Catherine Bennett suggests that Brown's book teaches us that we're about to have another pious Christian as Prime Minister. I think what it teaches us is that politicians today think their political principles somehow have greater legitimacy with the public if they claim they owe more to the writing of Micah rather than Marx, or some other atheist writer. I sincerely hope they're wrong about this but I'm not sure they are. The political culture in the United States is such that I can't imagine an avowed atheist ever winning a presidential election. We are nowhere near that here, thank goodness. But God forbid, if you'll pardon the expression, we ever arrive at a place where a politician's unbelief comes to be seen as an electoral liability.
I think it’s more that if you believe in any secular authority you become liable to accusations of being ideological and through that to accusations of extremism. Professing religious belief enables you to be seen as being opposed to bad things and in favour of good things in a large, vague way without implying any particular commitment. And also you get to plead forgiveness in advance for when you screw up, because your motives are pure. I think it’s one of the little quirks of the postmodern condition in that professing faith signals that you don’t believe in anything in particular, but at the same time aren’t some kind of unpleasant hardcore cynic. Ambition and self improvement are officially Good Things in current political discourse, but need to be tempered by some vague flap at the notion that there’s more to life than material values. Religion might not be one of the best ways to do this, but it is one of the most convenient. I suppose you could equally profess an attachment to “humanist” values with the same ends in mind, provided these values stay sufficiently undefined.
In fairness it should be pointed out that the origins of the left in Britain are religious, if you go back all the way to the civil war, Milton, the Levellers and other popular beat combos of the time. But that crowd were very keen on liberty and egalitarianism, neither of which are too fashionable these days.