How would the real Mary Magdalene have reacted to her posthumous reputation? Not very kindly, one suspects. Our only historical source, the New Testament, does not even hint that she was a prostitute, and she’s unlikely to have been placated by Christians telling her: ‘It’s OK, we think you were a reformed whore.’

No one in the Bible has been so elaborately misrepresented. In addition to not being an ex-prostitute, Mary of Magdala was not Mary of Bethany, the sister of Martha and Lazarus, who anoints the feet of Jesus with ‘about a pint of pure nard, an expensive perfume’ and then wipes it up with her hair. Nor was she the ‘woman taken in adultery’, the one told to go and sin no more. Nor was she the wife of Jesus. That is a fantasy of early Christian heretics that has been seized on by modern conspiracy theorists who imagine Jesus and Mary travelling to the south of France and founding the Illuminati before being spirited away in a black helicopter. It made Dan Brown very rich.

What do the Gospels tell us about Mary of Magdala? That she was known as ‘Magdalene’, had seven demons cast out of her by Jesus, was present at the foot of the cross, discovered the empty tomb and was the first person to whom the risen Lord appeared. You’d have thought that this was enough to be going on with — but no. We will not leave the poor woman alone.

Just this month, the media reacted with feverish excitement to a book called The Lost Gospel: Decoding the Ancient Text that Reveals Jesus’ Marriage to Mary the Magdalene. How many falsehoods can you fit into one title? Yes, there is a sixth-century Syriac text, familiar to specialist scholars, that relates a weird-but-boring legend about the Old Testament patriarch Joseph and his wife Asenath. It makes no mention of Jesus or Mary Magdalene. So it’s not lost, not a gospel and it most certainly has not been ‘decoded’ by the book’s authors, Simcha Jacobovici and Barrie Wilson. More about that later.

Meanwhile, Mary Magdalene is preparing to make her debut at the London Coliseum. She is the major character in The Gospel According to the Other Mary, an oratorio by John Adams that receives its world stage premiere on 21 November in an ENO production by Peter Sellars, the American director. Sellars also wrote its libretto, which juxtaposes biblical verses and the writings of 20th-century poets and campaigners for social justice.

That doesn’t sound very authentic, but it’s only fair to make a distinction between deliberate myth-making and creative responses to the fiction of the reformed prostitute. Since the Middle Ages, the Mary Magdalene of tradition has proved irresistible to artists. At the heart of the Gospel lies a call to repent, and according to the Catholic Church — which encouraged the conflation of Mary of Magdala, ‘the woman taken in adultery’ and the non-existent whore — this great saint was repenting for shocking sins. ‘Pure by virtue of repentance, she nevertheless remains a woman with a past,’ writes the historian James Carroll. ‘Her conversion, instead of removing her erotic allure, heightens it.’

Painters in particular have experimented with the subject. In ‘The penitent Magdalene’ by the Florentine Carlo Dolci (1616–86), she gazes to the heavens with a childlike innocence, clutching her alabaster jar of ointment; her scarlet robe looks more like the vestment of a cardinal than the costume of a prostitute. In 19th-century paintings she is a femme fatale, still a bit sluttish even as she bathes the feet of the Lord. But the most intriguing representations of Mary Magdalene date from the Reformation. To quote the American journalist Chris Herlinger, reporting on a Mary Magdalene exhibition in New York in 2002, ‘Catholic painters depicted her as a defender of Roman Catholic sacraments …while Flemish painters, feeling the tide of an emerging Protestant culture and wanting to downplay the importance of Mary, Jesus’s mother, chose to re-emphasise [her] penitence’.

In other words, in addition to giving prurient Christians a naughty thrill, the Magdalene has been conscripted into theological and ideological warfare. Modern feminists bore on about her endlessly. And Sellars hardly conceals his own agenda. The Gospel According to the Other Mary has Mary and Martha join Cesar Chavez — a folk saint of Mexican–Americans and old white hippies — in a march by the United Farm Workers. Mary sings from the autobiography of the Catholic social activist Dorothy Day. In places this is powerful: the work opens with Day’s description of a drug addict in withdrawal, howling as she beats her head against the bars of her cell. But when Mary tells us that ‘the surplus that comes in we will give to unemployed people in our neighbourhood’ — well, it would take a remarkable composer to immortalise those words, and John Adams doesn’t pull it off.

We shouldn’t be too hard on Sellars. He talks engagingly about the Marys that make up his heroine. He achieves the small miracle of discussing feminine spirituality in (relatively) unpretentious language —‘it’s a love of Jesus that takes practical form, care of the body while men are discussing theology, and you could consider that to be the perfume that fills the room’. It’s true that Sellars’s quotation sounds dated. He was the enfant terrible of 1970s Harvard and it shows. But that’s OK: nothing in the text is as embarrassing as, say, Leonard Bernstein’s Mass. The difference is that Lenny could write a tune and Adams can’t. Adams’s most celebrated opera, Nixon in China, works because the characters are fabulous and the orchestra’s post-minimalist chugging pushes forward the plot. In The Gospel According to the Other Mary the vocal lines move up and down as predictably as recitative. It’s difficult to see how any staging can rescue them.

So has the Magdalene been insulted again? No, because Sellars’s starting point is that he’s dealing with a composite figure — a jumble of Marys. He’s done his homework and he doesn’t pretend that his work is something that it isn’t. For a genuine affront to Mary of Magdala, look no further than The Lost Gospel. Its co-author Barrie Wilson is professor of religious studies at York University, Toronto. His identification of Joseph and Asenath as Jesus and Mary Magdalene is potty conjecture aimed at the bestseller lists. Academics who are seduced into this sort of project shouldn’t be allowed anywhere near a classroom. Presumably Wilson possesses the technical skills to decipher a Syriac manuscript. If so, he has — appropriately — well and truly prostituted them.