In an age when the literary landscape is dominated by cookie-cutter page-turners and lit-fic lite, penning a 600-page novel about a man’s anguished search for an ‘authentic encounter with God’ might appear to be career suicide. Such books are about as fashionable as morris dancing and as rare as unicorns. The fact that the Costa-shortlisted novelist Neil Griffiths manages to make theology urgent, hip, even sexy in his capacious, breathtaking third book is testament to his abilities. As a God Might Be goes ‘right down to the foundations’ of belief: it explores how faith is found and lost, and how it might elude even its most committed pursuer.

When Proctor McCullough, a comfortably married forty-something business consultant with six-year-old twins, finds himself suddenly ‘afflicted’ by spiritual yearnings, he deserts his family and leaves London for the West Country with the intention of building a church. He finds the correct spot by ‘instinct’ (we later learn that the ground was consecrated in the 1650s); it’s significant that the site is ‘a thousand paces before the cliff’s edge’. This sheer drop plays a role that is both symbolic, as one of Gerard Manley Hopkins’s mental ‘cliffs of fall’, and actual.

Recruiting a ragtag gang of young drifters and acolytes, McCullough sets about constructing his church with frightening zeal. Yet he also experiences an unsettling bafflement about the origins of his calling. While confessing to a genuine fear that ‘he’d been approached by God’, he has enough self-knowledge to suspect he’s having a breakdown or a straightforward midlife crisis. In quick succession he sees a priest, a Harley Street therapist and a neurologist. Most of the time, he simply doesn’t know what he’s doing: ‘He felt a little like fallen blossom himself, contingent and set adrift.’

As McCullough becomes increasingly estranged from his wife, Holly, and their children, the autistic Walter and his sister, Pearl, his callow votaries become a Greek chorus, commenting on his spiritual vacillations. Most outspoken of all is the alcoholic ex-con Terry, who provides a barrage of sceptical atheism, while the most morally troublesome (for both reader and protagonist) is Rebecca, a ‘hypnotically beautiful’ eighteen-year-old to whom McCullough finds himself helplessly attracted. Griffiths doesn’t shy away from the queasy nature of this, presenting his hero as both flawed and misguided. In the end, it is Rebecca’s mother, a ‘sleek, precise … infuriatingly calm’ poetry professor called Judith, with whom McCullough actually becomes romantically involved. After the book’s single steamy scene, a solemn epigraph from Cardinal Newman arrives like an ice bucket in the face, demonstrating how the juxtaposition of the sacred and the profane can always be used to amusing effect. It’s only when Terry commits an act of homicide, providing McCullough with a Dostoevskian moral dilemma, that the book becomes engrossingly serious and complex – a tour de force of psychological realism interleaved with nuanced theological debate.

In many ways, McCullough is in the tradition of literature’s bare-chested, vitalist seekers, its ascetic self-punishers and babblers – a Henderson or a Fitzcarraldo. Yet Griffiths resists making him a typically larger-than-life suffering dreamer, instead tethering him to a believable and intricately portrayed home life. As much as the temptation suffered by McCullough when faced with Judith and her daughter sets up a conflict between the earthly and the divine, the rich domestic scenes are always presented with narrative purpose. Walter, ‘in his own world’, is an analogue of his father, who often finds himself (as R D Laing defined madness) in a minority of one. When McCullough watches Walter retreating from waves on a beach, he wonders poignantly if ‘it was this that he himself was waiting for – this simplicity of experience, but on a cosmic scale’.

At the time McCullough’s ‘authentic encounter’ – with a Mosaic burning oak tree (‘nothing was being eaten in this fire. It was self-sustaining’) – occurs, the church is still unfinished. It’s enough to have had one epiphanic moment. The last hundred pages provide an electrifying argument for the value of such experiences above the ‘commoditised faith’ of organised religion – ‘the signal and not the noise’. McCullough delivers his central monologue at the bedside of a dying man, whose body is later burned on a funeral pyre on the beach below the ominous cliff, a scene alluding to the death of the atheist Shelley.

While the relentless philosophical inquiry becomes slightly attenuated towards the end, with characters occasionally, and inevitably, becoming mouthpieces for ideological stances, the book dextrously explores the central ideas of Christianity: original sin, the Trinity, God’s incarnation, the nature of evil, free will, the Resurrection, the efficacy of prayer. That a novel can encompass these knotty issues and still remain riveting, dramatic and aesthetically nutritious is extraordinary. Like Iris Murdoch, Griffiths presents a cast of characters inwardly divided by philosophical dilemmas, diverted from the cerebral by the corporeal and struggling with the enduring paradoxes of being and believing. This is a morally serious, exquisitely written, ambitious novel about the great search for meaning. It’s a novel for grown-ups and for readers like Larkin’s churchgoer, forever ‘surprising a need’ in themselves to be more serious.