Mid-March, and snow comes down in fat flakes. It seems winter will never relinquish its grip; you could almost picture a Pevensie child in oversized furs peering out from one of the centuries-old staircases. At Christ Church during the Oxford Literary Festival, there is no Lucy, of course – but there is someone Lewisian.

Alister McGrath is here to talk about his new biography of the Narnia author to a crowded lecture theatre amid the scent of wet woollens. There have been plenty of biographies of C.S. Lewis, from personal memoirs onwards, but McGrath said he sat down to his task by reading everything Lewis wrote, in chronological order. A plain, practical approach – and productive, too.

Trinity Term 1929 has long been a pivotal moment in the Lewis story – the moment this atheist Ulsterman found God. Lewis himself pinpointed that term in his autobiographical Surprised by Joy. But reading Lewis’s contemporaneous writings, said McGrath, “I began to realise something wasn’t quite right.” There was no sign of burgeoning faith at all in 1929, not even while Lewis’s father was dying.

McGrath reckons Lewis was in fact out in his estimate by a whole year, and suggests the loss of his father was actually a precipitant for a spiritual awakening in Trinity 1930. It turns out Lewis was always simply dreadful with dates. Put in charge of room allocations at Magdalen College, he made an appalling mess and his redoubtable military brother Warnie had to take over.

There are clearly other nuggets in C.S. Lewis: A Life, published this month: among them new findings on Lewis’s Great War friendship with fellow soldier Paddy Moore; and new arguments about whether Lewis bedded Paddy’s mother, and on the nature of his Shadowlands relationship with Joy Davidman.

McGrath fits the bill for a Lewis biographer. Also from Belfast, like Lewis he arrived at Oxford as an undergraduate and atheist but eventually became a Christian and a don – Professor of Historical Theology and Principal of Wycliffe Hall. Now teaching at King’s College London, he explained that he hopes to have grounded Lewis more thoroughly in the context of his Irish roots and of Oxford intellectual culture of his day.

For all his social conservatism, Lewis was also ahead of his time — for example, by blasting the inhumanity of eugenics. McGrath also made the startling observation that as a land defined by its sentient animals, Narnia itself stands as a protest against vivisection.

Festival schedulers put McGrath on at the same time as Philip Pullman, talking about his version of Grimm’s Fairy Tales. Perhaps they assumed that no one interested in hearing a theologian speak about Lewis would also want to see the atheist Pullman, who has called Narnia “one of the most ugly and poisonous things I've ever read”. Well, the schedulers were wrong. Whatever the upshot, it’s natural that these two giants of children’s fantasy should do battle for the heartland, and that people who care should read both.

But there was plenty else to tickle the reader’s fancy on this first festival Sunday: talks on Joan of Arc, on English gardeners in the Second World War, on the reality of relief aid; Winnie-the-Pooh entertaining children, Peter Hitchens debating drugs. There was advice for writers on how to make it through the slush pile and even, for those hardy enough, a literary walk through the slush pile that Oxford had become in the thawing snow.

I took pot luck and met an Italian proto-fascist of almost unfeasible charm and wickedness. This was Gabriele D’Annunzio, subject of a new biography by Lucy Hughes-Hallett, The Pike – so named, she said, because he was like that lurking opportunist, pouncing voraciously on whatever swam by. In D’Annunzio’s case, as often as not, this meant women: musicians, actresses, ducal daughters, even one lifelong lesbian who “made an exception for him” and told him how his heaven would be inhabited by a gigantic octopus composed of women’s legs but with no head. He took lovers and flung them aside with gusto, despite being short and ugly with teeth of three different colours.

Hughes-Hallett laid out D’Annunzio’s life from tyro to tyrant as if setting off a string of firecrackers. He launched his career as a poet by anonymously tipping off a newspaper about his own death. Italy was soon in deep mourning for this unknown genius – just in time for his telegram announcing that he yet lived (and magnanimously forgiving the newspaper for its error).

D’Annunzio wrote dazzling poetry and turgid, bombastic plays. He was a pioneering aviator and he egged Italy on to join the Great War, which in his own words he “adored”. In the aftermath he established a shortlived fiefdom in Fiume (now Rijeka in Croatia) which was, said Hughes-Hallet, rather like Haight-Ashbury for its drugs and sex – except that here the people “wanted to make love and war”. When it all came crashing down, it left many of D’Annunzio’s followers primed – with their black shirts, lightning flashes and straight-armed salutes – for Mussolini.

There are few festivals where one can revel in such contrast, walking straight from a talk about one of Oxford’d finest fantasists to immediately encountering a womanising proto-fascist. But that, it seems, is what makes the Oxford Literary Festival so special — snow or otherwise.

John Garth is the author of Tolkien and the Great War.