Geza Vermes has had a life in two halves. The turning point came in 1957, with a crisis that could almost justify a tabloid headline: "Jewish-born Catholic priest leaves Church after falling in love with married woman."

In the event, Vermes left his French seminary, moved to England, got married and began his climb up the academic ladder. He became widely known as an authority on the Dead Sea Scrolls and the historical background to the New Testament, and he concluded an illustrious academic career as the first professor of Jewish studies at the University of Oxford.

Since retirement in 1991, he has produced 12 more books, including the trilogy The Nativity (2006), The Passion (2005) - and now, in time for Easter, The Resurrection. All three, he explains diplomatically, are based on "the premise that the historical Jesus and the Christ of faith are two different things, though both of them exist and both are legitimate within their own confines". The project also raises questions about the links between Vermes's scholarly work and his remarkable life.

Vermes seems a natural optimist and believes in Providential Accidents, to quote the title of his gripping 1998 autobiography. The past 50 years have blessed him with great professional and personal satisfaction. But this could hardly have been predicted at the start.

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He was born to Jewish parents in 1924, although his mother's family (and later his father) "embraced Christianity for social reasons, naively or optimistically believing that a baptismal certificate would protect them from rampant Hungarian anti-Semitism". At the age of 18, Vermes opted to train for the priesthood, out of sincere conviction but also because the "six-year philosophical and theological curriculum" seemed the best way to satisfy his deep thirst for education. When the Holocaust came to Hungary in 1944 and snatched away his parents and many other relatives, Vermes was helped by a particularly noble and courageous cleric, his former parish priest who had become Bishop of Gy?r.

He survived the war hidden in a Budapest seminary and then, bereft of family, decided to join the Fathers of Sion, first in Leuven, Belgium, and then in Paris. Although Vermes left the priesthood in 1957 (and soon "grew out of" Christianity), he took with him some very ambivalent feelings.

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"On the one hand," he writes in his autobiography, "I felt - and still feel - a sentiment of gratitude for the help and protection I had received during the years of ordeal in the 1940s." Yet he was equally aware of "the darker sides of the church" and notably "her less than exemplary behaviour towards the Jews over the centuries".

It is very tempting, as Vermes knows well, to see his deep recent engagement with Christian origins as partly rooted in his own experiences. Yet he argues that it arose from the sheer logic of his research.

"I started with the Dead Sea Scrolls, then moved on to Jewish biblical interpretation, adopting a wide focus that included the New Testament as one minor element." Then, at the request of others, he devoted "20 years of slave labour" (1965-87) to producing an updated edition of Emil Sch¨¹rer's classic History of the Jewish People in the Age of Jesus Christ. It was only when he finished work on the first of three volumes that he "took a year off and said to myself: 'I now know much better what life in Palestine was like, so how would the Gospel picture of Jesus look if it was dropped into that ocean of Jewish history, religion and culture?'"

Vermes's "first academic love" was the Dead Sea Scrolls, study of which was long hampered by what he once called "the academic scandal par excellence of the 20th century". The first scrolls were discovered in 1947 or 1948, and responsibility for their publication was given to a small, largely clerical team under Father Roland de Vaux. (It also included a man called John Allegro, whom Vermes describes as "very much a maverick", who later became famous for his speculations about the role of the hallucinogenic mushroom Amanita muscaria in the development of the early church.)

Their work continued in secret, at a snail's pace, and eventually almost ground to a halt. Other scholars, denied access but desperate to look at this fascinating material, found their lives slipping away ...

So scandalous was the delay that some sniffed a conspiracy. Did the unpublished scrolls contain a deep, dark secret that could rock the foundations of Christianity? Vermes will have nothing to do with such Da Vinci Code-style speculation and offers a number of far more straightforward explanations: the team was too small, some material was not allocated to individual editors, some was handed over to doctoral students, and overall control was nonexistent, with no one required to publish anything by a particular date.

But was there nothing in the Scrolls to upset Christians? "Only if you have a very funny idea of Christianity," Vermes replies, "if you think it is totally unique and fallen from Heaven. But not if you have any sense that the events of the New Testament happened in real historical time and under specific circumstances."

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This is a field that Vermes has now made his own. His first major contribution was Jesus the Jew (1973). The very idea, he recalls, "was then something new and is now a clich¨¦. At the time, the title produced a frisson. 'Jesus the Jew?' people would say. 'Jesus was a Christian!'" Yet the rabbinic writings of roughly the same period reveal a number of charismatic Jewish healers and preachers similar to Jesus - little-known figures such as Hanina ben Dosa and Honi the Circle-drawer.

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Vermes has continued his "deconstructing of Christian tradition" through many other books. Although they naturally tend to appeal to "liberal and progressive Christians and Jews" rather more than old-fashioned traditionalists in either camp, he suspects that few Christians have lost a firm faith as a result of reading his work.

But he may, he admits, have had an impact on "people who had doubts about the way Christianity was handed down to them from the pulpit and were probably already on the fringes. Some of them write to tell me: 'For a long time I somehow felt as you say, but I was not in a position to formulate my feelings properly.' My books have helped them clarify their own minds."

With The Resurrection, of course, Vermes comes to the heart of the Christian faith. In the words of St Paul, "If Christ has not been raised, then our preaching is in vain and your faith is in vain ... ". Unfortunately, the different Gospel accounts are contradictory. One on its own might count as "anecdotal evidence; together they wouldn't stand up in court".

This is hardly news. By the 2nd century, Christians had noticed the inconsistencies between the Gospels and cobbled together a single narrative, which, says Vermes, "nearly managed to suppress the individual Gospels, but not quite. If it had been successful and we just had four Gospels in one, then all these complications would have disappeared."

Nonetheless, Vermes's verdict is unsparing. "Not even a credulous nonbeliever is likely to be persuaded by the various reports of the Resurrection," he reports. "None of them satisfies the minimum requirements of a legal or scientific inquiry."

Many atheist or Jewish scholars would be happy to end there, with Christian claims dismissed as nonsense. But Vermes, although forensic in his approach to Christian sources, is seldom overtly hostile to Christianity. The Resurrection concludes with a brief epilogue arguing that the notion of "resurrection", interpreted in a more metaphorical sense, remains meaningful and moving.

Is this partly a reflection of Vermes's own background in the priesthood? Perhaps. Vermes himself, however, puts the stress on another historical puzzle in need of a solution.

"I've been told a number of times by serious Christians: 'If everything is as you say, how can you account for the continuation of this faith? There must have been something that kept what seemed to be a failed movement not only going but very successful.'"

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Although the 83-year-old Vermes has reached the end of the Gospels, he has clearly not reached the end of the story. He is hard at work on a new challenge, "trying to find out how the Christianity established as a dogmatic philosophical religion from roughly the 5th century derives from the preaching of Jesus".