The Da Vinci Code is a theological thriller, which makes it an unlikely bestseller in what is often said to be a secular age. Like that other bestseller, The Lord of the Rings, this book is almost completely lacking in love interest, or indeed, in sex of any kind. A rite of hieros gamos, sacred marriage, which took place some years in the past, is briefly and almost clinically described; and the hero and heroine indulge in only one, very seemly, embrace on page 587.

But bestseller it is, indeed, one of the bestselling books ever. Before considering what makes it so popular, it may be helpful to give a resume of the plot. Jacques Saunière, a curator in the Louvre, is murdered in the gallery, but has time before he dies to leave a number of cryptic clues to the secret he guards and which is the cause of his death. Robert Langdon, professor of religious symbology at Harvard, with the help of the curator's granddaughter, Sophie, who is a trained cryptographer, sets out to unravel this mystery. Saunière turns out to have been the Grand Master of the Priory of Sion, a society dedicated to guarding a secret tradition: that Jesus Christ was a mortal man, not the Son of God, that he married Mary Magdalene, who bore him a child, and that their descendants were still alive. The investigators assume that the Catholic Church knows that this secret knowledge represents the true version of the Christian faith and would feel threatened by the possibility that the Priory might make their knowledge public. Robert and Sophie suppose that the Vatican, and more specifically Opus Dei, is responsible for the murders of Saunière and of three other members of the Priory who have been killed (offstage). In the end this suspicion proves to be wrong, although it is true that a member of Opus Dei had been manipulated by the villain into carrying out the killings.

The secrets of the Priory of Sion turn out not have been very well kept after all, for both Robert Langdon, and an English scholar, Sir Leigh Teabing, on whose expertise he calls, have already worked out precisely what those secrets are, even though they are not members of the Priory, and the clues left by Saunière for the most part confirm what they already know. Teabing is described as “a former British Royal Historian”, which I take to mean that he has been a Fellow of the Royal Historical Society. They explain the mystery to Sophie, an exposition which takes up chapters 55, 56 and 58 of the book. Stated briefly, they disclose that Jesus was a mortal man, but a great religious teacher. “Understandably his life was recorded by thousands of followers...More than eighty Gospels were considered for the New Testament.” He married Mary Magdalene: “Jesus was the original feminist”, says Teabing. “He intends for the future of his Church to be in the hands of Mary Magdalene.” The Church, which its founder intended should propagate devotion to the sacred feminine, was subverted by Constantine the Great. Wishing to make Christianity the religion of his Empire, and needing to sell this idea to his many pagan subjects, he decided to synthesize reverence for Jesus of Nazareth with the cult of Sol Invictus, the unconquered sun, to which he subscribed himself. He therefore put pressure on the Church Council of Nicaea in 325AD to define the Divinity of Christ and, in support of this, the imperial Church then adopted the four canonical Gospels as the authoritative accounts of Christ's life. Mary Magdalene was no longer remembered as the wife of Jesus but as “the woman in the city who was a sinner”. The other seventy-six Gospels and their teachings were declared heretical, and the Church became recognizably the male-dominated Catholic Church, with strong misogynistic tendencies, which exist today.

The Da Vinci Code is a novel and it might seem pedantic to point out that the account of Church history given by some of the characters is misleading. But Dan Brown has prefaced his book with this statement:

Fact: The Priory of Sion – a European secret society founded in 1099 – is a real organization. In 1975 Paris's Bibliothèque Nationale discovered parchments known as Les Dossiers Secrets, identifying numerous members of the Priory of Sion, including Sir Isaac Newton, Sandro Botticelli, Victor Hugo and Leonardo da Vinci.

The Vatican prelature known as Opus Dei is a deeply devout Catholic sect that has been the topic of recent controversy due to reports of brainwashing, coercion and a dangerous practice known as “corporal mortification”. [The address of its New York headquarters is then given.]

All descriptions of artwork, architecture, documents and secret rituals in this novel are accurate.

In Teabing and Langdon's account, however, the information which they give about Jesus' original teaching and the way it was radically changed by Constantine the Great is seriously flawed. Brown's characters assume that the pre-Constantinian Church was a single community, that all its members shared the same beliefs, and that dissenting groups only sprang up when Constantine forced that Church to change its teachings and some members refused to do so. In fact, there were as many different Christian sects in the centuries before Constantine as there are today. The largest of them called itself the Great Church, or the Catholic, that is universal, Church, and that was the one which Constantine favoured and ultimately joined. Its members certainly believed in the divinity of Christ long before Constantine's day and were, indeed, persecuted by the Roman authorities for doing so. The fullest surviving account of the baptismal rite for adults in that Church is found in The Apostolic Tradition, written by St Hippolytus of Rome in c220 AD a hundred years before the Council of Nicaea. Among the questions which candidates were asked by the officiating minister was this: “Dost thou believe in Jesus Christ, the Son of God, who was born of the Holy Spirit and the Virgin Mary, who was crucified in the days of Pontius Pilate, and died, and rose the third day living from the dead and ascended to the heavens?”. The Council of Nicaea, convened by Constantine the Great in 325, was not concerned with establishing whether Jesus was divine, because there was no disagreement about that among the members of the Great Church, but with debating whether the divinity of Jesus the Son of God differed from that of God the Father (they decided that it did not). Moreover, although the full canon of the New Testament was not finally agreed until later in the fourth century, the Great Church had accepted the four Gospels of Matthew, Mark, Luke and John as the sole authentic accounts of Jesus' life in the course of the second century.

Teabing is correct in asserting that there were other Gospels available in the early Christian centuries. He claims that these have now become accessible once more through the discovery of the Dead Sea Scrolls and the Nag Hammadi codices. The Dead Sea Scrolls are an irrelevance, because they have no Christian content, but the Nag Hammadi collection of texts contains a large range of apocryphal Christian writings (that is, books which have not been included in the canonical New Testament). Although most of these were already known before the Nag Hammadi documents were discovered in 1946, those codices in many cases provide fuller and better versions of the texts. Broadly, the texts fall into two groups: those which set out to fill in gaps in the New Testament narrative (eg, accounts of the childhood of Jesus, or of the latter activities of the Apostles), and those which claim to represent the true teachings of Jesus, which the Great Church has misrepresented. The latter group are normally referred to as Gnostic writings because the authors claim a special knowledge (gnosis) of Jesus' teachings which the Great Church lacks. These writings, like the four canonical Gospels, do not date from the time of Jesus, as Teabing implies, but were all written down much later. Teabing cites passages from two of them in support of his views about Jesus' true teaching – the Gospel of Philip and the Gospel of Mary (ie, Mary Magdalene), of which only a fragment survives. Both quotations are accurate, but they do not support the inferences which Teabing (and the Priory of Sion) wish to draw from them. The Gnostic Churches, which considered that these writings represented Jesus' authentic teaching, did indeed claim that he had given a more prominent place to women among his followers than the tradition of the Great Church was prepared to admit, and that Mary Magdalene was the recipient of at least one special revelation from Jesus after he had returned to his Father.

Gnostic Christianity existed in a wide variety of forms, but all Gnostics shared a common world-view: that the phenomenal universe had come into being as the result of a cosmic accident, which had trapped spiritual souls in physical bodies and cut those souls off from participation in the Pleroma, the fullness of the Godhead. The Gnostic symbol for the human condition was “gold in the mud”: gold was the soul, and mud the human body. All Gnostics also agreed that Jesus, far from being an ordinary human being, was a divine messenger, sent from the Pleroma to give men the knowledge (gnosis) of their true spiritual condition and how to overcome it. The Gnostic Gospels give no support to the view that Christ was simply a human teacher who preached the importance of the sacred feminine. (There are many other instances of wrong information about Christian doctrine and early Church history in The Da Vinci Code and the interested reader should consult Bart D. Ehrman's Truth and Fiction in “The Da Vinci Code” , 2004.)

There is no evidence in the Gnostic Gospels, any more than there is in the canonical Gospels, to support the view that Jesus fathered a child. The idea that Jesus was descended from the royal house of David is, of course, found in the New Testament, but the belief that he and Mary Magdalene produced a royal bloodline would seem to derive from Michael Baigent, Richard Leigh and Henry Lincoln in The Holy Blood and the Holy Grail (1982). Leigh Teabing (whose name is a partly anagrammatized amalgam of two of the authors of The Holy Blood) admits his indebtedness to this source: “To my taste, the authors made some dubious leaps of faith in their analysis, but their fundamental premise is sound, and to their credit they finally brought the idea of Christ's bloodline into the mainstream”. This book was much hyped, by the BBC among others, when it first appeared, though its sales never approached those of The Da Vinci Code. The authors adduced no evidence about the bloodline of Christ which would satisfy standard historical criteria.

Teabing and Langdon in The Da Vinci Code have to offer to Sophie, and to the reader, an explanation about how the royal bloodline of Jesus and knowledge of it has been transmitted to the present day. Their account is reminiscent of one of the exam questions in 1066 and All That: “Fill in two of the following: Blank, Blank, Simon de Montfort”. After the Crucifixion, the widowed Mary Magdalene fled to France, where she gave birth to Jesus' daughter, whom she called Sarah. This is simply an adaptation of the medieval legend of how Mary, with her sister Martha, her brother Lazarus and their servant Sarah came to Provence. St Sarah is still venerated by the Gypsies of Provence. Each year they attend a festival of re-enactment, organized by the Catholic clergy, at the place where the saints are said to have landed, during which they ride into the sea and carry a statue of St Sarah ashore. Teabing then moves on several hundred years and relates how a descendant of Sarah married a Merovingian King of France, so that for a time the royal bloodline occupied a position of earthly power. The Merovingians were overthrown in 751 by Pepin, the father of Charlemagne, and Teabing skims over another 450 years to reach 1099, when the First Crusade captured Jerusalem and Godfrey of Bouillon became ruler of the Holy City. Teabing claims that he was a descendant of the Merovingian Kings, and therefore of Jesus, though in fact, Godfrey was, through his mother, a descendant of Charlemagne whose father overthrew the Merovingians.

Godrey of Bouillon, we are told, founded the Priory of Sion in 1099 to preserve the secret knowledge that had hitherto been preserved in his family as an oral tradition. Some thirty years later the Order of Knights Templar was founded and it is implied that they were subordinate to the Priory of Sion. They made their headquarters on the Temple Mount, and conducted excavations on the site of Solomon's temple where they found four enormous trunkloadsf of documents, dating from before the time of Constantine the Great. These authenticated the Priory of Sion's version of the life and teachings of Jesus, and were shipped back to their headquarters in Europe. In 1291, the Crusader Kingdom was lost to the Saracens, and the Priory of Sion and the Knights Templar withdrew to the West. There, in 1307, the Templars were arrested and put on trial on the orders of Philip IV of France, with the connivance of Pope Clement V, and their Order was suppressed in 1312. The Priory of Sion was, however, able to take charge of the vital trunkloads of documents which the Templars held and also to protect the sacred bloodline. The Priory still exists and a list of its Grand Masters is given on page 431 of the book. Yet although the Priory is dedicated to honouring the cult of the sacred feminine and although women may become Grand Masters, apparently no woman has done so since the death of Iolande de Bar in 1483. All her successors have been men, most of them very well known: Sandro Botticelli, Leonardo da Vinci, Isaac Newton, Claude Debussy and Jean Cocteau among them.

This account of the historical transmission of the secret teaching is not very satisfactory. The narrative of the first 1,000 years offers little detail and no proof. The evidence adduced from the crusading period is at first sight more circumstantial. Certainly the Order of Knights Templar was founded in 1128 and had its headquarters on the Temple Mount in the al-Aqsa Mosque, but they were not in any way connected with the Priory of Sion. The Order of the Temple was, from its inception, directly subject to the pope. It was suppressed in 1312 as the result of pressure brought to bear on Pope Clement V by King Philip IV of France. Its members were charged with heresy, sodomy and the worship of an idol called Baphomet. Few scholars think that these charges had any substance, and it is generally supposed that the chief offence of the Order was that it had great wealth and no real function after the loss of the Holy Land. Although some contemporaries believed that the Templars were guilty as charged, nobody supposed that they had been suppressed because they had secret knowledge which conflicted with the Church's teaching. (The documents of the trial have been critically examined by Malcolm Barber in The Trial of the Templars.) It was not until the eighteenth century, when the newly founded Masonic Orders began to claim that the Templars were one link in the chain which united them to Solomon's Temple, that conspiracy theories about the trial of the Templars began to appear. Masonic groups saw in the Church's suppression of the Templars an early example of its hostility to their enlightened beliefs. while conservative Catholics, such as Joseph von Hammer Purgstall, in his Mysterium Baphometi Revelatum (The Mystery of Baphomet Revealed) of 1818, denounced the Templars as a group of Ophite Gnostics who had sought to undermine the true Christian faith. The view of the Templars' role in preserving the secret teachings of Jesus and his bloodline described in The Da Vinci Code has no support in medieval sources, but is a modern variation of these nineteenth-century conspiracy theories about the Order.

There had been a Priory of Sion in the Crusader Kingdom. When the First Crusade captured Jerusalem in 1099 they found the church of Our Lady of Sion in ruins. At some point before 1112 it was rebuilt and a community of Austin Canons was endowed to serve it. The church stood on what was thought to be the site of the house in which Jesus had celebrated the Last Supper, and the Cenaculum, the chapel of the Upper Room, built by the Crusaders in c1180 to commemorate this, may still be seen there. After the loss of the Holy Land in 1291, the Austin Canons of Mount Sinai retired to their estates in Europe, and their headquarters were at Orléans until 1619, when the Priory was suppressed and Louis XIII transferred their property to the Jesuits. The modern Priory of Sion is an esoteric society which has no connection with the Austin Canons of Crusader Jerusalem. Although documents detailing its earlier history have been deposited in French archives, these also appear to be modern. Certainly none of them has been examined and authenticated by professional historians. The claims made by the Priory about historical continuity with the crusader Priory of Sion, and the list of famous Grand Masters, remain uncorroborated.

In The Da Vinci Code the Catholic Church is cast as the implacable enemy of the Priory of Sion and its secret teaching down the ages. Not merely did the papacy ruthlessly suppress the Knights Templar, it also showed its hatred of the sacred feminine by persecuting witches, but “those deemed ‘witches’ by the Church included all female scholars, priestesses, gypsies, mystics [and] nature lovers”, so Robert Langdon claims. They are all said to be victims of the papal Inquisition – but that is patently untrue, for Protestants persecuted witches with as much zeal and as little discrimination as Catholics did. (There was no papal Inquisition at work in Salem in 1692). This passage is typical of Brown's approach to Church history. There is a notable absence of any reference to Protestantism in the book. As Protestants share with Catholics a belief in the divinity of Christ and the authenticity of the canonical Gospels their reaction to the secret of the Priory of Sion would be as hostile as that of the Vatican – though this is a possibility which Brown seems anxious not to explore. When talking about the Catholic Church in the present day, the hostility of Brown's characters is directed chiefly at its conservative wing and particularly at Opus Dei, which is treated as a sinister organization, though one which admittedly performs many good works. Brown's approach to it resembles that of Alexandre Dumas to the Jesuit Order in his historical novels.

Although some readers may identify with the anti-Catholic prejudices of Langdon and Teabing, I suspect that this has had little to do with the book's popularity. The Da Vinci Code belongs to the genre of occult mystery novels, which range from Bulwer Lytton's Zanoni to the works of H. P. Lovecraft and Dennis Wheatley. The only difference is that Dan Brown is dealing with a “white” mystery – the secret teachings of Jesus. Such works, always popular, appeal to the Gnostic which is lurking in many of us, the desire to be part of an elite with privileged information. The narrator in Foucault's Pendulum by Umberto Eco speculates about this attitude: “hadn't Agliè spoken of the yearning for mystery which stirred in the age of the Antonines? Yet someone had just arrived and declared himself the Son of God ... made flesh, to redeem the sins of the world. Was that a run-of-the-mill mystery? And he promised salvation to all: you had only to love your neighbour. Was that a trivial secret? ... And yet [those who yearned for mystery] turned deaf ears. Is that all there is to it? How trite”. There are two other reasons why this book might attract a large readership. First, it is constructed as a series of interconnected puzzles. The solution to one set confronts the investigators with another. Readers are not asked to solve the clues, but if reasonably well educated they will get satisfaction from understanding them: Leonardo's diagram of Vitruvian man, the epicene figure of St John in his “Last Supper”, the significance of the Fibonacci sequence of numbers, the male and female quantities of poetic scansion. A reader who does not know about some of these points will almost certainly have friends who do, or, if not, will be able to surf the internet for answers. On one level the book is a kind of intellectual game, and will appeal to the many people who enjoy quizzes.

The second reason is very different. Although it starts as a thriller, the book develops into a Grail Quest. The Holy Grail has been a potent force in the Western imagination ever since Chrétien de Troyes wrote his Perceval in c1181. While the young hero is sitting at a dinner in the castle of the Fisher King, a procession enters, led by a page carrying a lance from which blood drips, followed by two squires with golden candlesticks, followed by a maiden, holding aloft a Grail, made of gold, studded with jewel and giving out “so brilliant a light that the candles lost their brightness”. Although Perceval later learns from a hermit that the Grail contained a eucharistic host with which the Fisher King's father was fed, Chrétien died leaving the story unfinished, so wye never learn what precisely the Grail was. This uncertainty is compounded by the fact that Wolfram von Eschenbach, writing a generation later, claimed that Chrétien had falsified the Grail story, but that he had learned the true version from Guiot the Provencal. Wolfram's Grail is a stone, brought from the Earthly Paradise, the spiritual powers of which are renewed every Good Friday when a heavenly dove lays a eucharistic Host upon it. Thus although in many later versions the Grail is treated as the chalice of the Last Supper, the descriptions given in the two earliest sources and the divergences between them has left space for alternative interpretations.

In The Da Vinci Code we are told that the chalice of the Grail signifies the womb, and in particular the womb of St Mary Magdalene which transmitted the royal bloodline of Jesus, son of David. In that sense the Holy Grail (or San Greal) would signify a person, someone who carried in themselves the royal blood (or Sang réal). The statistical probabilities are that if Jesus really had a daughter who had issue, thousands of people would be able to claim descent from her 2,000 years later, but the elitist Priory of Sion seems to envisage that there will only be one descendant. The Grail in The Da Vinci Code is said also to be the tomb of Mary Magdalene, in or beside which the Priory had deposited for safe keeping the four trunks of documents brought back by the Templars from the Holy Land, which proved the authenticity of her cult. The second half of the book is a Grail Quest in which Langdon and Sophie hunt for the shrine. It is surprising, though, that the professor of religious symbology at Harvard did not know that the same legend which reports that Mary Magdalene ended her days in France also says she is buried in the basilica of Vézelay near Auxerre, where, indeed, her shrine may still be seen.

There is no obvious attempt to model The Da Vinci Code on a medieval Grail romance: nevertheless, there are certain parallels. Then Grail quest is an individual adventure in search of spiritual fulfilment, and those who undertake it encounter a series of seemingly irrelevant obstacles and difficulties. Not the least of these is the fact that the Grail castle is very difficult to find. Langdon and Sophie experience the same problem. The final clues which they solve lead them to the Rosslyn Chapel near Edinburgh, but in the best medieval quest tradition they discover that although it had once been there, the Grail is there no longer. They come then to realize that: “You do not find the Grail. The Grail finds you”. At the very end of the book Langdon believes that he does experience the Grail in the form in which he understands it: “For a moment he thought he heard a woman's voice ... the wisdom of the ages ... whispering up from the chasms of the earth”. The phrase is more reminiscent of Rider Haggard's She than of The High Quest of the Holy Grail, but that may be part of Dan Brown's appeal.

During the Middle Ages people could see the chalice of the Last Supper in the cathedral of Genoa, where it had been deposited by crusaders from the Holy Land. Similarly the Holy Blood, which according to some versions of the legend the Holy Grail was said to contain, was preserved in reliquaries at Bruges and at Westminster Abbey; while the eucharistic Host, present in the Grail in other versions, was to be found on every Catholic altar when Mass was said. But the Grail itself could not be found in one place, having an independent life in the Western religious imagination which the Church authorities could never fully control, and of all the symbols from Christian tradition it is perhaps the one which can most easily be transposed into a post-Christian context. According to recent religious surveys, some 70 per cent of the population of Britain claims to value spirituality but to have no religious affiliation. Although such people would probably be unimpressed by the religious ideas of the Priory of Sion, they might well find the Grail quest symbolism congenial.