Alastair Campbell, Tony Blair's former spin doctor, famously 'doesn't do God', as he reminds the Archbishop of Canterbury at the beginning of a revelatory interview for GQ magazine.

So, 'Tell me why I should?' he asks.

Perhaps surprisingly, Justin Welby advises him to skip the intellectual arguments and go straight to 'the person of Jesus Christ'. It means, he says, opening your life to the presence of Christ by his spirit - and there's a mystical element to it.

Campbell is the famously profane ex-journalist said to be the model for the foul-mouthed and hyper-aggressive Malcolm Tucker in the long-running satire The Thick Of It. Describing himself as a 'pro-faith atheist', in a 40-minute intervew he grills the Archbishop on core doctrines of the Christian faith such as the virgin birth, heaven and hell and whether he ever has doubts (of course, Welby says, mentioning an occasion when he stood by a mass grave in South Sudan. 'But I come back to the person of Jesus Christ. It is all about Jesus Christ. The moment you get away from Jesus Christ you lose the plot in Christian terms.')

Previously released sections of the interview included a question about whether Welby ever got depressed. He replied: 'I think if you had asked me a year ago I'd have said no, and ten years ago I would have said absolutely not. But what was that phrase Churchill used? "Black dog".

'There is an element of that. I think as I am getting older I am realising it does come from time to time. I have those moments - you would know this - when objectively everything is fine, but you think you are, beyond description, hopeless.'

Welby also ran into controversy when he said he could not give a 'straight answer' as to whether gay sex was wrong.