Alpha: Cult or no cult, it is getting Anglicans very hot under their dog-collars

Jonathan Aitken has done it three times. The Archbishop of Canterbury, George Carey, thinks it is "superb". Fitness expert Rosemary Conley describes it as "one of the greatest initiatives in recent times". To some, the Alpha course, of which there were five in 1992 and now 18,775 worldwide, is just what the church needs in a day when the Baal of celebrity attracts more followers than Jesus Christ. To others, Alpha is a cult, whose brainwashed followers have been cajoled into embracing an emotional comfort-blanket of unthinking conformity.

An opportunity to decide which it is will be provided by ITV's 10-week television series, Alpha: Will It Change Their Lives? Starting tonight, the series, which is presented by Sir David Frost, documents what happens to a mainly agnostic group of 10 people as they take the course. It will be followed by a £1m advertising campaign for Alpha, with posters at 75 Tube stations, on 3,000 buses and on 1,500 billboard sites around the country. The series has already led the National Secular Society to complain to regulators that ITV is showing "a 10-hour advertisement for the Alpha course".

There is no denying Alpha's success. Its seeds were sown when Sandy Mitchell, the vicar of Holy Trinity Brompton, an Anglican church in London's Knightsbridge ­ HTB to aficionados ­ took on a curate named Nicky Gumbel in 1986. By late 1990 Gumbel was running Alpha, already established by another HTB clergyman. Agnostics, Christians and the plain curious are all welcome. Participants on the 15-session course eat together, listen to a talk on a Christian theme, and participate in an act of worship. In smaller groups they discuss issues in more detail. By the end of the course, many are speaking in tongues.

It is Gumbel who is credited with Alpha's exponential growth. He is described as being more influential within the Church than the Archbishop of Canterbury, an influence that has caused resentment. "It's Noddy theology," says one Anglican priest. "It's all about 'me and my God'." Another says that people have been "alienated" by the "insidious marketing of Alpha as the definitive expression of the Christian faith".

Such remarks are dealt with easily by Gumbel, who along with many of HTB's congregation, has been at a Home 2001 gathering in Pontin's near Lowestoft in Suffolk for the past week. "We regard criticism as a blessing," he says. "And if it's justified, then we can learn from it." As befits a former barrister, Gumbel is adept at deflective answers. He insists that Alpha is within the orthodox Christian mainstream, eschewing labels such as "evangelical". "What unites us is greater than what divides us," he says. "The issue today is the same as in the first century: is Jesus the universal redeemer?" No Christian is going to disagree with him on that. Confronted with difficult questions, he says: "It's hard to give one-sentence answers. We don't have any ultimate answers."

What Gumbel does have is charisma. That, along with the demonstrative nature of the religion he espouses, is what arouses suspicions. Is he a good man, or a dangerous man? Those at Pontin's are in no doubt. The families and many single young professionals who attend HTB greeted Gumbel enthusiastically when he told the story of a woman who had parted from her husband. She did the Alpha course, as did her daughter. Prompting the two women to tell their tale as surely as Bob Monkhouse used to induce contestants on Family Fortunes to recount embarrassing incidents, Gumbel brought the crowd to a crescendo when it turned out that the husband was there and had been reunited with his wife. Only the stony-hearted could fail to have been moved by the story.

Alpha's adherents defy the geeky God-squad image. They range from City bankers and BBC producers to ex-cons. Many are attractive and well-dressed. Not much in evidence are the starched elderly ladies of the declining country parish, or the awkward youths of the college Christian Union. "We're living at a time when the big questions are out there," says Gumbel, whom everyone calls Nicky. "What happens when I die? Why am I here? What about guilt and forgiveness? There is a spiritual hunger, a searching for something beyond the material. People outside the church want to talk about these things, and it's hard to do that in the pub. Alpha provides a place to explore."

Some would say that it provides an opportunity for the needy and vulnerable to be taken in by a cult. To this accusation Gumbel replies: "Come along and see for yourself. Everyone is entitled to do so." Many sceptics have done just that, and have left admiring Gumbel despite themselves. What is beyond doubt is that we're going to be seeing a lot more of him.

'Alpha: Will It Change Their Lives?': ITV tonight, 10.45pm