On June 2, 2000, John Humphrys and his girlfriend Valerie Sanderson had a son, Owen. Humphrys was 56. He had two older children – they are now 40 and 38 – but the arrival of another child so late in life came as a shock.

“Men have a postnatal state too,” he says, “particularly much older men like me. You enter this really vulnerable state . . . you expect it in your twenties, it’s part of the process, the normal process, but I was much older. All your attention is focused on this new human. It’s an incredibly powerful effect.”

Two years later Humphrys was at an awards ceremony. “It was at a posh London hotel and I was due to present an award. They showed this film about kids suffering from whatever and I started blubbing. I never do that. Having a child late in life exposes you in a way you thought you’d moved beyond.”

On September 1 2004 Chechen terrorists seized a school in Beslan in Russia. On September 3 the siege ended in a shootout in the course of which 186 children died. Humphrys, without consulting his editor on Radio 4’s Today programme, called the office of the Archbishop of Canterbury, Rowan Williams, and asked him to come on the programme the next day to discuss this horror. Williams agreed.

“I’d never before spontaneously picked up the phone like that. Luckily the editor said yes. It was not about getting the story. Mostly I really wanted to know what he would make of this.”

As a journalist, Humphrys has observed and discussed many aspects of life in the human slaughterhouse. But, because of Owen, Beslan was different.

He was brought up in a terraced house in Splott in Cardiff, the son of a French polisher and a hairdresser. He never could work out his father’s views on religion, but his mother took him to church regularly. He absorbed the language of the Bible and the Book of Common Prayer, but he never found faith. In time this became a sort of almost atheism.

He didn’t believe but he prayed.

“I was – am – dissatisfied with my failure to come to any specific conclusions. I know how pathetic and lame this sounds and I am embarrassed by it but I continued praying. I’ve prayed ever since I was a child and I continued to do so but I was increasingly puzzled by it.”

Owen, Beslan and his own religious ambivalence thus converged on the Williams interview. But it didn’t work. Humphrys was not satisfied with the answers to what was the oldest and most painful religious question of all: how can a good God permit such evil? So he then put together radio series of three interviews – with Williams, Jonathan Sacks, the Chief Rabbi, and Professor Tariq Ramadan, a Muslim scholar. It was called Humphrys in Search of God, though, as one wit pointed out, it might just as well have been called God in Search of Humphrys. Neither search was successful. Again he was unpersuaded.

“The interviews left me back where I started – well, rather further back as they had spectacularly failed to satisfy me on any intellectual level. Their arguments did not stack up. In the end they did concede you had to believe first. This has always been a problem for me. Faith is faith. If you have to satisfy the intellectual argument, then it’s not faith.” There was a huge response – “sackfuls of letters” – and a publisher asked him to turn the interviews into a book. He was reluctant but agreed to do it on the basis that it would simply consist of transcriptions. But then he started reading the new wave of militant atheists – Richard Dawkins, Daniel Dennett, Michael Onfray and Christopher Hitchens – and he lost his temper.

“Perhaps it was a mistake to read one after the other, to gorge myself, but it was the unfairness of it all. I thought they’d no right to do that . . . I was surprised how offended I was by their attack on people who believe, but it was personal.”

He felt it was “grotesque for clever people like Dawkins” to present themselves as some kind of martyrs for the atheist cause.

“Nobody is denying them their atheism and nobody has done for a very long time in this country. I doubt whether any of Dawkins’s immediate relatives have been burnt at the stake. Hitchens presents religion as not just wrongheaded but as something so dangerous it must be eradicated. And then they say even these people who profess to be believers actually know it’s not true. How dare they? That’s unacceptable. I think it’s dishonest.”

The book was no longer to be mere transcripts, it was to be a very personal account of the spiritual predicament in which Humphrys found himself.

“What you believe comes down in the end to what’s inside you so it has to be personal. I don’t see how you can do it otherwise.” He was a praying atheist for whom the arguments of the religious patriarchs had failed miserably but also for whom the savagery of the opponents of religion were manifestly unjust. The truth was, of course, that he was profoundly uneasy. He couldn’t bear the uncertainty. He wanted to do something about it.

To understand this you have to understand two things about the man – he is staggeringly, perhaps neurotically, energetic and he didn’t go to university. His energy is beyond belief. Sparks seem to fly off him. He talks incessantly, questioning himself and anybody who happens to be in the room. As we spoke, he boiled beetroot and potatoes and made me sample raw organic carrot – “That’s a proper carrot!” The latter half of my recordings are partially inaudible due to loud crunching noises. The energy meant that the one thing he couldn’t do when confronted with his predicament was let it be.

Anybody who studied almost any nonscientific subject at university would step back from the problem of evil and the existence of God. It is a sea infested by leviathans from Augustine and Aquinas down to Luther and Kant. The whole of western culture floats unsteadily on this sea. But Humphrys, the nation’s grinning, yapping, undereducated terrier, just dived in. And the amazing things is, when he’d wet everybody by shaking off the water, he came out with Kant in his mouth.

“You do know,” I say to him, “that with this book you’ve done Kant?”

“I know,” he cries, “somebody else told me that! And they told me I must under no circumstances read any Kant.”

Immanuel Kant, perhaps the greatest philosopher of the modern era, did not believe any of the proofs of the existence of God. But he was a believer and his evidence was the “moral sense within” – conscience. That is exactly what Humphrys concludes.

He tells, for example, the stories of Lisa Potts and Irena Sendler. At the age of 18 Potts defended her infants’ class from a madman with a machete, sustaining appalling injuries. Sendler smuggled 2,500 Jewish children out of Nazi-occupied Poland. She was caught by the Gestapo but freed by partisans. At the age of 97 she said: “I continue to have pangs of conscience that I did so little.” Such stories, Humphrys thinks, undermine the materialistic account of the world.

“As for me,” he writes, “it is difficult to understand the existence of conscience without accepting the existence of something beyond ourselves.”

And so no, having been less inclined to belief after the interviews, he finds himself more inclined after the book. He’s still not there, of course, but his determination to speak out against the posturing of the militant atheists has given him back the role with which he feels most comfortable – a praying, failed atheist.

Only Humphrys could do all this, of course. He is a unique figure in national life. He shrugs and says it’s just because he’s been on the Today show for longer than anybody else. But the truth is there is something special about this man. He is the opinionator, the cause of opinions in others. With his jaunty, commonsensical manner and his determination to unearth every badger from every burrow, he lures people into debates – about the world certainly, but also about himself. I’ve had more discussions about the meaning of Humphrys than I care to remember. Last week he was even in the thick of it because of his holiday home – it’s in the Peloponnese, the place where Greece has been burning.

He is self-deprecating about all this – “I’m a hack, that’s it really, and I do feel slightly guilty occasionally for imposing” – but the terrier ego, the obsessive belief in his own destiny to expose all that is hidden, keeps taking over. It was God that was hidden in this case and he remains so despite the best efforts of Humphrys. Owen, however, continues to ask more awkward questions than his father ever could.