The British General Election has been full of surprises, and the outcome of this Thursday's vote seems less predictable as each day passes.

One of the most notable features, of course, has been the steady rise in Jeremy Corbyn's approval ratings – while at the same time, those of Theresa May have declined.

A number of explanations might be offered for this. But whatever else might be said, the fact is that the Labour Party leader is embodying a fundamental truth about how to exercise leadership and inspire others while the Conservative leader isn't.

Probably neither Corbyn nor May have ever heard of the acclaimed theologian Walter Brueggemann (described on Wikipedia as 'an American Protestant Old Testament scholar ... who is widely considered one of the most influential Old Testament scholars of the last several decades').

But one of his best-known aphorisms sums up this current election campaign surprisingly well. Featuring in his 1987 book, Hopeful Imagination: Prophetic Voices in Exile, it says simply this: 'People are not changed by moral exhortation, but by transformed imagination'.

In other words, people don't like to be nagged or exhorted into doing the 'right thing'. They are much more likely to change their behaviour and views as a result of having an alternative, positive vision set out in front of them.

Now say what you like about Jeremy Corbyn (and many do), but ask people what he stands for and most likely they will be able to tell you. They will mention things like raising taxes on the richest and on corporations in order to abolish student tuition fees, spend lots more on schools and hospitals, and end austerity. Many people thus associate with him a particular vision – a positive vision – of how the future could be.

Ask people what vision Theresa May offers, however, and pundits will tell you that voters and even Tory election candidates themselves are struggling to nail it. As an article in the London Times on Saturday observed, research of social media posts, even those supporting the Conservatives, showed that in content and substance they 'have little to hold on to. They are varying noises of support around the themes of May, strength and remembering to vote Conservative. [But] few in their number are offering a reason (my italics) for that vote'.

In other words, it's what George W Bush memorably called 'the vision thing'. All this is not to say that the Conservatives do not have a vision which could transform people's imagination – but, if they do, it has not been clearly articulated. Nor is it to say that the Labour vision is necessarily deliverable, when leadership competence and financial realities are taken into account.

But when it comes to setting people's minds on fire with a vision of a brighter future, Jeremy Corbyn wins over Theresa May hands down. It may be that this is a lesson that is too late for her to learn. And as the campaign has revealed, she is plainly in fact an extremely shy person who, whatever her other talents, finds engagement with the general public tricky. She is also more of a details person than a big picture one.

However, regardless of one's politics, people do require a vision. It's notable that Jesus in his teaching is constantly teaching in parables – transforming people's imagination – and speaking of the Kingdom of God, that embodiment of life as it should truly be, rather than just popping up and saying, 'Do this' or 'Don't do that'.

Even the ten commandments, which when read as a standalone list do look simply like 'moral exhortations', actually come in the context of who God is and his transformative, radical grace in rescuing his people.

There's a place for both, of course. I think Brueggemann slightly overstates his case. But the substance of his point remains. And that is one of the main reasons Jeremy Corbyn has moved up the popularity ratings, while Theresa May has lost quite a bit of her shine.

As the Bible put it memorably many centuries ago: 'Where there is no vision, the people perish,' (Proverbs 29v18). And in a democracy, they are less likely to give you a landslide, too.

David Baker is a former daily newspaper journalist now working as an Anglican minister in Sussex, England. Find him on Twitter @Baker_David_A