Reading the first sentence of Terry Eagleton's review of The God Delusion in the October 2006 edition of the London Review of Books was not unlike watching a gunfighter kicking over a table of cards in an otherwise well-ordered saloon. "Imagine," fired Eagleton, "someone holding forth on biology whose only knowledge of the subject is the Book of British Birds and you have a rough idea of what it feels like to read Richard Dawkins on theology."

And that was only the opening volley. Further down the page Eagleton proceeds to shoot up Dawkins's failure to do justice to the complexity of the God he sought to rout ("He seems to imagine God, if not exactly with a white beard, then at least as some kind of chap"), his literality and lack of imagination ("Dawkins occasionally writes as though 'Thou still unravish'd bride of quietness' is a mighty funny way to describe a Grecian urn") and his belief in the progressive nature of history ("We have it from the mouth of Mr Public Science himself that aside from a few local, temporary hiccups like ecological disasters, famine, ethnic wars and nuclear wastelands, History is perpetually on the up").

Entertaining, even exhilarating stuff. But no great surprise to those who've followed Eagleton's career in any detail. He has a reputation for entering other people's rooms and kicking over their cards. He appears equally happy whether outraging conventional students of literature at Oxford with his vigorous espousal of critical theory, confounding his long-time Marxist allies with his periodic dabblings with spirituality, or lambasting Martin Amis for his suggestion that British Muslims "must suffer" for the actions of suicide bombers. (These comments, said Eagleton, were "not unlike the ramblings of a British National Party thug").

Neither does the degree of Eagleton's intellectual aggression seem to be modified by past friendships. In his new book Reason, Faith and Revolution: Reflections on the God Debate, he is not content with amplifying his LRB attack upon Dawkins. He widens his target to include a new antagonist he calls Ditchkins, a composite of Dawkins and Eagleton's old International Socialist drinking mate and author of God Is Not Great, Christopher Hitchens.

It was enough to make me think, as I made my plans to interview Terry Eagleton, that I might be unwise to try to gain his attention or interest by stressing our biographical affinities, our shared attendance at northern Catholic schools, our one-time virtual comradeship in the ranks of International Socialism, even our common interest in the work of such cultural theorists as Roland Barthes or Michel Foucault.

None of this, after all, was likely to obscure the considerably less acceptable news that I was to interview him for a magazine which not only laboured under the intellectually suspect title of New Humanist but was also a product of an organisation called the Rationalist Association. (Eagleton is particularly exercised by the New Atheists' tendency to conflate reason and rationality. "We hold many beliefs that have no unimpeachably rational justification, but are nevertheless reasonable to entertain.")

But when we finally sat down to talk in a Dublin hotel (Eagleton currently divides his life between Dublin and Derry and a string of international universities) he looked so relieved to be out of the torrential rain that was pouring down outside on St Stephen's Green that I decided to take a chance and play the Catholic card. As an ex-Catholic myself, I said, I couldn't help but wonder why you were quite so generous about your Catholic schooling in your autobiography The Gatekeeper.

"I valued the way it taught me to think analytically, to not be afraid of analytic thought, however nonsensical some of the content surely was. There was an opportunity to argue."

But how could he square that relatively sanguine memory with the requirement at Catholic schools to memorise and recite the absurd one-line strictures contained in the standard catechism?

"I agree that the catechism was a way of short-circuiting thought. But it wasn't all there was. I also remember a religious teacher in the sixth form, a rational enlightened man, quoting from an awful textbook called The Fundamentals of Religion that we had to learn like a garage mechanic boning up on parts. He came to a passage which dismissed Buddhism in two sentences, looked up, and said, 'That's shoddy scholarship'. That phrase resounded in my ears. It wasn't typical. But it did happen. It was possible."

And that had been enough to keep him within the fold at school?

"It wasn't so bad that I had to rebel. No period of Joycean rebellion. It was repressive but it was not aggressive or violent."

But hadn't he as an intelligent sixth-former sometimes wanted to kick against the awful certainty of Catholic doctrine, its sheer unreadiness to entertain the idea that there might be something in other religions or ways of thought?

"Well, there is a bad side to certainty but there's also a good side. People with my background don't automatically thrill to the idea that we don't know what we think about anything. I was taught by people at Cambridge who got an almost erotic frisson from the idea that they didn't know what they thought and could afford not to know. Whereas I came from a background where it was thought that there were certain things you really had to get sorted out. There's a difference between reasonable certainty and dogmatism."

But at Cambridge he'd embraced Marxism. How had he managed to tie that in with his old Catholicism?

"It was a time when the Church was going through a massive renewal. It was called the new theology and it gave me good reasons to stay in the church when most right, decent people would have left in disgust. I was challenged head-on by a number of Dominican clergy who would say, 'Okay, so you're joining the International Socialists. Okay, so we quite agree with that revolutionary project. But it's just that Christianity from within its own revolutionary perspective can see that that project has certain limits to it.' For the first time I was not only hearing an intellectually persuasive interpretation of Christianity but also one that made sense politically to me."

Memories like this constantly inform Eagleton's passionate criticism of the "New Atheists". Whereas he has spent months and even years of his life debating theology with clever believers, the likes of Dawkins and Hitchens suddenly arrive on the scene and sweep away the entire philosophical content of religion with a derisory wave of the hand. Eagleton might now be ready to talk of religion as an allegory and to question along with Dawkins and Hitchens the literal truth of the Bible. But what he can never overlook in his opponents is their failure to ever engage in intellectual debate with the likes of the Dominicans who changed the course of his own life at Cambridge. It is because they never exposed themselves to this type of theological debate that they can now be indicted for having "bought their atheism on the cheap". They are, in the equally scathing words of other Eagleton enthusiasts, nothing more than "discount store atheists" or even "schoolyard atheists".

But what precisely have these alleged cheapskates overlooked? In his LRB review, Eagleton provides a namecheck. "What one wonders are Dawkins's views on the epistemological differences between Aquinas and Duns Scotus? Has he read Eriugena on subjectivity, Rahner on grace or Moltmann on hope? Has he even heard of them?"

This is good knockabout stuff but, as Anthony Grayling pointed out in his LRB letters page response, charging Dawkins with failing to read theology "misses the point that when one rejects the premises of a set of views it is a waste of time to address what is built on those premises". Or, as Richard Dawkins himself put it to me during my interview with him for New Humanist in early 2007, "Somebody who thinks the way I do doesn't think theology is a subject at all. So to me it is like someone saying they don't believe in fairies and then being asked how they know if they haven't studied fairy-ology."

Eagleton can, of course, fight back against this kind of rebuttal if he can only show that there is something in theology which undermines the arguments against religion made by the new atheists. And this indeed is what he is up to in Reason, Faith and Revolution when he seeks to show, with the use of the theologian Thomas Aquinas, that the God so readily dismissed by Dawkins and Hitchens is not a god that many theologians, or indeed believers, would recognise. While God for Dawkins and his ilk is "some kind of mega-manufacturer or cosmic chief executive officer" who set the world in motion and now directs it from his home in the clouds, Aquinas was quite ready to entertain the possibility "that the world had no origin at all". Dawkins and Hitchens are equally theologically illiterate in their view of religion as a failed attempt to explain the world. "Christianity was never meant to be an explanation of anything in the first place." God is not a mega-manufacturer. "He is rather what sustains all things in being by his love and would still be this even if the world had no beginning."

I quoted this passage back to Eagleton and asked for some elaboration. Did Aquinas really reject the Biblical story of the creation of the world? "Oh yes. He paid a great deal of attention to scripture but he was a mainstream Catholic and not a fundamentalist so he didn't take it literally. For Aquinas, God the Creator is not a hypothesis about how the world originated."

Indeed, religion can be seen a persistent rebuff, a no-go area, for all those rationalists who spend their lives in the search for causes. God, Eagleton cheerfully concedes, is "every bit as gloriously pointless as Ditchkins tells us he is. He is a kind of perpetual critique of instrumental reason."

If this argument already seems "fanciful" (Anthony Grayling's preferred term for Eagleton's theology) then you will need to tighten your safety belt as we move on to Eagleton's account of this "pointless" God's involvement with the world.

God, Eagleton tells us, may not have created the world like a mega-manufacturer but he is the reason why there is something rather than nothing. He is what sustains all things in being by his love. He made the world not for any instrumental reason but simply "for the love and delight of it".

When I first encountered passages like this in Reason, Faith and Revolution I couldn't help but remember a sketch from Beyond the Fringe in which Peter Cook (and I think it was Jonathan Miller) parodied the Bishop of Woolwich's assertion that God was not an old man in the sky by pedantically discussing, with much waving of the arms, his actual whereabouts. "Is he over here, then?" "No, not exactly over there." "More to the left, then?" "Yes, that's right. But down a little."

Eagleton's God is equally elusive in these quasi-mystical passages. What can it mean, for example, to say that God sustains all things by his love and that he made the world simply for "the love and delight of it"?

Faced with such musings from Eagleton about the nature of God, some critics simply throw up their hands in dismay. The biologist PZ Myers, for example, describes such vague definitions as "blathering pseudo-scholarship" and accuses Eagleton of basking in his lack of clarity: "his own contradictions are worn with pride as symbols of ineffable profundity."

It is easy to sympathise with Myers's irritation but perhaps there's a danger in not persisting with Eagleton's arguments, in not trying a little harder to understand the complex even devious manner in which he is trying to rescue religion from the strictures of the New Atheists.

For it's an uneasy truth that Eagleton's counter-attack has been warmly welcomed by a range of people who might not otherwise seem like natural apologists for religion. Part of the reason for this appreciation is undoubtedly stylistic. Eagleton, as students of his books on critical theory will know, is a powerful and engaging writer, perhaps no more so than when, with bursts of comic vituperation which recall Kenneth Tynan at his best, he is seeing off those he regards as second-rate opponents.

But probably more relevant is the sense among many readers and critics that Eagleton is providing a welcome antidote to the rather simple-minded conception of religion that Dawkins and Hitchens selected for their demolition jobs. He is rather like a wise old schoolmaster explaining to two eager young students that the significance of Hamlet is hardly exhausted by describing it as "a revenge drama".

So literary theorist Stanley Fish rushes to praise Eagleton for puncturing "the complacency" of the New Atheists while movie critic Andrew O'Hehir describes Reason, Faith and Revolution as "one of the most fascinating, more original and prickliest works of philosophy to emerge from the post 9/11 era", and Piers Paul Read allows in the Observer that Eagleton is more learned that his opponents and "broader in his thinking".

But is the emperor really as well dressed as such critics allow? I decided to press him further about his theology. "You say that God made the world simply for the love and delight of it. But you don't mean 'made' in the usual sense of the word as you've already insisted that God did not create the world."

"That's right. Aquinas is saying that the relationship between God and the world is about the fact that the world is in some ways His. Not in the sense that my shoes are mine because I manufactured them but because at the centre of the world lies his love and freedom. God didn't create the world. He loved it into being. Now what that means, God knows, but that's exactly what Aquinas was saying. The concept of God is what will not let you go. He will not let you slip through his fingers. It's that kind of unconditional love. If you like, that's impossible. We can only know conditional love, but if you are to have some kind of authentic idea of God that's the place from which you have to start, not seeing God as some kind of manufacturer."

I was intrigued by Eagleton's admission of uncertainty, his parenthetical "God knows", in this piece of elaborate theology. It prompted me to suggest that he was not so much accepting the details of Aquinas's account as using it as one way to argue that religion was one domain of thought which was incompatible with science. It occupied another register. Science, for example, couldn't even begin to answer the question of why there was a world in the first place, why there was something rather than nothing, why what we did have was intelligible to us. But did this mean that we needed to call upon religion to provide to such fundamental questions? Wasn't this more properly the domain of philosophy?

"Yes, I think that's fair enough. It was Leibniz after all who raised the question of why there is anything at all. If that is a coherent question, and some philosophers think it isn't, then it has received an answer from the theologians. It is because of God. Now that might not be right but it is a question that theology tries to deal with."

But were such questions ever considered by the mass of believers who were the intended audience for Dawkins and Hitchens?

Eagleton was conciliatory. "I don't want to deny that there are a lot of simplistic ways of thinking in religion. And yes, maybe I do have a more sophisticated view of religion than many believers but, hey, most people's understanding of evolution is not like Mr Dawkins's understanding of evolution and most people's understanding of Marxism is not one that you or I would want to defend."

But he'd still want to insist that the average believer was more sophisticated than Dawkins and Hitchens allowed? He had, after all, spoken very passionately in defence of the religion of his forebears and his inability to accept that the creed to which they devoted their lives was worthless and void.

"Well, it's not exactly a knock-down argument from reason but something that has been believed by millions of people for thousands of years is not just garbage. It may not be right or true as Dawkins or Hitchens see it but there is something in it that has to be interpreted."

And, of course, as Eagleton makes plain in his book, also something to be celebrated. Even the "cruelties and stupidities that the Irish Church has perpetrated do not prevent me from recalling how, without it, generations of my own ancestors would have gone unschooled, unnursed, unconsoled and unburied." Dawkins, however, in a book of almost four hundred pages "can scarcely bring himself to concede that a single human benefit has flowed from religious faith."

But wasn't his complaint misplaced in that both Dawkins and Hitchens were committed to liberating all those who, far from benefiting from religion, had had their lives and their thoughts constrained rather than elevated or enlarged by dogmas? I had, I told Eagleton, been very impressed, in my own conversation with Dawkins, by his stories of those who had approached him after reading The God Delusion or hearing one of his public lectures, and expressed fervent thanks for the manner in which he changed their lives for the better.

"Listen. If Dawkins has emancipated people, freed them from the religious closet as it were, then all credit to him. Loath as I might be to compare Dawkins to Jesus Christ, in this he resembles the heroic figure in the New Testament who comes to sweep away all the fetishism and sickness and cynicism of the neurotic religionists."

This was Eagleton's first mention of the figure of Christ in our conversation even though much of the argument in Reason, Faith and Revolution is about how Christianity has betrayed its origins, how its original emphasis upon the transformational capacity of love makes common cause with socialism.

You want to save Christianity from the Christians?

"Yes, I quote my father who insisted that Jesus Christ was a socialist and that any Christianity that is not on the side of the dispossessed against the arrogance of the powerful and rich is utterly untraditional. Dawkins and Hitchens write about Christianity and never link the words God, justice and love. That is either a sign of their obtuseness or a sign of the massive self-betrayal of the Christian movement. It has got to the point where intelligent people like them don't understand that Christianity is not about how many months you get in purgatory for adultery. It's about a love and a thirst for justice that will bring you to your death. There's nothing lovely about it."

Eagleton believes in Jesus, or rather in the symbolic power of Jesus the revolutionary who urged his followers to feed the hungry, love their enemies, give away their possessions and visit the sick, and was finally tortured and killed for such advocacy. If, he argues, we want an image, a signifier, that captures the ugly awful truth of human history then we could not do better than choose the tortured body of the innocent Christ - the crucifixion.

Even one of Eagleton's sternest rationalist critics is prepared to allow the potency of this image. PZ Myers, in a review which elsewhere describes a day spent reading Eagleton's new book as frustrating, horrible and awful, admits that "the symbol of the crucified Christ is a powerful one. It appeals because we can identify with suffering ... it resonates."

But Myers quickly finds this merit cancelled out by the actions that have been carried out in the name of the suffering Christ. Something more uplifting is required if we want a signifier for the human condition. "Imagine the culture we would live in now if, instead of a dead corpse on an instrument of torture, our signifier was a child staring in wonder at the stars."

Eagleton has no time for such uplifting images. Our history, he believes, demands an image that constantly reminds us of our failures to set the world to rights. This is not for Eagleton a fatalistic denial of the value of attempting to improve the world, or indeed a denial of the manner in which liberal endeavours have enhanced our various freedoms. But it is a powerful iconic reproof to all those who have perverted liberalism - an ideology, asserts Eagleton, with its roots in Christianity - into a belief in unilinear progress.

And who subscribes most tenaciously to this story of universal moral progress? Eagleton looks no further than his familiar opponent.

"Dawkins," he contends, "has a Panglossian vision of progress. A view from North Oxford. Indeed for all his self-conscious modernity he turns out to be something of an old-fashioned Hegelian believing in a Zeitgeist (his own word) involving every increasing moral progress with just the occasional 'reversal'. History is perpetually on the up. Not even beaming tambourine-banging evangelicals are quite so pathologically bullish. What is this but an example of blind faith? What rational soul would sign up to such a secular myth?"

(When I confronted Dawkins in 2007 with his description of the Holocaust as "a temporary setback", he at first insisted that it was still appropriate to believe in general moral progress. He thought that the idea of such progress was "plausible" but agreed that my scepticism deserved attention. It was, he finally said, "a fair cop".)

It is Dawkins's stated belief in the inevitability of progress that, according to Eagleton, marks him out as a particular kind of humanist.

"Dawkins deeply believes in the flourishing of the free human spirit which makes him a liberal humanist rather than a tragic humanist. He believes that if only those terrible guys out there would stop stifling and shackling us, then our creative capacities would flourish. I don't believe that. As a Marxist I reject that simple liberationism. I'm not again humanism. I'm for a humanism which recognises the price of liberation. And that's what I call tragic humanism. The only idea of emancipation worth having is one that starts from looking at the worst, that starts from Swift's race of odious little vermin. If you're the kind of humanist who can understand what Socrates meant when he said it would been far better if man had never been born, you're on. A humanism like Dawkins's and possibly that held by Hitchens isn't worth all that much. It's too easy."

As I checked my watch and saw that my time with Eagleton was up I reflected that while Christ had certainly taken a long time to enter the argument, poor old Marx had nearly missed his entrance altogether. But it was only after I'd manoeuvred an unwilling Eagleton into the arms of the New Humanist photographer in the hotel lobby that I began to recognise the import of Marx's absence from our conversation.

Marx, of course, sees all religious ideas as social and historical products. This doesn't empty them of all significance, turn them into mere epiphenomena. Marx readily allows that such ideas are vehicles for expressing suffering and the suffering they record is very real indeed. Religion is "the heart of a heartless world, the soul of soulless circumstances" as well as being "the opium of the people".

But oddly this appears to be a Marxism too far for Terry Eagleton. He may readily draw our attention to the material facts of capitalism, imperialism and inequality, and may continue to criticise those who fail to recognise that Islamophobia has little to do with religion and almost everything to do with western material interests, but it seems that he cannot in the context of his argument with Hitchens and Dawkins, or indeed in the context of his own religious biography, go the whole hog and allow that all religious ideas are illusory.

Reason, Faith and Revolution is, after all, primarily an exercise in retrieving or resuscitating those religious ideas that are not "true" in some vulgar scientific way but that nevertheless have an intrinsic potency, a real value, for those who hold them. As the Marxist critic John Molyneux has argued, this leaves Eagleton in an uncomfortable position. "In demonstrating his understanding of the liberal theologians' concept of an immaterial, impersonal god of love and tolerance in contrast to the Old Testament god of vengeance, Eagleton leaves decidedly open the possibility that this liberal god may actually exist, or be worthy of worship. For a Marxist the loving, caring, impersonal god of Dietrich Bonhoeffer and radical Jesus of Terry Eagleton are both just as much human creations, illusory projections, as the unpleasant bigoted gods of Ian Paisley or Osama Bin Laden."

There is, then, a fascinating double repression going on in the pages of The God Delusion and Religion, Faith and Revolution. Dawkins, the thoroughgoing scientist, abandons a central tenet of science - testability - in order to proclaim his belief in moral progress, while Eagleton, the thoroughgoing Marxist, is forced to relinquish a fundamental tenet of Marxism - its materialism - in order to find religious ideas of sufficient intrinsic value to counter Dawkins's alleged caricature.

In a last throwaway remark Eagleton told me that he was unhappy that Dawkins had never taken up his invitation to debate the subject of religion. After this attempt to bring their views into some sort of alignment I can't help feeling that another sort of event might be more appropriate, a contest in which the protagonists' insistence on playing by different rules was recognised from the start. How about a match? I can see it now in all its wonderful confusion. Oxford United FC versus Derry Hurling.

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Reason, Faith and Revolution: Reflections on the God Debate by Terry Eagleton is published by Yale University Press

Photos by Cliona O'Flaherty