Michel Benoît's new thriller depicts the Catholic Church as a nest of corruption and lies since the days of St Peter

The shadow cast by the phenomenal worldwide success of Dan Brown's The Da Vinci Code looms large over The Thirteenth Apostle, Michel Benoît's tale of Vatican shenanigans, newly arrived in Britain after achieving bestseller status in France and Spain. Not that Benoît, himself, is prepared to acknowledge any overlap. Even for a moment. "When I read that book of Dan Brown, I said to myself: 'It is not possible, on such a topic, to say such shitty things.' His scholarship does not exist. I am a scholar."

He says it with a stereotypical French arrogance, but it is undoubtedly true. Benoît is a former Benedictine monk who studied for a doctorate in theology in Rome and went on to teach the New Testament in a monastery. Yet his tale of a cultish, underground society at the heart of the Catholic Church, prepared to kill to protect an ancient secret, which if revealed might sink the whole Christian enterprise, will have a very familiar ring to anyone who has read Brown's thriller. To reveal more would give away the plot, but lets just say that Jesus turns out to be somewhat less than he has been cracked up to be.

Equally resonant for Brown's many fans will be the backdrop to Benoît's sensational story of historical manipulation and cover-up at the heart of the church. He presents a picture of endemic corruption within institutional Catholicism that stretches from a murderous Saint Peter at the very start of its history down to a villainous Cardinal Catzinger in the 21st century. Surely the current Pope must have grounds to sue, I ask? "You have a bad mind," Benoît smiles. "You English. What a vicious mind!"

Tall and athletic, in his late sixties, Benoît (not his real name – "my real name is to difficult to pronounce," he tells me, declining to give me the chance to try) is a charmer. As we sit talking in the back room of a small south London bookshop, he is clearly enjoying defending the history behind a book that his publishers are describing as "dangerous" for Catholics to read. Once he has dismissed Dan Brown, Benoît offers an alternative point of reference for his first work of popular fiction: Umberto Eco's The Name of the Rose. "That is my reference. I love this book. Eco is one of the best men who knows Italian history in the 14th century."

Certainly there is a good deal of scholarship lying behind The Thirteenth Apostle as it juxtaposes the power struggle that convulsed the early church in the decades after Jesus's death with a present-day plot to stop the publication of a long-lost account of that battle, detrimental to St Peter and every pope who followed him. Yet Benoît's book seems to be just as firmly rooted in the ex-monk's own history.

As a precocious teenager, he joined the ancient Benedictine order in 1958. He had already completed a degree in pharmacology and had declined an invitation to work with the Nobel Prize-winning biochemist, Jacques Monod, in order to pursue what he saw as his vocation. He entered the abbey of Saint Benoît sur Loire (from which he derives his pen name) and was, evidently, an awkward customer from the start. At the time, all trainee monks went on to ordination as priests, but he took advantage of a new ruling by the church, to refuse, the first Benedictine ever to do so, he claims.

Not knowing what to do with him, his superiors sent him to study in Rome for four and a half years. This period is the basis for his allegations of corruption at God's business address on earth. "I heard much more than I say in the book. I am, if anything, under the reality. My aim was not to make a trash book."

His studies drew him to the gospels and Jesus. "And I was told that Jesus was a Jew. It was the first time I had heard it. I had been told before that Jesus was born in Rome, maybe had a Greek education. That was the politically correct version."

Now I don't want to come over all Catholic here. Certainly there has been, is and no doubt will be much that goes on in the Vatican that hardly lives up to the high ideals that the Catholic Church preaches. It has also historically had a problem with Jesus's humanity, despite the whole point of his incarnation on earth being that he was both human and divine. But to suggest that nobody in Rome in the late 1960s had ever mentioned that Jesus was a Jew overlooks Nostra Aetate, one of the key documents of the landmark Second Vatican Council, held in Rome from 1962 until 1965, which addressed both Christian anti-Semitism and Jesus's Jewishness.

Whatever the truth of the matter, Benoît's scholarly enthusiasms after his time in Rome clearly went off in what his superiors took to be unorthodox directions. When their efforts to redirect him to the monastery garden failed, they eased him out. He left the Benedictines in 1984 and has written an account of his ill treatment, published in France in 1989 as Prisoner of God.

His main work these past 23 years, though, has been to continue to research the historical Jesus. It was after he had published his findings about the real Jesus in France that the idea for The Thirteenth Apostle took shape in his mind. "My book had sold 5,000 copies which I am told is good for that sort of book but for me was a misery. So I said: 'Look, I want to say some things. Let's do a thriller. A novel of shit – crime, sex, everything.' And I enjoyed it. It was a lot of fun. But fun with guts. I cannot say the historical material is true because there is no truth in history, but it takes the approach of truth, based on scholarship about the texts on the beginnings of the church."

To give an example of Benoît's method that doesn't spoil the ending: Judas Iscariot is tricked into betraying Jesus to the Temple establishment in Benoît's account. To stop him talking afterwards, Peter, the leader of the apostles, knifes him to death. Judas, it seems, is an obstacle to Peter's ambition to head the church and go down in history.

Where, I ask, is the evidence for this? "If you read the official gospel, there are two accounts. I take off the glasses of faith and read them as I would any other text. One account is in Matthew's gospel where it is said Judas kills himself by hanging. And one is in the Acts of the Apostles where it says his guts fall out. And who tells that story? It is Peter. You have to ask why he tells it. You have to be a good scholar."

I suggest that no court would convict Peter on such evidence. "I stand for it as a historical hypothesis," Benoît protests, "as a scholar. I have studied this. You talk of facts. But the only fact is that Jesus died. With the rest you have to take text, context and commonsense." Which – though I don't say it aloud to Benoît for fear of another explosion of scatological language – sounds awfully like something Dan Brown might say. *

The extract: The Thirteenth Apostle, by Michel Benoît (Alma £7.99)

"...At the request of the Cardinal Prefect of the Congregation, you have been informed of a little problem that has arisen recently in France, in a Benedictine abbey that is under very strict surveillance. You gave me carte blanche to deal with it... the monk whose recent remarks were giving us cause for alarm is no longer in a position to harm the Holy Catholic Church."

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The Gospel According to Judas by Jeffrey Archer and Frank Moloney

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The Sixth Lamentation by William Brodrick

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