There is a detail in Leonardo da Vinci’s painting The Last Supper that requires no decoding by Dan Brown fans. An overturned salt cellar lies on the table in front of Judas Iscariot, signifying the bad luck of spilt salt and branding the disciple who is about to betray Jesus Christ. But after 2,000 years of vilification, Judas’s luck has changed: he’s been exonerated.

Well, not quite. A tattered papyrus manuscript found in the Egyptian desert, hailed as an authentic copy of the lost Gospel of Judas, purports to reveal that far from betraying Jesus, Judas sacrificed himself for his master. Experts are taking the claims with a pinch of, er, salt.

The ancient Coptic document, dated to the 3rd or 4th century, portrays Judas as the disciple closest to Jesus, singled out to receive mystic knowledge and hand the Messiah over to his persecutors so that he would be freed of his physical form.

According to the 26-page gospel, unveiled last week in Washington by the National Geographical Society, Jesus told Judas: “Step away from the others and I shall tell you the mysteries of the kingdom. It is possible for you to reach it, but it will grieve you a great deal.”

Later, before the fateful Passover holiday, Jesus told Judas: “You will exceed them all. For you will sacrifice the man that clothes me.” Convinced scholars believe that Judas emerges as a champion who is envied and resented by the other disciples, eventually becoming a symbol of treachery.

This dramatic reinterpretation is the latest attempt to stand the Bible story on its head. There have been efforts to rehabilitate Pontius Pilate, the Roman prefect of Judaea who handed Jesus over for execution, and a raft of fanciful books have persuaded a lot of people that Jesus married, had children and swanned off to live in France.

But if the Gospel of Judas is true, parents will be free to resume naming children Judas, a name common during the time of Christ but anathema to most Christians ever since. Prayers for receiving the Eucharist will no longer mention Judas’s betrayal. And of course the Bible will have to be rewritten.

None of these outcomes is likely. Sceptics point out that the new gospel was apparently written between AD130 and AD170, long after the real Judas lived, whereas the gospels’ account of events, set down within decades of Jesus’s death, have a more authentic air.

The original text appears to be the work of a 2nd-century gnostic sect and echoes its preoccupation with secret knowledge leading to salvation. Most damningly, it was considered for inclusion in the New Testament canon but received the thumbs down in about AD300, when Irenaeus, an early Christian bishop, denounced it as heretical.

Yet speculation about Judas has always abounded, partly because the gospels say little about the circumstances of his calling or his part in Jesus’s ministry. He has served as the perfect scapegoat, caricatured by anti-semites in paintings and literature as the stereotypical Jew with red hair, as in Shakespeare’s Shylock. His treachery for money was regarded as a typical piece of Jewish venality.

Judas was the disciples’ bagman, who carried their money in a purse and kept a close eye on the shekels. It was Judas who objected to Mary anointing Christ’s feet with expensive perfume, which he claimed could have been sold for 300 pence and given to the poor. His professed concern was put in a different light by the Gospel of St John, which stated: “Now he said this, not because he cared for the poor; but because he was a thief.”

The most fertile source of theories about Judas is his surname, Iscariot. A widely accepted view is that in Hebrew this proclaimed him “a man of Kerioth”, a town of Judah, which set him apart from the other apostles, who were all from Galilee. This may have added to their later antipathy.

A more intriguing hypothesis is that Iscariot is a transformation of the Latin sicarius, or “dagger-man”. The Sicarii were a group of assassins among Jewish rebels intent on driving the Romans out of Judaea. So Judas could have seen Jesus as the Jewish leader who would lead a revolt.