But considering that it's just 100 yards away from the cosmopolitan bustle generated by the Natural History Museum, the Science Museum and the Victoria & Albert Museum, the home of the vicar of the Holy Trinity Brompton Church is decidedly serene.

Sitting at the end of a pretty Kensington mews - the sort of pretty Kensington mews that anyone can live in, provided they are worth a few million - the Vicarage is a roomy detached house dating from the 1960s. Inside, the feeling that one has just stepped out of the frenetic metropolis and into the sleepy suburbs, is not dispelled.

Preparations are being made for a meeting of the Deanery Chapter - whatever that may be - and foil-covered baking trays await a blast of heat before the dog-collared hordes arrive for lunch. A well-tended garden stretches out beyond the kitchen window.

The Vicar, Nicky Gumbel, and his wife Pippa are all affable smiles, as they ask a few sociable and sunny questions, and make coffee. I'm charmed as their gentle probing quickly reveals that we have a friend in common, and disarmed as I'm told that she is actually one of the two church-wardens here at Holy Trinity.

The sense that I've just stumbled upon a timeless village community in the heart of the capital, has just been burnished to a shine. Yet all is not quite what it seems.

The Reverend Nicky Gumbel is far from parochial. On the contrary, he is sometimes described as the most influential vicar in England, perhaps the world, and for good reason. When he was first posted here in 1986, as a curate, Gumbel naturally became involved in the little course that had been run by the church since 1973, for Christians who wanted further to explore their faith.

In 1990 he took the course over completely. Under the leadership of Gumbel, who was formerly a barrister, the Alpha Course has "gone viral", and is now run from churches and living rooms around the world. Its literature, almost exclusively written by Gumbel, is published in "about" 198 languages.

An Ipsos Mori poll in October found that in Britain alone, five million people have been on an Alpha Course or know someone who has, and that 24 per cent of the population can identify Alpha as a Christian course. The poll also found that one million Britons are interested in attending a course.

If anyone can be credited with having modernised English Anglicanism, reinvigorating a somewhat moribund church, then it is Gumbel. "I get," he says, with a beaming grin, perched lankily in his bookish, ordered study, "the credit and the blame."

Gumbel's quip suggests that he understands perfectly well that the course is viewed by many as highly controversial, and is relaxed about it. But the presence during our interview of Alpha's press officer, Mark Elsdon-Dew, confirms that the organisation is actually highly disciplined and professional.

Alpha is far too big now to be run from this vicarage, or from the large and welcoming church down the lane. Even the little office-block next door to the vicarage became too modest for the needs of Alpha. Now its 150 staff work out of a large, well-appointed, modernised Georgian building opposite the V&A. Gumbel, in a couple of decades, has constructed quite an impressive spiritual empire.

"I was very reluctant to take Alpha on at first, because then it was for people who were already Christians," Gumbel explains. "I was more interested in people outside the church, really. The way things worked out here, I had to take it on. But in the second group I took, I had a group of people who were totally outside the church.

"They were a mixture - some describing themselves as agnostic - but none were regular church-goers. Their lives were radically changed. Most of them are now leaders, you know? And that was when I began to realise that this course could be used for people who were outside the church."

This focus on people "outside the church", makes Gumbel a proselytiser, the very sort of person that people have in mind when they declare that: "I don't mind if people choose to be religious, I just can't bear it when they try to ram it down other people's throats."

"I think that we really try not to ram it down people's throats," says Gumbel. "The idea of Alpha is, well, the way we describe it now, is as a way to explore the meaning of life, an opportunity. Nobody has to explore the meaning of life. We don't force anyone to do Alpha. All we say is that if you do want to do it, we want to make it available to you, wherever you are, initially in this country, and then anywhere in the world.

So if you live in Beijing, we want to make sure that you can do Alpha if you want to do it." Gumbel has not plucked "Beijing" from out of the air, as an example of Alpha's reach. "Yeah. Alpha runs in China. I don't know exactly how it runs. But we did a conference in Hong Kong for 2,000 Chinese leaders, and the Government were represented there.

"We've been talking to the Government for some years. It's slow, but they are changing. The best way for Alpha to go into China is through the official channels, certainly in the initial stages it would be much the best if we could work in that way with them.

"There is a huge spiritual hunger in China - more than anywhere I've experienced anywhere in the world. You can see it among the Chinese here in London. They are so interested and open and responsive. And our brief experience of Chinese people is of responsiveness to Christ. There is almost no resistance in a sense. They are so eager to hear."

Resistance, of course, is something that all Christians in this country have become used to. For people who view Christianity with suspicion or distaste - and there are many - the idea of a British Christian course ploughing into China and meeting with little "resistance" might seem almost sinister.

I must confess that I don't really share that view, although I do understand it. I have to admit that I'm in the final stages of a 10-week basic Alpha Course myself right now, so I'm in a good position to attest that it doesn't feel like some sort of brainwashing cult that preys on the feeble-minded. Doing an Alpha Course is challenging and serious fun, even if you're a dyed-in-the-wool atheist like me.

The structure is laid-back, and runs on a similar format, whether you are taking part in a course catering to hundreds of people - like Gumbel's own course, run out of Holy Trinity - or just a handful - like the one I'm on, which takes place in the living room of a Pastor whose son is a friend of my son. Everyone has dinner, then one of the teachers talks for half-an-hour about that evening's Gumbel-prescribed topic.

Afterwards, there is a discussion, shaped by the leading questions of the teacher, who - in my case anyway - is totally untroubled by robust rebuttal. Yet despite its seemingly free-wheeling nature, the Alpha Course sticks to a tight internal logic. There is no need to explain the existence of God, because nothing explains the story of Christ and his teachings, except the explanation that Christ himself gave, that he was the Son of God. Accept that, and then logic dictates that you accept everything.

There is heavy reliance on C S Lewis's argument, that there are only three possible ways to explain Christ's life, death, resurrection and fulfilment of prophecy - that he was mad, that he was bad, or that he was who he claimed to be. Christians believe that he could not have been mad or bad, because his teachings made - and make - such good moral sense that they have never been surpassed. So there's only one option left.

Gumbel comes as close to contemptuous as such a genial chap can get of Richard Dawkins's "fourth option" - that Christ was "honestly mistaken". "The argument in The God Delusion is that anyone who believes there is a God is deluded. If you believe there is a God, you are deluded, but Jesus, who believed he was God, was not deluded, just honestly mistaken. So to believe there is a God is a delusion, but to believe that you are God could just be an honest mistake."

Gumbel looks both irritated and incredulous, and neither emotion appears to diminish when I suggest that Dawkins is simply displaying a reasonable respect for the status of Christ as a genuine historical figure of immense importance and stature. "Well, I think he has a respect for Jesus, which is a good thing. But I think the argument that there's another category is a false argument. If we came across someone out there who honestly believed they were God, we would say: 'You're deluded.'

"You can't prove that Jesus is the Son of God. You can't do a mathematical proof, or a philosophical proof, or a scientific proof. But it's more akin to the kind of evidence that I was used to as a lawyer. The evidence you put before a jury is historical evidence, the jurors weren't there, but they listen to witnesses. It's recent history, but the jury has to make up their minds whether they believe that person, or not. That's a step of faith. You can't prove things in a scientific or mathematic way."

Or beyond reasonable doubt? "Well, they've dropped that now, so it's satisfied so that you feel sure. But you have to look at the evidence and make up your mind, and I think it's the same with the Christian faith. It doesn't force you to believe. Where I part company with that line of argument, whether it's with Richard Dawkins or with anyone else, is where it says that all faith is irrational. I don't think faith is irrational. I think faith is based on reason and there's good reason to believe."

Actually, I accept what Gumbel says here. The course he has developed is clever, in that it helps you to accept, at the very least, that those who choose to believe in God can do so rationally, even if you choose yourself not to trust the evidence that they trust. If I've learned one really important thing from doing Alpha, it's that you don't have to be mad to be a Christian, and that in fact, madness doesn't even help.

From my experience, the course leaders - we have four - are adept at fielding questions and bringing matters back to the values that are viewed by Christians as essentials. Like Gumbel himself, they steer discussion away from controversy and back to the healing power of love. Again and again, despite my intellectual objections, I find myself feeling like a stubborn stick-in-the-mud, unwilling to go with the spiritual flow because I'm too mean-spirited to open my heart to Jesus.

I've been surprised by how much power the course exerts. Sometimes a leader will say something - that, say, human strife is always, at root, caused when people choose to love themselves and not others - and it strikes you quite profoundly as a simple, ineffable truth.

When you ask why people can't just adopt that credo, without necessarily going for the personal relationship with God bit - they'll just say that you can, of course, and that Jesus admired such people and described them as men and women of peace - but that it's a lot easier to live that way when you've tuned in to God's message. Christianity then starts to look like a radical lifestyle choice, something that you refuse to join in with because you're blandly conventional, hampered by a failure of imagination, and scared of looking like a wuss.

In truth, the course has made me feel a little envious of Christians. They live in the strangest and most alien of worlds - a world where everything makes sense. Oddly, the lure is not that they have all the answers. Instead it is that they have dispensed with the need for questions. Their moral universe is stable. For them, it all works. Unsurprisingly, since he has personally shaped the course, this interpretation chimes with Gumbel's own brief description of his conversion.

"Jesus said: 'I came so that you might have life, and have it in all its fullness.' And that was my experience. I came from a background where I didn't have a faith. I wasn't unhappy. I wasn't desperately going round saying: 'I need to find meaning and purpose in my life.' But I did find it. There was a spiritual hunger that was satisfied in this relationship with God through Jesus and through the experience of the Holy Spirit and the love of God. "The Christian faith is very simple at its heart. God loves you and the experience of God's love is a very profound thing.

It's a life-changing thing to be loved and accepted and to experience forgiveness and the result is a love for God that changes your priorities in life so that that relationship becomes the number-one priority. So I want to read the Bible. I want to hear what God is saying to me today.

I want to pray. I want to develop that relationship. "Jesus said: 'The first command is love God, and the second is love your neighbour.' So the priority of your life is to love, starting with your wife and your children, then spreading out to the world. To be a Christian is to want to make a difference to your family, to your neighbourhood, to the society around, and ultimately to the world."

If you like your amateur psychoanalysis to be glib and simplistic - and I have to admit that I do - then you have to assume that Gumbel's intense interest in bringing this message to people "outside the church" flows from his own position as an outsider-convert. An atheist when he went to Cambridge, from Eton, Gumbel converted to Anglicanism in his first year, after reading the New Testament.

He worked as a barrister for some time, worshipping at Holy Trinity, but there is not even the most workaday trace of Anglicanism in his background, or in his family history. "My father was an agnostic," says Gumbel. "He was not a believer. He was a secular Jew. He had probably been baptised, though I don't know for certain. He was brought up in Germany as the Nazis began their rise to power - but he left in 1929 when he was called to the bar over here. He always loved England. "He fought in the war, he was an officer in the British Army during the war. He just got his parents out in 1939 and his sister. But he never spoke to me about the fact that he was German - he didn't have an accent, he spoke perfect English - or that he was Jewish. He never spoke about either of those things.

"My mother told me when I was 14 that he was Jewish and German and that I was never to speak to him about it and I never did. Because of those things there was very little he could talk about, because everything led back to that. You couldn't talk about his school, his university ... You couldn't talk about him at all. So, the weather was the main topic of conversation because that was the only thing that didn't lead back to it."

Gumbel laughs heartily when I suggest that he was used to having a distant and unknowable father figure in his life, though he is quick to insist that: "He was also an amazing man who I loved very much and miss hugely." No amount of joking changes the fact that Gumbel left the bar and started his training for ordination into the Church of England the year after his father died, when he was 27.

Gumbel's father may have loved England. But he would surely be astonished and impressed by his son's quiet dominance of the far-reaching English institution that is Anglicanism. Gumbel's influence, however, has extended even beyond Anglicanism, and he is pleased that Alpha runs in the Catholic, Methodist and Baptist Churches as well.

"All we can do is do everything we can to promote unity. To the person outside the church, disunity is so off-putting. We have to stop fighting, because what unites us is infinitely greater than what divides us. We all believe in the divinity of Christ. We all believe that he died for us. We all believe in the resurrection. We all believe in the Bible and the historic creeds, and that's a huge amount of unity. My plea is always: 'Let's stop arguing about the secondary things, and unite around the really important things, and get that message out to the world.' "

It's clear that the promotion of unity is everything to Gumbel. So I feel like a heel when I tell him that I've noted that his organisation offers a course in post-abortion counselling. Gumbel squirms uncomfortably. He knows perfectly well that not all Christians would necessarily agree with the idea of such a course even being offered. "We do believe in the value of every human life from the moment of conception," he says.

"That is what we believe in. But also we are trying to show God's love for every human being, regardless of what's happened in their lives. We all make a mess in life, and we're trying to help people ... "I haven't been on that course and it's not the sort of course that I could attend ... I don't even know where it meets. It's very confidential and private. People don't necessarily want people to know that they're on it. But I do know from everyone involved in this church that it will be about moving forward and healing."

I suggest that it is difficult to understand how people can attend such a course without believing that in accessing a straightforward, practical and perfectly legal operation, they have done something inherently wrong. What can be healing about that? "My own feeling is that tolerance and truth have to be balanced, and that there's always a debate about love and truth. We're supposed to speak the truth in love. For some people the emphasis is so much on truth that love seems to get thrown out of the window. For others the emphasis is so much on love and tolerance that truth seems to get thrown out of the window. I'm sure I don't always get it right, but it's that balance that we're seeking to find.

"Unity is a very high priority but in terms of priorities, one of the great, urgent issues in our world today is that 30,000 children are dying every day of starvation. That seems today to be the great moral issue facing our world. For all those children to be dying every day of starvation is evil. Just in terms of the scale of it, it is so horrific. And it's not that nothing can be done. It really is possible to do something about these things.

The world has enough resources to deal with these issues. Universal primary education costs about the same as Europeans spend on ice-cream every year." Gumbel looks pained when I suggest that religious affiliation can be a hindrance to charitable efforts, not a help. He becomes agonised when I mention the recent murder of Gayle Williams, the young woman who was killed in Afghanistan because her assassins believed her to be a proselytising Christian. He recovers quickly though and says that he has found working for the Blair Foundation, Tony Blair's low-key, multi-faith organisation, to be a positive experience.

"We share a common agenda on certain things and can work together on them - poverty being a prime example. So we're working with the Blair Foundation in Tanzania, working with Muslim Aid and Jewish Aid to distribute bed nets to combat malaria, and I think that's a good example of where we can work together. The things I feel most passionately about unite us all.

"The key thing is a common vision, and the key thing about the Blair Foundation is that it's trying to unite people with a common purpose. There's no pointing saying we are all the same. Islam and Christianity are not the same. But we can work together on certain things." Gumbel is opaque on this year's much-maligned intervention by the Archbishop of Canterbury, Rowan Williams, on sharia law in Britain. He studiously avoids controversy himself, and says he declines to go on Question Time, "because they'll ask me questions, and if I say certain things I'll alienate half the churches running Alpha quite unnecessarily, just so that I can express my view, when my view is probably of no value whatsoever. It does no good at all for me to express my views on certain things, and potentially does a lot of harm."

He does, however, speak about the Archbishop with tremendous enthusiasm, even though his previous assertion could be read as a refutation of the idea that the Anglican Church should have any political dimension to it at all. "I think Rowan Williams is doing a really good job trying to hold it together and we should support him in every way we can. And he is a genius, a complete intellectual genius and a giant. He is so clever and understands so much.

"But like some really, really bright people, they see every argument and so what they write is so nuanced that it's quite difficult to understand and to follow. If you read any of his books, you'll find that at every sentence you have to stop and think: 'What on earth does he mean by that?' "Yes, I'm a great admirer of Rowan Williams. Personally, I like him very much. I have to admit his wife is on our staff here, and teaches at the theological college here. She's a really lovely person, he's a really lovely person, they're really lovely people, and they're very godly, they're very spiritual."

Once again, one is reminded that despite Nicky Gumbel's casual and open ardency, he is the architect and the spiritual leader of a very big, very influential, and very powerful organisation. We step out of the study, and into the jolly thrum of the Deanery Chapter meeting, full of relaxed yet earnest people, whose passion is "making a difference". Who can complain too much, really, about a network that cares only for the promotion of love?

When I tell my Alpha group that I've interviewed Nicky Gumbel, they exchange arch glances, and remind me how sceptical I was when they told me that they didn't believe in coincidences, but instead in the wisdom of God's guidance.

"How does it feel," one of them asks, "to suddenly find Alpha everywhere you go?" When I reply that it feels like entrapment, they all laugh away, like drains. They're not threatened in the least by atheists, and it seems only polite to respond in kind.

END