"The grail was the bowl made of precious material from which Christ and the apostles ate at the Last Supper or, in some versions, the chalice from which they drank. Joseph of Arimathea found it after Christ had been arrested. At the crucifixion, when the soldier pierced Christ's side with a lance, he used it to collect the blood. Its miraculous properties provided nourishment without limit, and kept men eternally alive. For centuries it was lost, concealed and guarded by a distant king, an evil magician, a brotherhood of knights, a cohort of angels, depending on which version you read. Finding it became the goal of Percival, Lancelot, Gawain and other knights of the Arthurian Round Table. It was a supreme test of Christian heroism and male purity, whose significance consisted precisely in the fact that it was out of reach, never to be found unless, perhaps, immediately lost again.
"And what, Mr Rossetti, were they going to do with the grail when they found it?" Benjamin Jowett is supposed to have asked the painter as he worked on the murals of the Oxford Union. It is a good question. The modern fame of these legends is due largely to Rossetti's contemporaries, mystics for whom they represented an escape from the visual and social ugliness of modern industrial societies into a world of powerful natural forces and reassuring moral absolutes. It is still hard to think about the grail stories except in the colours of Burne-Jones, Tennyson and Wagner.
The story was already seven centuries old in their day, and older still if we believe some authorities. Yet despite the heroic deeds of the American scholar RS Loomis, who spent his life striving against impossible odds to prove an ancient Welsh origin for the legend, all the evidence suggests that it emerged at the end of the 12th century from the fertile mind of the French poet Chrétien de Troyes. He used some incidental ideas he had found in older work, but basically thought it up for himself. The Welsh setting was due partly to the fame of Geoffrey of Monmouth, who had reinvented King Arthur about half a century before; but mainly to the fact that Wales was far enough away from northern France for almost any statement about it to be credible.
Chrétien's grail stories were left unfinished at his death in about 1191, and were continued by a variety of French romancers. Many of the best-known incidents, including the involvement of Joseph of Arimathea, were added by the early-13th-century French writer Robert of Boron. But the most famous and complete version was the massive German epic Parzifal, written shortly afterwards by Wolfram von Eschenbach. He rewrote Chrétien's story, transforming the grail into a miracle-working stone, adding a mountain of circumstantial detail and continuing the story for book upon book in ornate and convoluted German. Here was "holy German art" with a vengeance. It was of course in this form that the story came to Richard Wagner.
Wagner's vision of the middle ages was a romantic fantasy. Yet the striking thing about it is that it is an authentically medieval romantic fantasy. Chrétien and his followers wrote stories of personal moral redemption for an age equally obsessed with sinfulness and decay, and for a class whose way of life made personal redemption at the same time infinitely desirable and virtually impossible. The three main themes of the early grail legends, all of them put forward as answers to these dilemmas, were the veneration of relics, the power of the Eucharist and the spiritual aspirations of knighthood. Wagner discarded the relic-worship and the Eucharistic associations. But the rest is there: the corruption that grows out of experience, the wickedness of the flesh and the resulting association of evil with physical ageing and decay, the redemption of mankind by a single, uncorrupted hero. The grafting of religious ideals on to the crude and secular business of military service was one of the great educational achievements of the medieval church, even if the ideal rarely made contact with the reality. Perhaps the same thing could be done for 19th-century capitalists, no doubt with the same equivocal results.
Twentieth-century grail fantasies are harder to admire. Most of them have in common a perverse determination to believe that all potent legends must have roots in the earliest experience of mankind. This leaves no place for literary invention, and leads them to ignore the fundamentally Christian character of the grail legend. The rot set in with Jessie Weston, who, like many of her contemporaries, read too much of Frazer's Golden Bough for her own good. She believed that the grail legend was a survival from pre-Christian fertility rites. Others have argued for an origin in Zoroastrian belief, or Jewish passover rituals, or the darker side of the kabbalah - from anywhere, in fact, other than the spiritual tensions of the Christian middle ages. It is clearly possible for a writer to take a universal theme, such as innocence and corruption, and to express it in a wholly original fictional form. Yet many people who have no difficulty in accepting that Tolstoy made up the life of Anna Karenina seem quite unable to accept that Chrétien did the same for Sir Percival.
Oddly enough, the doubters represent the respectable end of the educated world. What is a sane man to say about the assorted Rosicrucians, Theosophists, occultists, satanists and conspiracy theorists who have since taken possession of the grail legend? Or that modern publishing phenomenon, The Holy Blood and the Holy Grail , which must rank as one of the silliest books of the 20th century? Or the various objets d'art periodically presented to the public as "the" holy grail, such as the notorious Glastonbury chalice "dug up by a grain broker", as Mark Twain observed, "at no cost of blood or travel, and apparently no purity required of him other than the average purity of a 20th-century dealer in cereal futures"?
Richard Barber has written a valuable and agreeably sensible account of the literary origins of the grail legend, as well as its subsequent fortunes. He is a serious scholar and a brave man, who is not afraid of making enemies, and has trodden on plenty of scholarly corns as well as a fair number of unscholarly ones. This is not a contentious or argumentative book. It bangs no drums and blows no trumpets, but begins and ends with the evidence. There is no grand theory. There is no one explanation that accounts for every feature of the grail legend. There is no key to all mythologies. There is no need for one. As a symbol of something unattainably good, the grail is a cumulative product of the imagination of different individuals at different times, continually reclothed and reinterpreted, but always representing the same challenging human aspiration.
· Jonathan Sumption's two-volume history, The Hundred Years War, is published by Faber.