One evening last January, gusts of icy wind and rain rollicked down Oxford Street in the West End of London, causing passers-by to seek refuge in brightly lit department stores. I, however, ducked into an inconspicuous doorway opposite BHS, entering a world far removed from the shoppers’ paradise outside. The door led to Regent Hall, Oxford Street’s only church. This was an ice rink until William Booth, the founder of the Salvation Army, converted it into a place of worship in 1882. The large, balconied auditorium was empty and dark, but there was activity outside the back entrance. As many as 30 homeless men, whom Booth would have described as “the least, the last and the lost”, were waiting for a Salvation Army drop-in centre to open.

They were jobless, destitute rough sleepers who spend the nights in doorways or on buses, overcoated and woolly-hatted in futile defiance of the cold, their few belongings crammed into bin bags or old backpacks. Among them were alcoholics, drug addicts, the physically sick and mentally disturbed. A few barely spoke English. Occasionally former soldiers turn up here, suffering from post-traumatic stress disorder, or defeated by unregimented life. “Some cases really get to you, especially when there’s very little you can do,” said Heidi Soljava-Duprat, the cheerful Finn who runs the centre. “It’s sad that we’re in 2015 and the problems are still the same as in Booth’s time.”

The doors opened at 5.30pm, giving the human flotsam a brief respite from the elements. Some sat in groups; others kept to themselves. They ate food donated by Nando’s, Eat and Starbucks. Volunteers handed out blankets, clothing and shoes and offered compassion and advice. For a few hours these men were treated with respect. A silver-haired Syrian who once worked for the BBC’s Arabic Service cried as he told me how he spends nights in churches since his wife ejected him. “This place is like my family home. It gives me great comfort.”

At 8pm the centre closed. The men lingered to the last minute. One or two begged – in vain – to stay. Watching them disappear into the night was “horrendous”, Soljava-Duprat said. A man called Kenny showed me his bag. “This is my pillow,” he said. “And these,” he added, tugging at his clothes, “are my bed.”

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When Booth founded the Salvation Army 150 years ago next month, he offered the destitute “soup, soap and salvation”, but the food and shelter this evening came with no quid pro quo. The Oxford Street centre did not offer its visitors salvation. Soljava-Duprat hoped that by showing them love and compassion they might turn to God of their own accord, but “no one is forced to receive the message”, she said. “Our job is to get them to a point where they can decide for themselves whether they’re ready to accept some kind of faith.”

A pawnbroker’s apprentice who became one of the most compelling revivalist preachers of his age, Booth had no such compunction. Saving souls was his life’s work. “The Founder” and his redoubtable wife, Catherine, the “Mother of the Army”, pursued that goal with extraordinary single-mindedness. They targeted the poor, the marginalised, the friendless and the fallen – those rejected by the established churches of Victorian Britain – constantly totting up the numbers brought to God.

Open-air meetings, brass bands, rallies outside bars and brothels, testimony from redeemed sinners: Booth did whatever was necessary to attract their attention. He took bawdy music-hall songs, changed the words and turned them into stirring hymns. He connived with the editor of the Pall Mall Gazette to buy a 13-year-old girl for £5 to expose the scandal of child prostitution. For Booth the “soup” and the “soap” were a means, salvation the end. “To reach the people whom we could not reach by any other means, we gave the hungry wretches a meal and then talked to them about God and eternity,” he wrote.

Booth made many enemies. Meetings were frequently attacked by mobs financed by the brewers and brothel-keepers whose livelihoods the Army threatened. The female Salvationists’ peaked bonnets were designed to act as protection from stones.

Respectable society, too, loathed the “Sally Army”, regarding it as fanatical, vulgar and ridiculous. Yet Booth, who resembled some long-bearded, Old Testament prophet in both style and appearance, gloried in persecution. The Army stirred people’s conscience by highlighting the horrors of the slums: it gained 10,000 full-time officers within a dozen years of its formation. By the time Booth died in 1912, aged 83, his organisation had spread throughout the world and he had met presidents and prime ministers; 65,000 mourners filed past his coffin at Clapton Congress Hall.

The Salvation Army has since become an integral part of British life, doling out tea and comfort during the two world wars; offering physical and spiritual sustenance after disasters and atrocities such as the terrorist attacks in London in July 2005; and carolling at Christmas. It is an institution that seems to have been with us for ever, but one that few know much about – which is why I chose to spend a week investigating its work.

I visited its churches, shelters and drop-in centres and was shocked by the number of outcasts I met. I interviewed numerous officers – all good, kind, selfless people who were dedicated to helping the desperate. But I also found a great religious movement that, in Britain at least, is shrinking, ageing and, frankly, struggling in this secular age.

“We need to do a lot better than we are,” Clive Adams, the Army’s British ­territorial commander, said when we met at its UK headquarters at Elephant and Castle, in south London. “Booth was willing to do almost anything to attain his goals. We need to get back to being less risk-averse and more bold in achieving ours.”

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William Booth, “the Founder”, began life as a preacher in the Methodist tradition and took an unashamedly militant approach to tackling social evils. Photo: Universal History Archive/Getty Images

Booth would still recognise his Army. It remains a quasi-military organisation with its own (severe) uniform, salute (index finger pointing to heaven), flag (flown on the moon during the Apollo 16 mission), motto (“Blood and Fire”), newspaper (the War Cry), ranks (from lieutenants to generals) and decorations (the Order of the Founder and Order of Distinguished Auxiliary Service). It enjoys the distinction of having been banned as a religious organisation by the Bolsheviks in 1923, and as a paramilitary group by Russia’s government in 2000.

Adams said that the military imagery remains appropriate: “Absolutely we’re at war. We’re in a war against evil, injustice, everything that marginalises people. We’re at war against sin.”

The Army remains an independent Christian church with its own doctrines and ethos. Its places of worship are called “corps”, and are clustered in Britain’s more deprived areas. Established to support the poor and illiterate, they spurn the elaborate trappings of mainstream churches.

The corps are mostly unadorned halls, without altars, pulpits or silver crosses – just “mercy seats” at the front where Salvationists can pray and testify. They have “corps officers”, not ministers, who spurn fancy vestments and theology degrees, and “songs” and “songsters” rather than hymns, psalms and choristers. Rousing music is still a central part of their services – “Sing so as to make the world hear,” Booth urged. There are no christenings, baptisms or communions, while funerals are celebrations because the deceased have been “promoted to glory”. The flag is never flown at half-mast.

As in Booth’s day, the Army’s stated mission is to “save souls, grow saints and serve suffering humanity”. To Salvationists, good and evil, heaven and hell, are not abstract notions. Most are genuinely distressed if friends or relatives die without finding God. Asked to define hell, Adams said: “Hell is where God isn’t, and to me that’s hell enough.”

Commander Adams with a portrait of Booth. Photo: Tom Pilston for the New Statesman

Nor have the Army’s attitudes to social issues changed greatly. It remains a profoundly conservative organisation that loves sinners but hates their sins. Its soldiers swear to abstain from alcohol, tobacco, drugs, gambling, pornography and extramarital sex, and until not so long ago could not divorce, marry non-officers or have mortgages (a form of debt). They must give “as large a proportion as possible” of their income to its ministries (their tithes are called “cartridges”). Thanks to Catherine Booth, the Army has always treated male and female officers as equals, but at traditional corps such as Regent Hall many women still tie their hair in buns and wear little or no make-up.

The Army opposes capital punishment, euthanasia, Sunday labour and abortion, except in extreme cases. It rejects Heritage Lottery Fund money because it opposes any form of gambling. Most controversially, it opposes gay marriage and considers homosexual acts a sin, though it opposes homophobia. “That may send an unfortunate signal,” Adams said. But: “We base ourselves on what we understand the Bible is saying.”

Above all, a resurrected Booth would applaud the Army’s continuing efforts to “care for the poor, feed the hungry, clothe the naked, love the unlovable and befriend those who have no friends”. It is probably the largest provider of social programmes outside the government – the precious, ultimate safety net for those who have reached rock bottom. With annual revenues of £280m, including more than £100m from donations and legacies, it runs a bewildering array of shelters and hostels, of detox, employment, training and advice centres, of homes for the elderly and prison chaplaincies.

It has 20 mobile units that attend major accidents or disasters to provide sustenance, staff reception centres and support the bereaved at mortuaries. It also runs Britain’s largest family-tracing service – a legacy of the “Inquiry Bureau” that Booth’s daughter-in-law set up to find runaway children in Victorian London. Last year it met 1,859 requests from parents of estranged offspring, long-lost siblings, people on their deathbeds and remorseful prison inmates, and it offers mediation when required.

As part of a government contract, the Army oversees 27 safehouses to support victims of human trafficking. It has helped 1,800 victims from 74 countries over the past three years, although Major Anne Read, who runs the service, said that is a tiny fraction of those living in “slave-like conditions” in Britain.

She described cases of eastern European and African women, forced into prostitution, locked in darkened rooms for years and raped several times a day. One of them who became pregnant was simply dumped on a motorway, “like a piece of rubbish”. She spoke of how men are forced to work all hours and live in filthy caravans. Families with eight or ten children are brought to Britain so someone can collect their benefits. Victims seldom escape, because they lack money, passports, English or any idea where they are. African women are scared into submission by “juju” ceremonies, or threats that videos of their prostitution will be sent home to their family.

“This is a very real manifestation of evil,” Major Read said. “The conditions people reach us in are absolutely shocking.”

They are profoundly traumatised, physically scarred, and often paranoid. “It’s appalling that so many people are living like slaves, when we tend to think that slavery is a historical issue that ended with William Wilberforce 200 years ago.” But she prays for the traffickers, and “that God changes the hearts of wicked men and women”.

The Army even runs its own bank, the Reliance, which Booth founded in 1890. It refuses to invest in the tobacco, alcohol, gambling or armament industries, paid its six top managers precisely £4,286 in bonuses last year, and does not issue credit cards lest it encourage debt.

Yet in one crucial respect today’s Army is very different from Booth’s: it scarcely evangelises any longer. Its members abhor the idea of proselytising. Open-air meetings are rare. With its charitable services mostly now run by professional employees, not soldiers, it can seem more like an NGO than a religious movement. Just once during my week with the Army was I asked about my own beliefs.

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I met Bev – a warm, intelligent, funny 54-year-old from Essex ­– at Greig House, a detox centre near Canary Wharf. She wept as she told me how she had spent decades fighting drug and alcohol addiction, and of the pain she’d caused her children. “I love all the people here. They’re the best in the world,” she said of the centre’s staff. But it was only on this, her fourth residential course, that she realised they worked for the Salvation Army.

Army officers say they are loath to exploit the vulnerable people they help. “We don’t run our night shelter as a recruiting tool. We run it because it’s flipping cold outside and we don’t want people to die,” said Lieutenant John Clifton, whose corps in Ilford, east London, provides winter dormitory accommodation for 25 homeless men and women. “I don’t think it’s ethical to take advantage of their physical need.”

Nowadays the shock troops of the Lord prefer to win converts through compassion and personal example and by gradually building relationships. “By stretching out a hand to mankind you’re showing them love and kindness,” said Major Muriel McClenahan, until recently the head of Territorial Emergency Services. “Because our motivation comes from faith, it opens the door to having those conversations [about Jesus]. You hope you’ve sown the seed that over time will germinate.”

Yet recruits and converts are hardly pouring in. The Army has 27,183 soldiers in Britain, down from 48,121 two decades ago. The number of corps has fallen from 823 to 706. The imposing William Booth College, on Denmark Hill in south London, opened in 1929 to train 800 officer cadets at a time, but at present it has fewer than 60 taking the two-year course.

The Army once boasted several thousand brass bands in the UK, and its own musical instrument factory in St Albans, but has fewer than 500 today. Conversely, it now employs 4,800 civilians – four times the number of officers – to run its social programmes.

The Army is ageing as well as shrinking. There is little new blood – most officers I met came from Salvationist families, and some were third- or fourth-generation Salvationists. They were overwhelmingly white, a conspicuous exception being Commissioner Adams, a mixed-race South African. Adams, 58, is a fourth-generation Salvationist who was raised in Cape Town under apartheid, joined the Army at seven and is married to an Abba-loving Norwegian, Marianne, who leads the Army’s women’s ministries in Britain. He is a likeable, self-effacing man who discourages people from using his title. After becoming territorial commander in 2013 he relinquished his driver so that the man could minister instead. Adams earns scarcely £1,000 a month, only about £300 more than his officers, and he finds his smart, Army-owned flat near Tower Bridge embarrassing. “We could easily move into a council flat down the road,” he said.

Onward, Christian soldiers: preparing to perk up Oxford Street. Photo: Tom Pilston for New Statesman

Adams is troubled by the falling membership: “An army needs soldiers.” Addressing the problem is a priority, he said. He believes the Army has become too inward-looking, complacent, insular, bureaucratic, white and middle-class. “The further from the source of a river, the more polluted the waters become. The further from the source of the movement, the harder it is to maintain that movement’s purity,” he said. “It’s been 150 years from our start so it’s a challenge for us to keep our principles and our focus alive . . . We need to get people back to basics. We’re not where we should be.”

He agrees that the Army should consider more fashionable uniforms and that it should reach out to gay people and racial minorities. “We’ve excluded instead of embracing . . . We’ve failed in our ministry to minorities,” he said. Adams wants the organisation to be more outspoken on some issues – the notion that the poor are responsible for their own plight, for instance – and says its failure to fight apartheid in his homeland still haunts him.

“We’ve hidden behind this thing about, ‘We don’t do cold evangelising any more. We don’t talk about the gospel overtly any more . . .’ We need to be sharing the good news about Jesus Christ. We’ve been neglecting this.”

Specifically, Adams wants more crusading soldiers and fewer “pew-warmers”. He would like fewer Salvationists to walk past beggars without stopping. “When we sing those Army war songs it must not just be a metaphor.”

More churchgoers should work in the charitable programmes, he said, and those programmes should have a stronger religious component, so that they address the spiritual as well as the physical needs of their beneficiaries.

“I would hope people were transformed, as opposed to changed. A social programme can change a person’s circumstances, but only the gospel can change a person from the inside out. The Salvation Army should be about transformation . . . We need to understand that people really can be changed by the gospel.”

On any given Sunday thirty years ago, Regent Hall held three services, and 500 worshippers would fill the old ice rink. Today it holds two and attracts roughly 150 people. But, like many other corps, it is trying to reach out. It has opened a coffee shop for weary shoppers, launched a community gospel choir, and still sends a brass band down Oxford Street on Sundays to entice people in.

And just occasionally lives really are transformed. An 83-year-old retired officer called Major Beryl told me how her father was walking along Oxford Street in the early 1920s, stopped to listen to the band, and was invited in. He accepted, found God, fell in love with a member of the congregation and married her in the Hall. They worshipped there every Sunday for decades, even during the Second World War when bombs were falling all around and the glass roof shattered.

Major Beryl’s father was “promoted to glory” while marching to the Cenotaph one Remembrance Sunday, but she kept coming. “Oh yes,” she chuckled. “This place has given me a lot to be grateful for.”

Martin Fletcher is a former foreign editor of the Times