When you are trying to change the world, it does not do to think small. “We have an open door in London, an open door in Canberra, and now we need one in Washington,” says Jim Wallis in a recent interview. Wallis is the US evangelical leader who is on a mission to spark a “revival for justice” that will take on the great issues of global poverty, inequality and climate change.

He is already causing a stir in the US, where his book, God’s Politics, was a bestseller, lambasting the shortsightedness of the Christian Right while arguing that secular liberals ignored the progressive potential of religion. He now advises Barack Obama, whom he has known for ten years, on how to reach out to evangelicals and put his religious troubles behind him.

Wallis was in London to stir up British interest in a movement that has already recruited tens of thousands of enthusiastic adherents in the US — and done much to dispel the guntoting, gay-bashing image that many outsiders have of evangelicals.

Along with his fellow “progressive evangelicals”, Wallis believes that injustice and suffering are far more central to the Christian message than the narrow focus on sexual morality which has preoccupied the religious right. He claims the message is resonating — indeed he believes we are on the verge of a new great awakening to rival the evangelism of the 19th century and William Wilberforce’s crusade against slavery.

“Now a new generation of evangelical students and pastors is coming of age,” he writes in his new book, Seven Ways to Change the World. “Their concerns are the slavery of poverty, the sexual trafficking of God’s children, environmental ‘creation care’, human rights and the image of God in such places as Darfur and how the Prince of Peace might view our endless wars and conflicts.”

But can Wallis’s unabashedly faith-based crusade really make any headway in assertively secular Britain? The challenge is one that he welcomes. He believes, though, that many groups are receptive to his message. There are powerful evangelical churches, like Holy Trinity, Brompton, home to Nicky Gumbel, inventor of the Alpha Course, where Wallis preached during his visit to Britain.

At the other end of the commitment scale, Wallis talks of his meetings with young people who described themselves as “spiritual but not religious” who are inspired by the message of global change. Even so, it is hard to make the case that a great awakening is under way on this side of the Atlantic.

Somewhat surprisingly, though, Wallis sees Gordon Brown as an ideal global leader for the movement. “I’ve never met a head of government who has such a deep understanding and passion about global inequality,” Wallis says. “Globalisation has an inherent logic but no ethics. He talks about the ethics.”

The admiration is mutual: Brown has met Wallis several times and gave a warm endorsement to God’s Politics. Wallis is pained by the Prime Minister’s current travails. While he steers clear of offering advice on any of Brown’s political crises, Wallis offers the PM a few words of support. “A lot of us trust his moral compass on global issues.” Wallis says the two of them share a language in talking about these issues; “Isaiah rolls off his tongue. He knows his Scripture.”

Wallis recalls a meeting with Brown where the then Chancellor railed passionately at him: “ ‘For the first time we have the knowledge and the resources to end global poverty, but we don’t have the moral will — and that’s your job.”

This, Wallis believes, confirms his own theory about how change comes about: “A social movement pushing at an open door.” Leaders face countless distractions that stop them making the changes they want — unless social pressure forces them to be true to themselves.

Wallis includes the Archbishop of Canterbury in this category. “He’d love that the Lambeth Conference would focus on global poverty but what are the chances of that with arguments about homosexuality and the Anglican Communion?”

Even his friend Obama, Wallis says, “won’t be able to change things without social movements”.

That movement, he says, must be at its core a religious one. He insists that faith is at the heart of all great social movements from Wilberforce onwards. “Historically, it takes energy, spark and perhaps most important is the power and presence of hope, and that’s where faith comes into play,” he says. “The heart of faith is that God is active in the world and through people things can change.”

Though he is keen to stress that religion does not have a monopoly on morality, Wallis believes it is this sense of hope that faith inspires which stops constant disappointment from casting people into that cynical realism which he calls “a buffer against commitment”. The size of the task does not seem to daunt him. His appetite for appearances, preaching and conferences to spread his message is prodigious. He cites his attendance at the inauguration of Nelson Mandela as President of South Africa, saying: “I have seen too much not to have hope.”

While the millions who joined the Make Poverty History campaign have drifted away as the excitement has faded and promises have gone unmet, Wallis will not let the facts of poverty’s stubborn persistence deflate him: “Hope,” he says, “means believing in spite of the evidence, then watching the evidence change.”