One of the most spine-tingling moments in U2's Elevation tour - and there were a few - came when the band led enthusiastic crowds around the world in an unself-conscious chorus of the Hallelujah. Whenever this happened, the sweaty masses, mouths open in praise, seemed more like a congregation than a crowd. Not quite rock 'n' roll, not quite religion. But there was, as Popstars reject Darius might put it, an awful lot of love in the room.
It is impossible to imagine any other rock stars (and, no, Cliff Richard doesn't count) orchestrating what for a few minutes amounted to a mass prayer meeting, albeit with state of the art lighting and sound. "Hallelujah," sang Bono, "praise ye the Lord."
Presbyterian minister Steve Stockman is the author of a new book which sets out to explore the religious themes both in the band's music and in its conscience-pricking extra-curricular activities. Walk On, the Spiritual Journey of U2 examines what Stockman calls the "God dimension" of the band.
The Christian identity of three-quarters of U2 - Bono, Larry and Edge - has been touched on in most of the interviews given by them over the past 25 years. Using this material, Stockman traces their journey from the days when they were members of evangelical group Shalom right through to the crusading years when Bono appeared to have embarked on a one-man mission to save the world. From Bible studies as teenagers right up to the Jubilee 2000 campaign, no spiritual stone is left unturned.
Stockman was encouraged in writing his book, Walk On by a disgruntled e-mail correspondent who took him to task for daring to suggest that the authors of a song called I Still Haven't Found What I'm Looking For could be of a Christian persuasion. Surely, if they had found Jesus, their search was at an end. Surely, if they were Christian they couldn't curse and drink Jack Daniels. Could Bono, a man who dressed up as the devil for the Zoo TV tour, be seriously considered as an authentic, fully paid-up follower of Christ?
It is an argument Stockman, who works in the chaplaincy at Queen's University, Belfast, and presents a weekly BBC radio programme, Rhythm and Soul, has found himself challenging repeatedly in Christian circles. "The Church in general has not seemed to understand U2 and has dismissed them as people lost to the Church," he says.
Through his defence of the Christian motivations of the band on his website, he was approached by publishing house Relevant Books to write a book on the subject. And while some Christians will not be pleased, Stockman also reckons there will be an equal number of U2 devotees who will question his theological take on the band.
"I think some fans will resent me talking about U2 and God in the same context," he says. "But as a fan for 20 years I have always believed that you cannot separate U2 from their Christianity. To understand U2, you have to understand where they are coming from spiritually."
The book is essential reading for anyone who ever wondered exactly how many U2 lyrics are nicked from the Bible - lots, as it turns out. It is a spiritual companion to their lyrics by a man who has used the words of U2 in his sermons for years. It also contains Stockman's own personal assessment of groups he calls the "theological police", who denounce those who stray outside the confines of their limited Christian viewpoint.
"The book gave me a chance to air some of the views on the Church as well as views I have on U2," he says.
Stockman paints a picture of a band - though it has to be said that, unsurprisingly, Bono is the star player here - rooted in a Christian tradition. Charting pivotal events in the band's history, he makes a convincing case for its unique place in an industry where God is uncool.
In his quest to explain the worthiness of the band, and particularly of Bono, some might feel Stockman goes a bit too far. After dissecting the lyrics to Peace on Earth from the latest album - "Jesus, could you take the time/To throw a drowning man a line" - he writes that the words show Bono's faith to be honest, courageous and vulnerable.
"This is why the church needs Bono," he continues. "This is why everybody needs Bono."
Not everyone will agree, but Stockman's underlying point that the Church needs to look beyond the traditional messenger, the traditional message even, is as perfectly argued as his T-shirt, which reads: "I don't need religion, just the love of God."
"People who tell me that U2 are not real Christians are missing something so obvious," he says. "They, particularly Bono, are campaigning to change the world; they sing about wanting to create a heaven on earth. Those people who don't believe they are Christians are missing their radical gospel agenda, which is trying to create another, better kingdom. They are missing the truth, which is we should all be trying to make a kingdom like the one Bono is trying to create."
So does he believe Bono has been anointed to rock stardom (the Pope did borrow his shades after all)? "I do say in the book that I believe there is a vocation there," he says. "The question is whether he would have done it if he weren't in a rock band.
"I believe this was the way the band could make the best of their abilities and infuse what they do with a Christian essence. Could they have done it if they were in a church? Perhaps not - and maybe that is the key."
Stockman's book focuses mainly on Bono, Larry and Edge, following their roots as evangelical Christians, their struggle within that group and their subsequent scaling back of previously very public declarations of faith. But Stockman does include a quote from a recent interview with Adam Clayton, the only member of the band not to have declared a Christian faith.
"I don't know what it is," said Clayton on the magic that was worked in stadiums across the world during the Elevation tour, "but I definitely know when it's there. It doesn't happen every night, but some nights, there's a sense of community and fellowship. And people have said there's a spiritual aspect to what's happening in the house."
It will be interesting to see how Bono and the rest of U2 respond to Stockman's assessment.
"I am half nervous and half intrigued about Bono's reaction," he says. "I've argued this thing with people all over the world for years, Christians asking me whether U2 are still believers. Now I just have to say, there you go, read the book."
Walk On: the Spiritual Journey of U2 by Steve Stockman, published by Relevant Books, is available from www.amazon.com ($13.99 in US). The Relevant Books website is at www.relevant-books.com