Christianity is a materialist religion. Its central doctrine, the Incarnation, asserts that in the life of the man Jesus, God united himself to human flesh and thereby opened matter itself to the possibility of divinisation. So Christians place their eschatological hope not in the survival of a disembodied soul, but in the resurrection of the body, the transformation of spirit and flesh into another order of being. The magnificent exhibition just opened at the British Museum explores the historical, cultural and artistic consequences of this belief. The cult of relics lay at the heart of Christian practice for a millennium and a half, and remains to this day for many believers in both East and West an important expression of their faith. The exhibition traces the history of the veneration of relics and the strange and beautiful objects it inspired.
Some of the most wonderful artefacts of the Middle Ages are on display, assembled not only from great collections such as that of the Vatican or the Cluny Museum, but from churches and individuals for whom these exhibits are not museum curiosities but sources of sacred power. The first object the visitor encounters on entering the exhibition is a glorious gilded 12th-century reliquary bust of St Baudime, on loan from the village of Saint-Nectaire le Haut in the Auvergne. Its usual resting place is not a museum but the parish church: it was sent on its travels to Cleveland, Baltimore and now London with some reluctance, and on its return it will be processed home with prayers and singing, incense and flowers. The final object in the exhibition is the Vatican's relic of the Mandylion of Edessa, a likeness of the face of Christ supposedly created by direct contact with the living Jesus. It is on loan not from the Vatican galleries, but from the Pope's private chapel, an object of devotion rather than a work of art. And a few weeks before the exhibition opened, Orthodox believers from the Georgian community in London came to the museum led by their bishop, to venerate one of the tiniest items on display, a reliquary pendant of St Demetrios, one of the great saints of Georgia.
All the essentials of the cult of relics are present in the very first eyewitness account of a Christian martyrdom, the execution by burning of Bishop Polycarp of Smyrna in AD156. The narrator tells how he and others "took up (Polycarp's) bones, which are more valuable than precious stones and finer than refined gold, and laid them in a suitable place, where the Lord will permit us to gather ... to celebrate the birthday of his martyrdom". The martyr was purified by suffering like gold refined in a furnace, his flesh transfigured by the presence of God's spirit. His mortal body might suffer, and be turned to ash, but in the light of eternity it shone like the imperishable jewels it would resemble at the resurrection.
So the martyrs' tombs were conduits of joy and blessing, trophies of the victory of these champions of Christ and an anticipation of Heaven itself. That conviction sent early Christians flocking to the shrines in search of healing and help, and eventually led to the holy bones being brought into the heart of the cities (against all the instincts of pagan antiquity) and placed under the altars on which Christ's own body was consecrated in the Mass. Soon every altar was required to contain relics, and the bodies of the saints were divided to meet the demand. The saint was beyond the restrictions of this world, a patron as fully present to his clients and venerators through a finger or a rib as in a whole skeleton, so that even small fragments of a holy body came to be treasured as a meeting place of Earth and Heaven.
Relics were processed in the liturgy, travelled to on pilgrimage and, very soon, kept and carried around by individual Christians as powerful talismans. Solemn oaths and contracts were sworn on them, they were dipped in water that was drunk to heal disease, they were carried into battle to confound the enemy. But relics, however gruesomely they had originated, were never valued as mere souvenirs of horror and suffering. They were tokens of the glorious destiny of the redeemed. And if the bones of the saints were indeed as precious as jewels, then they must be shrined appropriately - in ivory, gold, silver, precious or semi-precious stone - materials that signalled the relics' future transfiguration.
Relics exuded power: proximity and touch were important, but it was not essential to see them. In the early and high Middle Ages, they were often kept for veneration in closed containers - purses, caskets, coffins or ornate boxes, the boxes often gabled to resemble a church. Such reliquaries both concealed the relic itself and, by enclosing and surrounding it, symbolised not the process of dismemberment and decay that was the martyr's death, but the gathering together of the redeemed that was the Kingdom of God. Towards the end of the Middle Ages, these kinds of closed containers increasingly gave way to reliquaries designed to expose the relic to view, crystal monstrances and shrines that elaborated the symbolism of the relic, like the sumptuous gold and enamelled shrine of the Holy Thorn on display here, made for one of the greatest patrons of 15th-century French art, Jean, Duke of Berry, and displaying the Thorn between the knees of the tiny figure of Christ the Judge.
From the 12th century, "speaking reliquaries" became common, containers dramatically and sometimes grotesquely fashioned in the shape of a body part - head, foot or arm - often with a crystal window that emphasised the dismembered fragments they contained. Reliquaries and relic chapels displaying ranked rows of bone and other body fragments became increasingly common, their multiple open chambers similarly drawing attention to the fact of the dismemberment of the holy bodies from which the relics were derived, and thereby insisting on their materiality. One late medieval reliquary on loan from the Basilica of Santa Croce in Rome brings together these apparently contradictory kinds of display by assembling dozens of relic fragments around a Byzantine mosaic icon of the dead Christ, an image that became known all over late medieval Europe through myriad copies. In the icon, the Saviour's broken body is constituted by the tiny tesserae of the mosaic, just as His mystical body the Church was made up of the bodies of the saints, present here in the rows of relic fragments that the shrine displayed.
But visitors to this sumptuous exhibition are likely to be struck by how few of these glorious objects in fact retain the relics they were made to hold. The reliquaries themselves have become relics, in many cases bearing the scars of their survival against the odds. Many of the relics most treasured in medieval Europe were themselves the spoils of violence and betrayal, looted from the churches of Constantinople during the Fourth Crusade to feed mounting Western fever for "authentic" relics, especially of the True Cross. And the West has its own internal history of religious destruction. The wonderful bust of St Baudime once glittered with jewels and Roman cameos, where now there are only empty sockets. He at least is still venerated, having been rescued after the French Revolution from a local wine cellar. But most of the objects here have survived not as windows into Heaven, but as objets d'art, collectors' items stripped of the sacred treasure they once contained by reformation iconoclasm or French Revolutionary contempt for "gothick" superstition. Yet with or without their holy contents, these wonderful containers still have the power to stop us in our tracks.
Eamon Duffy is professor of the history of Christianity at the University of Cambridge, and a Fellow of Magdalene College. Treasures of Heaven: Saints, Relics and Devotion in Medieval Europe is on at the British Museum until 9 October.