In the unholy trinity of professional atheists, AC Grayling has always tended to be regarded as the good cop. Less coldly clinical in tone than Richard Dawkins, less aggressively combative than Christopher Hitchens, Grayling approaches the God debate with a gently teasing charm that could almost – but should never – be mistaken for conciliation. "Yes, I'm the velvet version," he chuckles.

So he insists that his new book does not belong in the same canon as Dawkins's The God Delusion and Hitchens's God Is Not Great. "No, because it's not against religion. There's not one occurrence of the word God, or afterlife, or anything like that. It doesn't attack religion, it's a positive book, there's nothing negative in it. People may think it's against religion – but it isn't." But then he says, with a mischievous twinkle: "Of course, what would really help the book a lot in America is if somebody tries to shoot me."

With any luck it shouldn't come to that, but Grayling is almost certainly going to upset a lot of Christians, for what he has written is a secular bible. The Good Book mirrors the Bible in both form and language, and is, as its author says, "ambitious and hubristic – a distillation of the best that has been thought and said by people who've really experienced life, and thought about it". Drawing on classical secular texts from east and west, Grayling has "done just what the Bible makers did with the sacred texts", reworking them into a "great treasury of insight and consolation and inspiration and uplift and understanding in the great non-religious traditions of the world". He has been working on his opus for several decades, and the result is an extravagantly erudite manifesto for rational thought.

In fact everything about Grayling is extravagantly erudite. We meet at his south London home, where he sits surrounded by teetering piles of books, great leaning towers of learning, and the conversation frequently detours into donnish tutorial mode. Spotting me glance at one of the volumes, which bears the title Epiphenomenalism, he launches at once into a detailed explanation of the concept – but then breaks off in delight as his dog trots in and rolls at his feet.

"Ooh, look at you, Misty!" he gurgles, bending to rub her stomach. "Ooh, you like that, don't you! Why don't you play outside? Oh, you want to stay and be interviewed? Ooh, you'd make an interesting interviewee, wouldn't you!" Then, moments later we are back in a tutorial. "If you're not careful," he smiles, "I'll explain the inter-substitutivity of co-referential terms salva veritate," and sure enough he does.

Who does he think will read The Good Book? "Well, I'm hoping absolutely every human being on the planet." He's sure that a lot of people will wonder just who he thinks he is, to have written a bible, but doesn't appear particularly troubled by this prospect. "The truth is that the book is very modestly done. My wife did give me a card," he giggles, "that said, 'I used to be an atheist until I realised I am God'. And I know that on Monty Pythonesque grounds there's a good likelihood that in five centuries time I will be one, as a result of this." He lets out another little chuckle. "But I certainly don't feel like one now, that's for sure."

The little jokes and kindly bearing can make Grayling sound quite benignly jovial about religion at times, as he chuckles away about "men in dresses" and "believing in fairies at the bottom of the garden", and throws out playfully mocking asides such as, "You can see we no longer really believe in God, because of all the CCTV cameras keeping watch on us." But when I suggest that he sounds less enraged than amused by religion, he says quickly: "Well, it does make me angry, because it causes a great deal of harm and unhappiness."

He is very cross, for example, with the question in the current census that asks: "What is your religion?" The British Humanist Society has just conducted a poll that asked those surveyed if they were religious – to which 65% said no. But when asked, "What is your religion?" 61% of the very same people answered Christian. "You see, they say, 'Oh well, nominally I suppose I'm Christian.' But two-thirds of the population don't regard themselves as religious! So we have to try to persuade society as a whole to recognise that religious groups are self-constituted interest groups; they exist to promote their point of view. Now, in a liberal democracy they have every right to do so. But they have no greater right than anybody else, any political party or Women's Institute or trade union. But for historical reasons they have massively overinflated influence – faith-based schools, religious broadcasting, bishops in the House of Lords, the presence of religion at every public event. We've got to push it back to its right size."

Atheists, according to Grayling, divide into three broad categories. There are those for whom this secular objection to the privileged status of religion in public life is the driving force of their concern. Then there are those, "like my chum Richard Dawkins", who are principally concerned with the metaphysical question of God's existence. "And I would certainly say there is an intrinsic problem about belief in falsehood." In other words, even if a person's faith did no harm to anybody, Grayling still wouldn't like it. "But the third point is about our ethics – how we live, how we treat one another, what the good life is. And that's the question that really concerns me the most."

It's only in the past decade that these three strands of thought have developed into a public campaign against faith – but it wasn't the atheists, according to Grayling, who provoked the confrontation. "The reason why it's become a big issue is that religions have turned the volume up, because they're on the back foot. The hold of religion is weakening, definitely, and diminishing in numbers. The reason why there's such a furore about it is that the cornered animal, the loser, starts making a big noise."

Even if this is true, however, the atheist movement has been accused of shooting itself in the foot by adopting a tone so militant as to alienate potential supporters, and fortify the religious lobby. I ask Grayling if he thinks there is any truth in the charge, and he listens patiently and politely to the question, but then dismisses it with a shake of the head.

"Well, firstly, I think the charges of militancy and fundamentalism of course come from our opponents, the theists. My rejoinder is to say when the boot was on their foot they burned us at the stake. All we're doing is speaking very frankly and bluntly and they don't like it," he laughs. "So we speak frankly and bluntly, and the respect agenda is now gone, they can no longer float behind the diaphanous veil – 'Ooh, I have faith so you mustn't offend me'. So they don't like the blunt talking. But we're not burning them at the stake. They've got to remember that when it was the other way around it was a much more serious matter.

"And besides, really," he adds with a withering little laugh, "how can you be a militant atheist? How can you be militant non-stamp collector? This is really what it comes down to. You just don't collect stamps. So how can you be a fundamentalist non-stamp collector? It's like sleeping furiously. It's just wrong."

If Grayling does have one fundamentalist article of faith, it is that all of us are capable of understanding philosophy. He grew up in a colonial family in what is now Zambia, where the grownups' chief preoccupation was adultery, leaving him free to bury himself in books. He first read Plato at 12, and says enthusiastically, "Anybody could read Aristotle's Nicomachean Ethics in the bath, it's great stuff!" – although I suspect his idea of an easy read may not be the same as yours or mine.

The author of 30 books, he is a professor of philosophy at Birkbeck College in London, and a supernumerary of St Anne's College, Oxford, as well as a UN human rights activist. But he is probably best described by that phrase that tends to make the British uncomfortable – a public intellectual.

"I spent the first half or more of my career in the ivory tower writing technical philosophy, but I recognised very early that academic philosophy is a very narrow part of the field. This is one of my big things: that philosophy belongs to everybody. Until 100 years ago philosophy did belong to everyone. Today, unfortunately, it's become very jargon-laden and scholastic, so it's become very specialised. But a lot of the stuff I've written has been trying to show people that this is part of the conversation mankind has with himself, about all the great questions. We're all intelligent monkeys, 99% of us are perfectly capable of understanding this, and I feel reasonably confident that given enough time and typewriters I could explain most of what goes on in technical philosophy to someone who has no background in it at all."

Is there a sniffy faction within the world of philosophy that takes a dim view of attempts to make the subject more widely accessible?

"Oh, I'm absolutely sure of it. But I also think that attitude has moderated considerably over time. Ten to 15 years ago, when I started to try to do this, I'm pretty sure there was a lot of sniffing going on." He does a bit of his own sniffing, though, a moment later, when I mention the popularity of bestselling writers whom he has described as quasi-philosophers.

"Hmm, yes, the [Alain] de Bottons and so on," Grayling murmurs rather sorrowfully. "He's a perfectly nice fellow, but it's not philosophy. It's cream-puff stuff. What worries me is that someone will go to it thinking, 'Ooh, this is an opportunity to think and find out something', and then they find that it's actually very shallow and doesn't have deep roots. And I do think that people who do this kind of thing should really have done some work and got engaged in something serious, and then they won't make too many mistakes when it comes to trying to introduce others to it."

Nobody could doubt that Grayling has "really done some work". He first had the idea for The Good Book as an undergraduate, and it certainly reads like the opus of an out-and-out workaholic. "I think all of my family would say I was to some extent a workaholic," he agrees, smiling wryly. He lives with his second wife, a novelist, and their 11-year-old daughter, but also has two grownup children from his first marriage, and one can't help suspecting that they all help him connect with a world that wasn't reading Plato at 12.

He attributes his workaholism to the death of his sister, who was murdered in South Africa in her 20s. His feelings towards the continent of his childhood, and of his sister's death, are so painfully tender that it's only in the last year that he has been able to eat any tropical fruit at all. "I've just been able to start eating some mango," he says quietly.

It's a rare moment when Grayling's scrupulously rational mind allows for a glimmer of something more emotionally subjective. But, of course, most people's lives and judgments aren't really guided by rigorous reason at all – which must be maddening to him. So I wonder what he makes of humankind's perverse attachment to non-rational impulses.

"I think they are failing in their responsibility to themselves as intelligent beings. By not being sufficiently reasonable. If you really press them, just ask them, aren't you glad that the people who built the aeroplane you fly in used reason? Aren't you glad that the pilots were trained according to reason? Aren't you glad that your doctor or train driver thinks about what they do and uses reason? And they will say yes. Then you say, 'Well, OK, if that's the case then how about applying it to your own life as well?'"

We've come to the end, and I have one more question. Can I ask, I venture tentatively, about your hair?

"Oh God, my hair."

He is invariably described as the lion-maned philosopher, so I'm curious to know how he maintains his magnificent locks. "Well, I don't really use very many products," he says. "It must look very artificial, but it isn't, and I do get a lot of stick for it. I put a bit of sticky stuff just to hold it up there – I don't know what the brand is, it's a sort of little thing of hairspray. I mean any sticky thing will do just to keep the front up."

It must require a lot of attention, though, doesn't it? "No, it doesn't really, but I do get a lot of stick for it. You see, I used to have very, very long hair in the 60s, so this is very restrained for me. But I said to my kids a few years ago, I'm going to shave all my hair off, I keep getting all this stick about it – I'm going to shave it all off. They said: 'No! You won't look the same, it won't be you.'"

He says he isn't remotely vain, but he does look like someone who cares a great deal about his appearance. "Ooh, well, that's very kind of you to say," he smiles. "I'm not self-conscious or aware of myself. I just give the wrong impression with this hairstyle. This may seem an odd thing to say, and I'm sure psychologists would pounce on this, but actually – well, actually, I don't sort of exist. The rest of the world does, and I'm really interested in it. If there's a group of people sitting round, and I think about it afterwards, I always fail to remember that I was there, if you see what I mean."

So when he sees himself in group photographs?

"Oh, I'm surprised to see there I am! Yes, very surprised."

• This article was amended on 4 April 2011. The original referred to Birbeck College. This has been corrected.