered, Dan Brown is making the same objection as Arius, and putting his finger on the logical problem in the doctrine of the Incarnation. Are the descendants of Christ meant to be divine? Patently not. But why not, if Jesus was God? The answer must be that Jesus was not of one substance with the father, and that is why the Catholic Church is so rattled. This book may be bilge, but it awakens an ancient and distinguished heresy. Dan Brown is the new heresiarch, and I vote that he, the Pope, Austen Ivereigh and the rest of us convene a new Council of Nicaea to settle the matter.