If a white suit has long been the trademark of Martin Bell, there is no doubt who is now firmly established in the minds of television viewers as the smart guy in the white hat. The magnificent panama that Diarmaid MacCulloch sported on his travels from Damascus to San Francisco is sitting on the desk in his book-lined study in Oxford University’s Theology Faculty.

“I can’t wear it any more in Oxford,” he chuckles. With his six-part series A History of Christianity now being screened on BBC4, before moving to BBC2 in the new year, the truly epic quality of the programmes means that this remarkably hard- working Oxford University professor is having to get used to being recognised when he steps out into St Giles’.

It has become compelling viewing even for those who may have not previously concerned themselves with how Saint Augustine’s idea of original sin shaped our views on sex, or the role of the Inquisition in re-inventing Catholicism.

And that’s largely down to the fact that it comes across as one man’s extraordinary journey.

“Well, yes, it was glorious fun,” he readily admits. “The biggest road movie ever — crawling around the churches of the world.”

There appears more than a touch of the adventurer about him as he seeks out Ghanaian prophetesses, ruined Syrian basilicas or pyramids in Mexico. And not even Indiana Jones, another man with a hat with a taste for crypts and early Christian relics, ever found himself having to face the Buddhist grannies in China.

Another grin crosses the professor’s face as he recalled provoking a riot in a rural corner of China, when the BBC crew tried filming some reputed eighth-century Christian remains in the inaccessible upper storeys of a pagoda.

It turned out that local Buddhist villagers, furious at the preferential treatment granted to Christian remains in the People’s Republic, had been locked in a bitter dispute with the Chinese government over allowing the site to be filmed.

Villagers took it upon themselves to stop the team climbing scaffolding that had been carefully put in place for the filming.

“In the end, our attempts to climb the pagoda were scotched by the fearsome old grannies of the village,” said Prof MacCulloch, apparently not in the least put out.

For, in a way, the incident served as a dramatic confirmation of his motivation in making the programme and spending two years writing the 1,184-page book A History of Christianity: The First Three Thousand Years.

“Religion is very important to people,” he says. “Religious belief can be very close to madness. My aim was to seek out what I see as the good in the varied forms of the Christian faith, while pointing clearly to what I think is foolish and dangerous in them.”

You might imagine that the task of telling the story of one of the world’s great religions in a hefty single tome, with the narrative ranging from Palestine in the first century to Korea in the 20th would be challenging enough on its own. (The book actually begins in Greece in 1000 BC rather than in a stable in Bethlehem).

But thanks to Guy Fawkes, no less, he landed himself with the additional job of simultaneously producing a major television series, which involved the little matter of travelling to 21 countries with a BBC production team.

He had been invited to contribute to a BBC Timewatch programme that was taking a fresh look at the Gunpowder Plot. “During a break from filming, someone asked, ‘can you think of anything else you could do?’. I said, ‘well, I just happen to be writing a history of Christianity’.”

Eyes lit up, and a three-year project away from his teaching job beckoned for Oxford University’s Professor of the History of the Church.

He had always maintained that exploring the byways of history is no bad thing and that historians can only benefit from getting out of libraries and seeing the places they are writing about. It turns out the story of Christianity is far more global than even he had imagined and he shows how Baghdad might have seemed a more likely capital for worldwide Christianity than Rome.

As things turned out, the discipline required in telling the story of Christianity in six television episodes helped him in the writing of his book, as he wrestled with the daunting problem of getting right “the big shapes and big structures”.

He says: “I spent a lot of my life thinking about those shapes. But I ended up going back to the book and revamping the whole thing. The television series certainly enriched the book.”

He ended up having to write large parts of the book while on location, travelling with a laptop so he could record places while they were still fresh in his mind, along with incidents such as appalling behaviour he witnessed in the Holy Sepulchre in Jerusalem early one December morning, as two rival ancient liturgies noisily proceeded simultaneously above the empty tomb of Christ.

“I particularly enjoyed the moment when the bearer of the Coptic censer swept with brio around the shrine to the very frontier of the rival liturgy and sent his cloud of incense billowing into the heretical Latin West,” he recalls.

If having to get up early was a drawback, it often meant he and the crew had some of the world’s most memorable places to themselves.

“Think of the Grand Mosque of Damascus or the Forest of Stelae in Xi’an entirely free of tourists.”

The professor recognises that he presents “an emphatically personal view” of Christian history in both the book and series, which last night were both celebrated at Oxford University Church of St Mary the Virgin, at an event attended by the university chancellor Lord Patten.

For Prof MacCulloch’s own religious journey has been a long one, having been born into a family where the church had been a three-generation family business.

His childhood was spent in the rectory of an Anglican country parish in Suffolk. “My dad was a country parson. We talked about history in the way other people talk about football. I remember my childhood with affection. It gave me the confidence to be who I am now.

“When I was a little boy, my parents also took me looking at old churches. But soon they realised they had bred a monster.”

A monster?

“Well, my appetite for churches was insatiable; no ecclesiastical building was safe from my probings. Eventually I went into therapy for the mania by taking up my role at Oxford. Then the BBC contributed further to rehab by giving me my own television series.”

His fascination with history had been greatly fuelled at Cambridge by the great Tudor historian Sir Geoffrey Elton, who in his hearing had once remarked that if historians are not sceptical, they are nothing.

“A terrifying man,” he recalls. “More like a headmaster than any headmaster I’ve met.

“His nephew, Ben Elton, is beginning to look like him, which explains why I’ve never been able to enjoy watching Ben Elton.”

A career in the church it seemed had once looked likely. I was ordained as a deacon and started down that road,” he said.

“But being a gay man, it was just impossible to proceed further, within the conditions of the Anglican set-up, because I was determined that I would make no bones about who I was.

“I was brought up to be truthful and truth has always mattered to me. The church couldn’t cope and so we parted company. It was a miserable experience.”

He remains a regular member of the congregation at St Barnabas Church, Jericho, where he plays the organ and sings in the choir. But there has been a clear retreat from religious orthodoxy and in the introduction to his book he describes himself as “a candid friend of Christianity”.

He has no doubt that Jesus Christ lived.

“That is not in question. There is as much evidence for his existence as there is for people in classical history that we know about.”

But some may be surprised to learn that the man who has spent so much of his life writing about Christianity, no longer believes Jesus to be the son of God in the traditional sense.

He told me: “I don’t find it plausible that he can be identified with God in the way that classical Christian belief claims. He was a figure of huge importance, but I cannot accept claims of his divinity. I cannot sign up to ancient doctrinal statements as an individual, even though I can recite them as part of a community in the liturgy of the church.”

The book burnings and death sentence pronounced on Salman Rushdie by Iran’s religious leader Ayatollah Khomeini following the publication of the novel The Satanic Verses had a profound effect on Prof MacCulloch’s view of sacred texts.

“What prices sacred books after that? That is no a way for human beings to behave towards each other. Sacred books cannot justify human beings behaving towards each other in that way. Although this is not the fault of religion, but people.

“I continue to find sacred books fascinating but not as the cornerstone of belief. Why should the Bible be different from other sacred books? The Bible's authority for Christians lies in the fact Christians have a special relationship with the Bible that cannot be altered, like the relationship of a parent and child.

“Once we see this, much modern neurosis about the authority of the Bible can be laid aside. Maybe the Bible can be taken seriously, rather than literally.”

But he insists that he remains in wonder about “how something so apparently crazy” can be so captivating to millions of people.

“What I have kept from religion is a sense of wonder. It is a great medicine for human pride and conceit.

“That is at the heart of Christian worship. It should encourage wonder.”

And so in its way should A History of Christianity.