LONDON — Mary Magdalene was a “co-Messiah” whose marriage and vigorous sex life with Jesus should be celebrated at the heart of Christianity, according to the authors of a new book who claim to have discovered a lost gospel.

A sixth-century manuscript translated from the ancient language of Syriac for the first time is credited with finally explaining what Jesus was up to in the decades before he appeared in the Bible as an adult. The book claims that Jesus’s sexuality was whitewashed from history by prudish early Christians, who also downplayed the importance of Mary Magdelene, his wife and the mother of his two children.

“She’s not just Mrs. Jesus, she is a co-deity, a co-redeemer, she’s called ‘Daughter of God’ as he’s called ‘Son of God,’” said Simcha Jacobovici, one of the authors of Lost Gospel, which was launched at the British Library in London on Wednesday. “We think of Christianity as sexless, this [Gospel] says that sex is sacred.”

Biblical scholars, religious groups and the Church of England were among those lining up to dismiss the book as the fantastical result of an over-active imagination, but the writers insist that they expected their historic discovery to be greeted with skepticism. They claim “theological protectionism” accounts for the hostile reception as well as a narrow-minded desire to maintain the traditional depiction of a celibate, divine Christ, rather than a man whose flesh and blood appears far more human.

Jacobovici, a Canadian-Israeli documentary-maker, and co-author Barrie Wilson, a professor at York University in Toronto, have based their book on a new interpretation of The Story of Joseph and Aseneth. The text has been previously been seen as a Jewish exploration of the Biblical story of Joseph (he of the Technicolor Dreamcoat), who married an Egyptian woman. By translating a 1,500-year-old version of the tale and a letter that accompanied it, the authors claim to have uncovered an encoded fifth gospel. They say Jesus and Mary Magdalene were given false names so as not to attract the attention of those upholding the strict doctrines of Paul the Apostle. “All other versions of Christianity that had survived for some 300 years were burnt, the people sometimes along with them, so they hid them in plain sight,” said Jacobovici.

The accompanying letter, which also dates back to around 550 A.D., says the story has a hidden meaning but it is cut off before any further explanation can be given. “This is not water damage, its censorship!” intones the voiceover on a Science Channel documentary that will be broadcast next month.

Wilson explained that, unlike some of the other forms of early religion, Pauline Christianity had been based on purity. “Paul’s theology of marriage could be said very simply: no sex before, during or after marriage,” he said. “Our gospel, in contradiction to Paul’s asceticism, talks about the importance of marriage and the intensity that it takes, that’s where the gospel—the element of good news comes in—it enriches the understanding of fulsome sexuality.”

He said the new gospel would help to solve the Church’s problems adapting to modern sexual politics and the Catholic question of accepting married or even female priests.

The Rev. Andrew Symes, executive secretary of Anglican Mainstream, a conservative church group, said it was typical that yet another reinterpretation of a religious text was seeking to liberalize the church’s position. “The arguments they use are purely sort of New Age neo-pagan trendy Californian stuff and a mixture of Eastern esoteric beliefs. It’s got nothing to do with what the Bible says,” he said.

It wasn’t just vested, conservative interests that dismissed the book, however. Diarmaid MacCulloch, professor of the History of the Church in the University of Oxford, told The Daily Beast that the claim to have decoded a new gospel was “to say the least, implausible.”

He said Jacobovici and Wilson were simply the latest in a long line of people hoping to find further Biblical insight. “The Bible has never said enough about the lives of its characters to satisfy piety, so pious folk make things up,” saidMacCulloch, who wrote A History of Christianity. “The difference now is that when such texts are ‘rediscovered,’ they are regarded as texts suppressed through Church conspiracy. We’re pattern-making animals, so we love a conspiracy theory.”

He refused to be drawn on whether the authors might be guilty of a deliberately deception. “I wouldn’t presume to question their motives,” he said.

The Church of England had no such qualms. Rev. Arun Arora, the Church’s director of communications, responded to the book’s publication in a pithy blog post that compared the work to Monty Python’s Life of Brian.

“It’s not Lost, it’s not a Gospel, it’s a very naughty marketing campaign,” he wrote, highlighting Jacobovici’s involvement with a string of equally far-fetched Biblical discoveries including the nails from Jesus’ crucifixion and the lost tomb of Jesus’ family.

“Ever since Dan Brown found an audience with the Da Vinci Code an industry has built up around ‘direct-to-the-public pseudoscholarship’ in which a combustible combination of conspiracy theorists, satellite channel documentaries and opportunistic publishers have identified a lucrative income stream,” Arora said.

When the suggestion that he was a serial hoaxer was put to Jacobovici, he told The Daily Beast that people were spreading lies about him in order to discredit his extraordinary discoveries. “One of the ways to deal with messages you don’t like is to attack the messenger,” he said. “Someone might say to me, why are you finding so many great things, why nobody else? I tell you why. Because I’m Jewish, I’m not Pauline—I don’t think inside a Christian box… I’m not a theologian, I’m not a Christian, and I see that in this world you can look at texts with fresh eyes and see new things.”

Despite the book’s hostile reception, the a spokeswoman for the publisher, Pegasus, said: “We stand behind our author’s research.”