A small cloud hangs over the drab Victorian church of Holy Trinity Brompton, in west London. It's tempting to see it as the wisp of suspicion that, rightly or wrongly, lingers over Britain's most powerful evangelical church. This is the birthplace of Alpha, the "informal, friendly and fun" course now practised in 169 countries, which numbers the new Archbishop of Canterbury among its recruits.

Ten days ago, Justin Welby was enthroned as leader of the Church of England. It was a new high in the history of HTB. The 56-year-old former oil executive is the first resident of Lambeth Palace to have found God through rock music and group therapy. Twenty years ago, evangelical Christianity was a fringe activity, associated with loony American cults. Today, 1.2 million Britons have attended an Alpha Course, and thousands attend HTB services every Sunday. It's British Christianity's biggest success story.

I've come to Brompton to meet Nicky Gumbel, vicar of HTB and chief architect of the Alpha Course. If Welby is the Church of England's new leader, Gumbel is his spiritual father. The two men share remarkably close stories, having known each other for more than 40 years. At 57, Gumbel is roughly the same age. Both attended Eton and Trinity College Cambridge, before pursuing high-powered careers which they gave up for the church.

Download the new Independent Premium app Sharing the full story, not just the headlines

The rectory at HTB is a classic Church of England affair. Not a handsome Georgian number, but knocked up in the Sixties in glass and concrete. Inside, it is light, airy, and tastefully upholstered in expensive fabrics. Gumbel is tall and handsome, with curly greying hair; he speaks with a clipped accent. He met his wife, Penny, through the church, and they have three grown-up children; the model of an active Christian family.

So, is Justin Welby the best Archbishop ever? Gumbel roars with laughter and points out he's only been in office a week. "I'm so biased because obviously he is a friend, and we love him." This is Gumbel's first interview since his friend became archbishop, and he is keen not to claim the appointment as an HTB victory. "It's not a difficult job," he says. "It's an impossible job. I actually think it is impossible, trying to hold everyone together. It needs a lot of support, and a lot of prayer."

Welby had been a bishop for less than a year when he was announced as the surprise replacement for Rowan Williams in November. His rollicking ascent through the ranks saw him leapfrog more experienced candidates, such as Richard Chartres, Bishop of London, and John Sentamu, Archbishop of York. Is Welby up to it? "I don't think anyone is strong enough to take it on," says Gumbel. "I don't think Barack Obama is strong enough to be President of the US. I don't think David Cameron is strong enough to be Prime Minister. You are under the constant glare of the internet and the media. You have to be on absolutely perfect form, 24 hours a day. But I think we have to believe [his appointment] is the providence of God."

They don't call themselves evangelical at HTB, and they certainly don't consider themselves a cult. But for some, there is something sinister about a church that, as part of its worship, encourages its members to lose control and "speak in tongues". This is the practice of speaking in a language unknown to you, often Arabic, as recorded in the New Testament. Then there is the gay question: for all the unstuffiness, HTB is as strict on homosexuality as the Catholic Church. The position is that sex outside marriage is a sin, whether hetero or homosexual, and since gay people can't marry, they can't have sex.

Mr Cameron's Bill to give gay people equal marriage rights is a major challenge to this line. Logically, I say, if gay people are to marry, then the sex they practise will no longer be a sin. "Well, it hasn't happened yet," Gumbel says. When I press him, he refers me to what Archbishop Welby said on his first day. Though against gay marriage, he said he was "averse to the language of exclusion", and "you see gay relationships that are just stunning in the quality of the relationship". He added that he had "particular friends where I recognise that, and am deeply challenged by it".

Gumbel is quick to point out that many of his friends are gay, showing me a photo of him with a group of his squash partners. Among them is the adventurer and Alpha convert, Bear Grylls ("sickeningly good at squash. He's good at everything"). Gumbel is clearly tired of being asked his views on homosexuality. "The position that we hold to here is exactly the same as the church worldwide." Which is what? "Look at what Justin said in his interview. Justin absolutely put it how I would want to. It holds to the orthodox position while being extremely gracious and accepting."

But what does he personally think? "The reason I'm reluctant to say is that you get question after question on the subject, and it's not the main thing. It gets taken out of context, and it's said that you are obsessed with the subject. I'm not obsessed with it. I'm not interested in it. I don't want to say things that seem nuanced and different from what Justin has said. He is leading the Anglican church. I am an Anglican clergyman, I'm 100 per cent behind him. And he's got to fight the battle. I don't."

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

The trouble is that this issue is not going away. Peter Tatchell has written an open letter to Welby accusing his position of being homophobic, and gay marriage is destined to become legal, whatever the Church of England says. Welby has indicated that he is open to discussion, and Gumbel too shows flexibility. "Everything is being re-evaluated all the time," he says. "Hopefully in life you change and you learn. You nuance things, you develop. The other thing is, we have people in our church who have totally different views on a number of things... and that is great. They don't all have to believe the same things."

If I came looking for a Blofeld-style figure masterminding Britain's Christian revolution, I couldn't have found a less sinister man. Gumbel is warm and friendly, and highly intelligent. He was born to a German Jewish father and a non-practising Anglican mother, and discovered religion for himself in his twenties. "My parents were a little bit worried. My mother said it's fine to become a Christian, as long as it doesn't change my life. And my father was immediately worried that I wouldn't become a barrister, which to him was the only thing you could be."

Gumbel did become a barrister, though he gave up six years later and retrained as a priest. Apart from his Jewish ancestry, his discovery of Christianity was all the more surprising, given that he had been a vocal atheist at school. The church, for him, had been "boring" at Eton. It wasn't until his first year at Cambridge that it spoke to him.

"My degree was then in economics, and I really had nothing to do. So I read the New Testament. And I could not believe what I was reading. I was so struck by it, by the story of Jesus. His character, his words, his parables. It just had the ring of truth."

In 1983, Gumbel began studying theology at Wycliffe Hall in Oxford, and became a curate at Holy Trinity Brompton in 1986. The Alpha Course had been founded in 1977, by the Reverend Charles Marnham, but Gumbel developed it into its current format. Courses involve sessions over a 10-week period, which are preceded by informal suppers. There is usually a weekend away, and participants include a mixture of believers and non-believers, some of whom never convert.

Gumbel admits to suffering from occasional doubt, saying "doubt and faith are two sides of the same coin". Only recently, he discovered the full extent of his father's Jewish ancestry, having been sworn never to ask him about it by his mother when he was 14. I wonder how he can square the existence of God with the murder of most of his father's family in the Holocaust. "There are no easy answers," he says. "You can look at it from the point of view of God giving us freedom. He gave us the freedom to love, but also not to love. He also suffered for us, on the cross, and suffered alongside us. But it doesn't explain the Holocaust."

The greatest challenge to his faith was in 1996, when he was playing squash with his best friend, Nick Hawkins. "He was 42 years old, a father of six, and he just dropped dead on the squash court. That was a very, very difficult moment." Did he consider jacking it all in? "No. I remember going for a walk at four in the morning, and I chose to go on believing. I said to God, I choose to go on believing in you."

Gumbel is usually described as a charismatic preacher, but he sees himself as shy and introverted. So how does he stand up and preach? "I find it nerve-racking, I really do. I do it because that's what I'm called to do. I much prefer writing." He has written several books, which have sold millions of copies worldwide.

This Easter, Gumbel is setting off on a tour, to Myanmar, India, Singapore and Indonesia. With 20 million people around the world now thought to have experienced an Alpha course, it's hard not to see Gumbel as a crusading evangelist. "I hate the word evangelical! If you torture me, I'm Anglican. But it's not helpful. We label people in order to dismiss them."

He is winding down his own speaking commitments, but remains HTB's figurehead, and will provide an invaluable support to his friend the Archbishop. "Justin spoke about optimism, and these are times of optimism and hope and resurrection," he says. "I personally feel very optimistic about the church, about the new Pope, about the new Archbishop. We are in a new season. There is so much interest. I've never experienced so many young people pouring into church."

As I leave the rectory, that little cloud is still above HTB. But it does look slightly smaller.

Alpha females

Spice Girl Geri Halliwell and former glamour model Samantha Fox are among the high-profile young professionals who have taken the Alpha Course. Fox said it changed her life while Halliwell says she was encouraged to do it by friends, having previously been agnostic. She wears a crucifix on a necklace.