"Who is C.S Lewis?" I asked one of my schoolmates at the Methodist College, Belfast, some time around 1969. Our headmaster had mentioned how much he had enjoyed reading one of his books, which had something to do with a lion and a wardrobe. It seemed an improbable plotline, and I wondered what on earth it was all about.

"Lewis?" my friend replied, a puzzled look on his face. "Isn't he an English writer?" It was not the most promising (or accurate) introduction to Lewis, and my momentary interest in him went no further. I was too preoccupied with studying science in the sixth form to worry much about lions or wardrobes.

At that stage, I was a rather ungracious and aggressive 16-year-old atheist, who took the view that any sensible person realised that science had long since eliminated belief in God. It was, therefore, something of a surprise when I found my intellectual world turned inside out only a few years later.

I had gone up to Oxford University to study science in much greater detail, assuming that it would confirm my atheism. After much mental anguish, I realised that Christianity made far more sense than atheism. Much to my embarrassment, I became one of a group of people whom I had, until this point, totally despised - serious religious believers.

As I began to think about my faith, friends suggested that I should read Lewis. Curious, I bought a few of his books in 1974, and scribbled the date of purchase on their title pages. I still have them, and they have remained with me ever since.

It is hard to put into words what I found in Lewis then, and continue to find to this day. Somehow, he seemed to present Christianity in a way that satisfied my intellectual longings, and stimulated my imagination. It was not just that he said some good things: he also seemed to say them rather well.

I still read Lewis today. Indeed, I reread him, finding many things that I missed the first time round. There always seemed to be added layers of meaning waiting to be discovered, good images to be used in sermons, or elegant turns of phrase to be turned over and savoured. I am hardly alone in this evaluation. Lewis's books sell more strongly today than at any point in his lifetime. He has become one of the most widely read religious writers of the 20th century.

IN WRITING C. S. Lewis: A life, I found myself warming to Lewis in a way I had not expected. He was no saint. Indeed, in many ways, I uncovered a flawed and damaged person - like the rest of us. Researching Lewis's life and writings brought home to me how God is able to take and use even the most damaged and flawed person, and do remarkable things through them.

Lewis, I began to realise, is as much a witness to divine grace in his life as in his writings. Reading Lewis also helped me to reconnect with my past. Lewis and I were both born in Belfast. We were both atheists as young men. We both went to Oxford University, initially as undergraduates, and then as dons.

So why does Lewis remain so popular? Sadly, there are those who still dismiss him as lightweight and populist: beneath serious academic or ecclesial consideration. That was certainly the attitude that I encountered at Oxford in the mid-'70s, when I began to study Christian theology.

Lewis was then ridiculed by the theological establishment as "outdated", "populist", and "theologically naïve." Yet I found that his writings offered a powerful intellectual, moral, and imaginative vision of the Christian faith which I found lacking in many of the lesser writers I was obliged to read.

And I soon found that I was in excellent company - even then, but much more so now. Rowan Williams's brilliant recent study of the Chronicles of Narnia, The Lion's World, is noteworthy for more than its important insights into Lewis's ideas. Lord Williams, easily one of the most significant theologians and churchmen in the UK, found something in Lewis that he did not find elsewhere. "I can only confess", he said, "to being repeatedly humbled and reconverted by Lewis in a way that is true of few other modern Christian writers."

EVEN back in the '40s, senior churchmen were beginning to recognise that Lewis was a remarkable lay theologian, able to connect with contemporary culture in ways that seemed to elude the national Church.

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When an apology was called for

Francis Spufford argues that C. S. Lewis was an uncomfortable apologist, that his efforts were very much of their time, and that his approach is now out of step with most British readers

Lewis soared to fame after the publication of The Problem of Pain, and the success of his first series of radio talks. Senior churchmen - including the Regius Professor of Divinity at Oxford University, Oliver Chase Quick, and the Archbishop of Canterbury, William Temple - came to regard Lewis and Dorothy L. Sayers as talented lay theologians, able to communicate effectively with British culture.

Although not himself a theologian in the professional sense of the term, Lewis's first honorary degree was a Doctorate of Divinity, awarded by the University of St Andrews, Scotland, in 1946.

Professor Donald M. Baillie, speaking at the award ceremony on behalf of the Faculty of Divinity, declared that Lewis had "succeeded in capturing the attention of many who will not readily listen to professional theologians", and had "arranged a new kind of marriage between theological reflection and poetic imagination".

Both these points have obvious relevance to the contemporary challenges and opportunities faced by the Church of England - and to understanding Lewis's ongoing relevance for the Church today.

In the first place, Lewis showed himself to be adept at what is now known as "cultural translation". He found himself able to find the right analogy, a telling way of putting something into words, that engaged both the imagination and reason. Mere Christianity showed that Lewis was able to translate his theology into the "cultural vernacular".

AS THE recent rise of the "New Atheism" made clear, the Church must find ways of connecting with the complex cultures of today's Britain. Lewis remains a role-model for this task.

Baillie's second point, about theology and the imagination, must also be taken seriously. Many would agree that Lewis's genius lay not so much in affirming the rationality of faith, but in demonstrating that rationality through an appeal to the imagination.

Lewis's move away from atheism - initially to theism, and then to Christianity - partly reflected his growing disenchantment with its imaginative deficiency. What he later termed his "glib and shallow rationalism" was intellectually unpersuasive, and existentially unsatisfying. "Nearly all that I loved I believed to be imaginary; nearly all that I believed to be real I thought grim and meaningless."

Lewis discovered that Christianity made sense of things - of what he observed in the world around him, and experienced within him. "I believe in Christianity as I believe that the sun has risen, not only because I see it, but because by it, I see everything else." For Lewis, the imagination enables us to grasp the rationality of the world - and, above all, the ability of the Christian faith to act as a lens, bringing things into focus.

We need to recapture the imagination of our culture. It remains important to show that Christianity makes sense, especially in the light of the rhetorically loud, but intellectually shallow criticisms of the "New Atheism". Lewis's Oxford contemporary, the theologian Austin Farrer, realised the importance of this point in the '60s. "Rational argument does not create belief, but it maintains a climate in which belief may flourish."

Lewis's real talent, Farrer argued, was "not proof; it was depiction". While it was important to reassure our culture that the Christian faith was reasonable, this was not enough to secure its acceptance. It had to inspire and excite by engaging the imagination.

LEWIS provides his readers with a reasoned defence of faith; yet he does so in a way that avoids reducing the appeal of faith to the cold steel of logic. Instead, we find a warm and vibrant appeal to the imagination, in which we are invited to see how Christianity gives us a new way of thinking and living.

As Farrer noticed, Lewis's apologetic approach looks like an argument; but, on closer inspection, it is an encouragement to see things in a new way, and thus grasp the rationality of faith through the imagination. Lewis makes us "think we are listening to an argument" when, in reality, "we are presented with a vision, and it is the vision that carries conviction."

Lewis's appeal to the imagination opens up a deeper and richer vision of reality, which Christianity cap-tures in its own way. He helps us to realise that its vision of reality is often difficult to express in words and concepts, as the doctrine of the Trinity makes clear.

Yet, as Lewis pointed out, our difficulties often arise from not seeing things the right way. Lewis's apologetic for faith often takes the form of a simple invitation to see things in a new way. We still have much to learn from him, 50 years after his death.

The Revd Dr Alister McGrath is Professor of Theology, Ministry, and Education at King's College, London. He is the author of C. S. Lewis - A Life: Eccentric genius, reluctant prophet, published by Hodder & Stoughton at £9.99 (Church Times Bookshop £8.99), and The Intellectual World of C. S Lewis, published by Wiley-Blackwell at £19.99 (Church Times Bookshop £17.99). He is taking part in the C. S. Lewis Symposium at Westminster Abbey on 21 November; details at www.westminster-abbey.org.