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Holy Thorn Reliquary (made around 1370), from Paris

I'm in Paris, and I'm in what was one of the greatest museums of medieval Europe. Although it doesn't look like a museum - it's a two-storey gothic building, with soaring architecture and spectacular stained glass. It's the Sainte Chapelle, the palace church of the kings of France, built in the 1240s to house what were then the most precious objects in the world. And supreme amongst those objects was without question the Crown of Thorns, placed on Christ's head before the crucifixion - and a relic of the utmost sanctity. For medieval Christendom the key purpose of life in this world was to secure salvation in the next, and relics of the saints offered a direct line to heaven. No relics were more powerful than those associated with the suffering of Christ himself - or more valuable. This amazing church, created to exhibit the king's collection of relics, cost 40,000 livres to build; the Crown of Thorns alone cost the king over three times that amount. It was probably the most valuable thing in Europe. The most precious gift that the king of France could make was a single thorn, detached from the Crown, and the object in this programme is one of those thorns, and the reliquary made to house it.

"It's not just an object, it's got the devotion of centuries behind it. It looks like a window into another world, which I think is what it is." (Sister Benedicta Ward)

This week's programmes are about how we talk to our god, or our gods. The objects are, if you like, just aids to conversation, but these conversations are about very big subjects indeed. They are about salvation, not only of individuals but of communities and even empires. Around six hundred years ago, religion and society all round the world were so closely connected that it would have been impossible for most people to say where one ended and the other began. Perhaps that's why unworldly hopes were so often articulated through worldly wealth, and given shape in temples and precious objects. It's a paradox that we see at its most extreme in the Holy Thorn Reliquary, now held here at the British Museum.

The Holy Thorn Reliquary is a one-foot (30 cm) -high theatre, made of solid gold and encrusted with jewels. In it we watch the terrifying drama of the end of the world, the day on which we, along with all the other dead, will be raised and will face judgement. This is a drama in which one day every spectator will be a participant. It's in three acts. At the bottom, as the angels blow their trumpets at the earth's imagined corners, graves open on an enamel hillside of vivid green. Four figures - two men, two women, naked, in white enamel and still in their coffins - look up and raise their hands in supplication. Far above them, at the very top of the Reliquary, sits God the Father, enthroned in judgement, among radiant gold and precious gems. And in between is the focus of the whole Reliquary.

For medieval Christians, the only hope of escaping the torments of hell lay in the redeeming blood that Christ had shed. At the very centre of the Reliquary is Christ, showing us his wounds. And here, just below him, before your very eyes and only inches away from you, is one of the long, needle-like thorns that caused that holy blood to flow. 'Ista est una spinea corone Domini nostril Ihesu Christi', reads the enamel label: 'This is a thorn from the crown of our Lord Jesus Christ'.

It is, I think, impossible to exaggerate how powerfully this object would affect anyone kneeling in front of it. The blood drawn by this worthless thorn will save immortal souls, and so nothing earthly can be too precious for it. Neither the sapphire it stands on, nor the rock crystal that protects it, nor the rubies and the pearls that frame it. This is a sermon in gold and jewels, an aid to intense contemplation and a source of the deepest comfort. Here's the Roman Catholic Bishop of Leeds, the Right Reverend Arthur Roche:

"I'm sure that the devotion to a relic such as this thorn has actually in some ways sanctified it. It certainly becomes a focus for the consideration, reflection on deeper things, as to the cost of suffering. Especially when you think that, if that thorn is authentic, then it was actually piercing the head of Christ during the course of his suffering and his crucifixion, and in some sense connects our suffering on this earth to his suffering for us. The focus gives us a strength to endure the things that we are presently going through."

There's no way now of proving that this was a thorn that actually pierced the head of Christ, but we can say with confidence that it's a type of buckthorn that still grows around Jerusalem, and the first mention of the Crown of Thorns as a relic is in Jerusalem, around 400. It was later taken from the Holy Land to Constantinople, the Christian capital of the eastern Roman Empire, where it was kept and venerated for centuries. But shortly after 1200, the impecunious emperor pawned the Crown to the Venetians for a mammoth sum. This shocked the crusader king of France, Louis IX, but it also gave him an opportunity. He paid off the debt, and redeemed the relic. So, although Louis as a crusader didn't conquer the Holy Land - the site of Christ's suffering - he did the next best thing, he acquired the Crown of Thorns. So great was its power in medieval people's eyes, that through it Louis was linked directly to Christ himself. To house his incomparable relic, Louis built not just a reliquary, but a whole church. He called it his holy chapel - the Sainte Chapelle.

The stained-glass windows of the Sainte Chapelle leave us in no doubt that Paris and the kingdom of France were to be permanently transformed by the arrival of the Crown of Thorns. St Louis is shown paired with Solomon, the Sainte Chapelle is his temple, and Paris has become Jerusalem. When the Crown arrived, it was described as being on deposit with the king of France until the day of judgement, when Christ would return to collect it and the kingdom of France would become the kingdom of Heaven. When this chapel was completed and dedicated in 1248, the archbishop proclaimed: "Just as the Lord Jesus Christ chose the Holy Land for the display of the mysteries of his redemption, so he has specially chosen our France for the more devoted veneration of the triumph of his Passion." The Crown of Thorns has played a long and fascinating part in the international politics of piety. It allowed St Louis to claim for France a unique status among the kingdoms of Europe, and every French ruler since St Louis has wanted to follow his example. Here's the historian Sister Benedicta Ward:

"To have a relic particularly connected with the Passion of Christ was the best thing you could have. But there were also relics of the saints, particularly the martyrs. I think they provoked a lot of envy, especially the French collections. The rivalry in England - 'We want to have a better relic than they have because we are a better nation than they are.' They're subject to all kinds of external influences. Like everything else, it can be part of commerce, can't it? Politics, commerce, exchange - yes, this is certainly all round the relics."

In this complex economy of influence, a thorn from the Crown became the ultimate French royal gift. In the late fourteenth century one was awarded to a powerful French prince, Jean, Duc de Berry. Looking at the Reliquary, we can be absolutely confident that it belonged to Jean de Berry - it's got his coat-of-arms enamelled on to it. But it also sums up many of his preoccupations. He commissioned some of the greatest religious art of the period, and he was a passionate collector of relics - he had the marriage ring of the Virgin, a cup used at the Wedding at Cana, a fragment of the Burning Bush and a complete body of one of the Holy Innocents, the children murdered by Herod. He was also an enthusiastic builder of castles and so, appropriately, the base of our Reliquary is a castle made out of solid gold.

This Holy Thorn Reliquary is without question one of the supreme achievements of medieval European metalwork, but sadly there is no way of knowing if it was in fact the greatest thing in Jean de Berry's collection. The bulk of his goldsmith's work was broken up and melted down within months of his death, when the English occupied Paris after the battle of Agincourt in 1415. The fact that this Reliquary survives tells us that he must have given it away before his death.

We're not sure who he gave it to, but by 1544 it was in the treasury of the Habsburg emperors in Vienna. And from there its secularisation begins - the gold, enamel and jewels become far more valuable and interesting than the humble thorn that they house. In the 1860s it was sent to a dishonest antique dealer for restoration. Instead of carrying out the repairs, the dealer created a forgery, which he sent back in its place to the Imperial treasury, keeping the original himself. Eventually the genuine Reliquary was bought by the head of the Vienna branch of the Rothschild bank, and it was donated to the British Museum by Ferdinand de Rothschild in 1898 as part of the Waddesdon Bequest, which now occupies the whole of a small gallery here at the museum.

You never get to the end of the Holy Thorn Reliquary. It's a theatre for the cosmic drama of salvation. It's a sermon on high medieval theology. And you could almost say it's a single-object museum, even if an incomparably lavish one. There's one exhibit, mounted on sapphire, displayed behind rock crystal, and labelled on enamel. But its purpose is the same as any museum's, to provide a worthy setting for a great thing. We can't of course know how visitors approach the objects on display in the British Museum, but many visitors surely still use the Holy Thorn Reliquary for its original devotional purpose of contemplation and prayer.

Certainly the veneration of the Crown of Thorns remains very much alive. It was Napoleon who decided that it should be housed permanently in Notre Dame, and there, on the first Friday of every month, the whole Crown of Thorns, from which our one thorn was taken over six hundred years ago, is still shown to crowds of faithful worshippers.

In the next programme we're still in the world of sacred Christian objects, we're in Constantinople, not with a relic but with an icon. We'll be with a beautiful and revered image that, astonishingly, is about whether or not there should be images at all... it's the painting of a holy row.