On religion, Jeremy Corbyn is interestingly moderate, circumspect — not the angry atheist you might expect. In a recent interview with the Christian magazine Third Way, he said his upbringing was quite religious: his mother was a ‘Bible-reading agnostic’ and his father a believer, and he went to a Christian school. ‘At what point did you decide that it wasn’t for you?’ he was asked. He replied very carefully, even challenging the premise of the question: ‘I’m not anti-religious at all. Not at all… I find religion very interesting. I find the power of faith very interesting. I have friends who are very strongly atheist and wouldn’t have anything to do with any faith, but I take a much more relaxed view of it. I think the faith community offers and does a great deal for people. There don’t have to be wars about religion, there has to be honesty about religion. We have much more in common than separates us.’

I predict he’ll go a bit further very soon, and start talking about the Christian basis of his socialism, and citing Jesus as an early role model. If so, he might just be able to build support beyond his obvious base. Enough to keep him in the job for a while. Progressive idealism is able to widen its appeal when allied with religious idealism. Without such an alliance, it looks rather narrow and frail.

This was Tony Blair’s key discovery. He won the trust of the middle class by presenting himself as an idealistic vicar who was a bit too cool for the church. Of course, this approach earned him plenty of derision as a sanctimonious git with a messiah complex, but that was a small price to pay for his electoral success.

When Brown took over, he was canny enough to grasp the importance of this religious aspect of contemporary progressive politics. He successfully rebranded himself as the ‘son of the manse’, the quietly moral offshoot of Presbyterian idealism. Well, it was successful for a while: his inherited moral compass seemed an heirloom that would seriously impress the experts on Antiques Roadshow.

And over the water, Obama won the presidency by confusing the roles of politician and preacher as no one had done for many decades. Maybe no one ever — because the civil rights movement had created a new overlap that he was the first to fully exploit.

Ed Miliband was too atheist. He tried to soften this with Judaism, but it didn’t work. ‘I have a particular faith,’ he once said. ‘I describe myself as a Jewish atheist. I’m Jewish by birth origin and it’s a part of who I am.’ His hope was that his Judaism would give him a vaguely religious aura, that it might reassure believers that he understood the importance of religion, despite his atheism.

But secular Judaism doesn’t work like that. To the average gentile, it looks like a particularly stark rejection of religion. Aren’t secular Jews (Marx, Freud, Woody Allen) the most energetic atheists of all? And talk of ‘faith’ is not reassuring once it’s divorced from mainstream organised religion. Didn’t Lenin and Hitler have bucketloads of faith?

The British electorate wants a party leader to be sympathetic to religion. A Tory can easily enough signal mild Anglican allegiance, as David Cameron does well. A Labour leader has a trickier job, for two reasons. Lots of people in his party hate religion. And his religious stance matters more, because he is more clearly in the business of selling a vision, persuading the electorate that something new is possible. The very concepts of hope and change have Christian roots — imagine the British left without its broad imagery of the New Jerusalem.

This is something that various left-wing intellectuals have been admitting over the past decade. Terry Eagleton has powerfully argued that progressive humanism has Judeo-Christian roots; recently the veteran lefty academic David Marquand, though non-religious himself, has suggested that a close alliance with religion might be the way forward for a new socialism. Even Russell Brand signals the shift: his new-agey, John Lennon-ish, expanded-consciousness version of political radicalism reflects a hunger for a more comprehensive idealism, of which a restructured tax system is just an aspect.

So the big question about Corbyn is whether he understands that the left is a quasi-religious movement. And that it can only appeal to the nation if it manages to seem in tune with actual religion. It’s hard for a non-believer to do this: saying that faith communities do a lot of good isn’t really enough. And it’s hard to imagine Corbyn undergoing a strategic conversion. So it’s hard to see how he’ll survive, in this new climate in which secular political idealism is not enough.