f you thought medieval religious art was all clasped hands and uplifted eyes, then prepare yourself for the gorefests that shudder through this brilliant new show like a broadsword hitting bone. On 29 December 1170, four knights sent by King Henry II entered the holy sanctum of Canterbury Cathedral with swords drawn and slew its archbishop, Thomas Becket, a flamboyant, charismatic politician who’d started his career as the king’s right-hand man then became a thorn in his side as a champion of church over crown. The murder – whether or not Henry really intended it – rapidly became notorious across Europe and Becket was revered as a modern martyr. Not figuratively but literally, being canonised as a saint just three years after his death. The medieval cult of Becket was promoted with shockingly realistic murder scenes on bejewelled caskets, the glowing pages of illuminated manuscripts and mystical stained glass. This exhibition has plenty to fascinate history buffs. But its glory is to make the art of the middle ages come alive. The emotional story of Becket’s slaying and the strangeness of the rites and rituals that celebrated him provide a direct human connection with the people and images of a remote world. Suddenly the art of that faraway time seems brutally contemporary. On a casket for a physical relic of Becket made in Limoges, France, just a decade or so after his death, yellow copper figures set against deep blue enamelling portray two startling scenes. The lower one shows the wicked knight striking down Becket, holding his huge sword in both hands and swinging at the archbishop’s throat while he prays at the altar. Above, monks mourn the murdered man, kneeling in tragically eloquent poses. None of these figures conform to later European artistic standards: they are elongated puppets in a pre-perspective realm of flat blueness. Yet the emotion is still raw after all this time. A painting from the illustrated Harley Psalter manuscript, setting the anguished monks against mystical gold and giving them poses whose poignancy anticipates the Italian genius Giotto, is even more moving. As the image of the murder in the cathedral hardened and spread, the details became more precise in their horror. That great Limoges casket at the start of the show was practically a news image by medieval standards, when information spread slowly through a world without printing. It suggests decapitation. But according to more considered accounts, a knight struck Becket on the head with his sword, severing a chunk of skull and brain. The blow was so hard the sword lost its tip. In an illuminated codex lent by the British Library, a piece of sword and semicircle of skull fall to the ground. The same two grisly fragments can be seen on a gilded casket that comes from Hedalen Stave Church in Norway. This is a marvellous object that mixes two worlds. On the front is Becket’s martyrdom. But the little chest is also decorated with two dragons whose gorgeous heads rise like the prows of Viking ships. After all the Viking age wasn’t so long ago and Christianity still felt new in early 13th-century Norway. Nearby is a massive stone font from Sweden. It too depicts Becket’s death but it also has gurning monster heads. Perhaps the violent story of Britain’s celebrity saint appealed to medieval Scandinavians who still told Viking sagas and remembered the heyday of pagan pillage. The confrontation between church and monarch that Becket embodied was a crucial moment in the consolidation of authority in medieval “Christendom”. It was a conflict the church won. Becket’s death was a political goldmine for the pope. Henry II and his successors had to kneel and abase themselves at his magnificent shrine in the new gothic style. As for ordinary people, they genuinely revered and trusted this British saint: “That hem hath holpen whan that they were seeke” – that helped them when they were sick, as Geoffrey Chaucer put it. His great social epic The Canterbury Tales is built around a group of pilgrims going from the Tabard Inn in Southwark to Becket’s shrine at Canterbury, telling stories to while away what was then a tough trek. There’s an early manuscript of it here, open at a portrait of The Wife of Bath, whip in hand. The hope of being cured that drew pilgrims to Canterbury – not just from London but the continent, too – is magnificently illuminated by the most spectacular artwork in the exhibition. A row of stained glass windows lent by Canterbury Cathedral show true life stories of people said to have been cured by Becket. Ralph the Leper swore by the saint after a visit to the shrine mended him. A peasant called Edward was even healed of being judicially blinded and castrated by his devotion to Becket: a glowing sequence of intensely coloured, and gloriously backlit, windows depicts him being held down while one man pokes his eyes out and another takes a knife to his loins. In another scene, he displays his healed body to the crowd in Canterbury while a man points to his magically restored penis. All the reliquary caskets reflect this same belief in saintly magic. They once held bits of bone or clothing said to be sanctified remains of Becket himself. Lead bottles also survive from Canterbury and the places far afield they were reverently taken. They held the diluted blood of Becket, to be drunk as medicine. And then it ended. In the early 1500s, Henry VIII responded to the pope’s refusal of a royal divorce by breaking with Rome. He personally went to Canterbury to oversee the destruction of Becket’s shrine – and carry home its treasures in a goody bag. The very mention of Becket was banned: a disturbing collection of vandalised books, from which Becket has been cut out or covered in red ink like a sea of blood, show how his legend threatened the new Tudor state centuries after his death. But it would be wrong to leave this exhibition thinking it a nostalgic tribute to a lost Catholic Britain. There were no “Catholics” or “Protestants” in the 12th century, and feudal Christianity itself had little in common with any living church. The Britain, and Europe, we see here believed in magic blood and potent bones. It was another country where dragons hid under church fonts and were slain by knights. The British Museum’s mind-expanding exhibitions have taken us to many cultural wonderlands, but this trip to medieval Kent is truly far-flung.