A cathedral window, a fragment of skull: this retelling of the murder of St Thomas Becket is sublime

In AD 1170, Thomas Becket, a former king’s favourite and a rebellious Archbishop of Canterbury, was assassinated in his cathedral. The fatal blow was so strong that it lopped off the top of his head and shattered the sword in two.

As a fascinating new show at the British Museum demonstrates, killing this critic of temporal power generated a religious phenomenon. People collected his blood: one man rushed some home, soaked up in fabric, to give to his wife. She was instantly cured of her paralysis, the first of hundreds.

Three years after his death, Thomas was made a saint, which is swift even for medieval standards. Reliquaries housing bits and pieces of him cropped up as far away as Sweden: he was an export, a political symbol and a tourist magnet, painting a picture of the 12th century that took me by surprise.

Forget the Monty Python image of toothless peasants gathering mud. Cosmopolitan England was part of an empire that stretched from the Scottish border to the Pyrenees; the London of Thomas’s childhood thronged with trade from China and the Middle East. At Canterbury Cathedral, he would’ve enjoyed a state-of-the-art water-pumping system, illustrated at this exhibition in the pages of a psalter. Far from being a Dark Age of intellectual stagnancy, Norman England bubbled with debate over the right way to govern, and Thomas, we are reminded, was an inspiration for Magna Carta.

The break with Henry II, which began when Thomas refused to serve as his chancellor as well as archbishop, frustrating Henry’s dream of an alliance between church and state, was as philosophical as it was personal. Henry wanted clerics tried in secular courts; Thomas wanted to preserve separate justice. I suspect the average punter today would agree with the king, and perceptions of Thomas capture the ebb and flow of British attitudes towards Catholicism.

The view of contemporary pilgrims to his shrine in Canterbury, recreated at the British Museum via a computer graphic, was that the church stood for mercy against brutal secular authority. Tourists would be pointed to the glorious glass windows telling the stories of Thomas’s miracles, here loaned to the Museum and arranged, for the first time in 350 years, into proper order so that we can enjoy a close-up look, and at eye-level too.

It’s the absolute highlight of the exhibition. In one, a man accused of theft is sentenced by the Crown to being blinded and castrated. Thomas appears above his bed at night and, miraculously, restores the man’s stolen goods. The message is radical: whatever a king might do to you, he has nothing on God or His forgiving Church. How was the Crown, just beginning to build an effective state, supposed to compete?

It’s no surprise that during the Reformation, Thomas was denounced as a traitor and his shrine torn down; his name was obliterated, the words “saint” and “martyr” were crossed out of religious texts. At Ashford in Kent, a statue of Thomas was cleverly saved from vandals by replacing his crozier with a wool comb and pretending he was St Blaise, and many images were hidden under floors or bricked up in church walls. For Catholics hiding abroad, he became a symbol of resistance.

Now that England is no longer a self-consciously sectarian society, Becket, along with Thomas More, has been repurposed as a defender of freedom of conscience, but that’s an ahistorical interpretation. Both men defended the rights of the Catholic Church because they believed its doctrines to be true and its moral authority supreme. Given that respect for the Church has all but collapsed in the wake of sex-abuse scandals, many visitors to the exhibition will find beyond the pale the idea of giving its clerics separate (never mind more lenient) legal treatment. The religious privileges that Thomas fought for are total anathema in a society that does not automatically assume Christianity’s claims to be true.

Murder and the Making of a Saint presents itself as a story in over 100 objects, and the collection is sublime: the blue and gold reliquaries are mesmerising, as is the vast array of religious tat that would’ve been sold to Chaucer’s pilgrims. A particularly glorious item is a surgical instrument case (c1520–30) that depicts two lances being driven into Thomas’s back. If you cannot make it to the show itself, then I implore you to buy the catalogue, which is one of the best I’ve ever read – scholarly and entertaining, a good history book in its own right.

But when one comes to the end of the exhibition, this Catholic was anticipating – yearning for – an emotional high, and I didn’t quite get it. We are presented with a bold silver statuette of Thomas with a sword stuck in his head, like Excalibur, and what is reputedly a piece of his skull. The provenance of the cranium, I’m told, is sound – in which case, to the believer, this is not just an object in a glass box, it is the fragment of a saint. Murder and the Making of a Saint is an intelligent, sensitive show, but an artefact of such profound holiness is best experienced in a church, contextualised by faith, and not in a museum.

From May 20 to Aug 22. Tickets: britishmuseum.org; 020 7323 8181?