In the middle ages, thousands of French Christians were burned to death for insisting that Jesus of Nazareth had been married to Mary Magdalene.

The Cathars' conviction that the Son of God was also a husband and father was one of the most persistent of heresies, fuelling centuries of secretive speculation, not to mention bestselling books such as Dan Brown's conspiracy novel, The Da Vinci Code.

Some treated these ideas as hokum, others as an outrageous attack on the Catholic Church. Now, however, religious sleuths Simon Jacobovici, described as "a self-made Indiana Jones", and his associate, Professor Barrie Wilson of York University in Toronto, think they have found proof that this was no heresy, but historical fact. Their find could rival the discovery of the Dead Sea Scrolls for sensation. And as with that event, they will inevitably be faced with naysayers, many with the authority of the Church behind them.

The Lost Gospel is their account of an accidental discovery that led to a reinterpretation of an early text which, when decoded, reveals Jesus and Mary's marriage. This text, known as The Story Of Joseph And Aseneth, can be traced close to the time of Jesus's life, and might even be the basis of the Gnostic branch of Christianity. For years, different versions of the Christian story struggled for supremacy, but eventually the Gnostics and others were stamped out by the orthodox Christian establishment. Rather than accept the testimony of those who had been Jesus's followers, on which Gnostic beliefs were based, the authorities preferred the ideas of Paul, who had never known him.

This explosive text, the authors stress, does not fall into the "long lost" category. It has lain in the British Library since the mid-1800s, labelled Manuscript 17,202. Written in Syriac, The Story Of Joseph And Aseneth purports to tell of the famous Old Testament hero and his marriage to a beautiful, high-born gentile. What sparked our investigative pair's interest, however, was that shortly after having it newly translated, and using multispectral imaging to decipher previously unreadable or deleted words, they had a Damascene moment. On a trip to Turkey, they saw a statue of the goddess Artemis, and found that its symbolism matched that in the story about Aseneth. Re-reading the text, they realised that the tale was not from the Jewish tradition, as had long been presumed, but was an early Christian story, in which the players were disguised. There is not space here to elaborate on their reasoning. Suffice to say, they present a closely argued case that the protagonists are Jesus and Mary Magdalene, and that this is an account of their meeting, marriage, and a plot against both their lives and that of their children, which they all survive.

This is nothing less, say Jacobovici and Wilson, than an early Gnostic Gospel. It was suppressed for reasons of fear, but expressed in the mythological manner distinctive to that tradition even when dealing with historical facts. No wonder the authors could barely control their excitement when in 2012, in the midst of their researches, Karen King, a professor at Harvard Divinity School, unearthed a fragment of Coptic papyrus, in which Jesus refers to "my wife". Here was confirmation they were on the right track.

What follows is an analysis of this deciphered text, and of the political context in which Jesus of Nazareth lived. They rightly claim it is a detective story, and while it would be stretching a point to say it is a rattling good read, it is fascinating, and challenging, in equal measure. What The Lost Gospel indubitably offers, however, is the explorers' thrill at getting closer to the people on whom Christianity was founded than anyone before. I say people, because not the least of the revelations of this text is the role played by Mary Magdalene. She was not only Jesus's dearest and cleverest disciple, we are told, but the keeper of the flame after his death. This is not a new concept, of course. Some years ago the novelist Katherine McGowan wrote that Mary, as Jesus's widow, was "the true spiritual founder of Christianity in the Western world". (McGowan, incidentally, believes she is directly descended from the holy couple.)

Beyond McGowan's eccentricity, there is much in this book that could worry or anger the devout Christian, not least the idea that the sacrament of communion originally referred not to drinking Jesus's own blood but to Mary's menstrual blood. The chapter on sacred sex will raise a few eyebrows, as will the explanation of why it was Mary Magdalene and not Mary the mother of Jesus who was the true virgin. But even more disturbing is their speculation that Jesus made some sort of political accommodation with the murderous would-be emperor Sejanus, in order to secure his place as King of the Jews. Only a miscalculation of the mood of those in power led to his crucifixion rather than his elevation. The character this suggests is far from the humble, spiritual and unworldly man of the conventional gospels, a fact even the non-believer might find hard to swallow.

As a piece of investigative reporting, The Lost Gospel is impressive and enlightening, and its focus on the political context in which the early Christians lived instructive. At times, however, the writers are carried away with their own theories, and their arguments are not always persuasive. Describing the Pauline view of Jesus as "death-obsessed" compared to the sexual emphasis of the Gnostics seems rather to overlook the significance of the resurrection for Paul and his ilk. Sacred sex is no doubt life-affirming, but rising from the dead is surely an even better trick.

By its conclusion, one has been reminded forcefully of the primitive and violent origins of the world's most powerful religion. It is also clear that, no matter how scrupulous this and later scholarship proves, such discoveries are unlikely to change the way committed Christians think. Quite simply, the train left the station a long time ago. The official tenets of Christianity have shaped civilisation and culture too profoundly to be tinkered with or revised. As a far from innocent Jesus emerges from this book, one realises yet again the extent to which people put their trust in a story they want to believe is true. Faith has little place for fact; otherwise, belief would just be a history lesson. How unsettling to consider that on such slender and partial truths, the world as we know it has been built.