Writing in the 4th century, Eusebius, the earliest historian of the Church, tells a story about Agbar, King of Edessa, a city now in the south-eastern corner of Turkey. The king desired the cure of an illness that afflicted him and so wrote a letter to Jesus. Jesus replied, promising to send a disciple to heal the king. Today, scholars do not think the letters, carefully transcribed by Eusebius, are genuine. But it is true that Edessa was one of the earliest Christian kingdoms. Agbar XIII, who reigned in the late 2nd century, was a Christian. Eusebius tells us that he had found the letters in Edessa and translated them into Greek himself. The legend may reflect the desire of Edessa to assert the origin of its church independent of Rome. 

In the century after Eusebius the story developed further. By this time Edessa boasted a portrait of Christ called the Mandylion (a word whose meaning is obscure). To provide a provenance for this icon, Agbar's messenger was now said to have painted the picture when he caught up with Jesus. Later, it was ascribed a completely miraculous origin. The icon protected Edessa for centuries before it was carried to Constantinople in 944. It disappears from the historical record after the sack of the Byzantine capital during the Fourth Crusade of 1204, but it was likely to have been carried home by the triumphant crusaders. There are at least three candidates identified as the original version of this icon. The one in the show at the British Museum comes from the Pope's private Matilda Chapel in the Vatican. The frame is baroque, but the picture within appears to be a 13th-century copy of a much older original.

Today, the Catholic Church has an ambivalent attitude towards relics. They are tolerated as an example of popular piety but have no special theological significance. A few attract continued veneration from the faithful, but this appears quaint to Protestants brought up to treat them as papist superstition. But we should remember that it was only an accident of history that expelled the cult of the saints from England. The strange and foreign objects in the British Museum's exhibition represent customs that were once as English as roast beef and brown ale.