Like his Oxford friend J.R.R. Tolkien, Lewis was deeply aware of the imaginative power of "myths" - stories told to make sense of who we are, where we find ourselves, what has gone wrong with things, and what can be done about it. A "myth," as Lewis uses the term, is not a false story told to deceive, but a story that on the one hand resonates with the deepest structures of reality, and on the other has an ability to connect up with the human imagination. Tolkien was able to use myth to saturate The Lord of the Rings with a mysterious "otherness," a sense of magic which hints at a reality beyond that which human reason can fathom. Lewis realized that good and evil, anguish and joy, can all be seen more clearly when "dipped in myth." Through their "presentational realism," these narratives provided a way of grasping the deeper structures of our world at both the imaginative and rational levels.

Lewis may also have come to realize the power of myth through reading G.K. Chesterton's The Everlasting Man, with its classic distinction between "imaginary" and "imaginative," and deft analysis of how the imagination reaches beyond the limits of reason. "Every true artist," Chesterton argued, feels "that he is touching transcendental truths; that his images are shadows of things seen through the veil."

For Lewis, a myth is a story which evokes awe, enchantment and inspiration, and which conveys or embodies an imaginative expression of the deepest meanings of life - meanings that prove totally elusive in the face of any attempt to express them in purely abstract or conceptual forms. For Lewis, God authorizes the use of myth as a means of captivating the human imagination and engaging the human reason.

Lewis thus declares that human beings construct myths because they are meant to. They have been created by God with an innate capacity to create myths as echoes of a greater story or "story of a larger kind." Early Christian writers spoke of the logos spermatikos, a "seed-bearing word" implanted within creation by God, preparing the ground for the definitive revelation of God in Jesus Christ. Tolkien and Lewis both - though in slightly different ways - work with the notion of mythos spermatikos, a "narrative template" embedded within the human soul as part of the created order. Once more, these prepare the ground for the definitive revelation of God in the story of Jesus Christ. This approach is not about Jungian archetypes (although they may perform a similar function); it is rather a fundamentally Christian insight about the deeper structure of reality, and the best ways of representing and experiencing it by those who bear the "image of God."

Lewis argues that, since "God chooses to be mythopoeic," then we in our turn must be "mythopathic" - that is to say, receptive to God's myth, recognizing and acknowledging its "mythical radiance" and offering it an "imaginative welcome." And, since God uses myths as a means of communicating both truth and meaning, why should not humans do the same? Particularly those wishing to encourage their culture to offer an "imaginative embrace" to the Christian faith? Lewis offers a powerful imaginative alternative to the dull over-intellectualized apologetics of his own generation, which limited the appeal of the Christian faith to our reason.

Steeped in the riches of medieval and Renaissance literature, and with a deep understanding of how "myths" work, Lewis managed to find the right voice and the right words to get past the suspicions of a "fully waking imagination of a logical mind." Somehow, Narnia seems to provide a deeper, brighter, more wonderful and more meaningful world than anything we know from our own experience. Though the "Chronicles of Narnia" are clearly a work of fiction, they nevertheless seem far more "true to life" than many supposedly factual works. These evocative stories help us grasp that it is possible for the weak and foolish to have a noble calling in a dark world; that our deepest intuitions point us to the true meaning of things; that there is indeed something beautiful and wonderful at the heart of the universe; and that this may be found, embraced and adored.

Tolkien's Lord of the Rings is about finding a master ring - the ring that rules the other rings, which then must be destroyed because it is so dangerous and destructive. At the deepest level, Lewis's "Chronicles of Narnia" are about finding a master story - the story that makes sense of all other stories, which then must be embraced because of its power to give meaning and value to life.

But which is the true story? Which are merely its shadows and echoes? And which are fabrications, tales spun to entrap and deceive? At an early stage in the Lion, the Witch and the Wardrobe, the four children begin to hear stories about the true origins and destiny of Narnia. Puzzled, they find they have to make decisions about what persons and what stories are to be trusted. Is Narnia really the realm of the White Witch? Or is she a usurper, whose power will be broken when two Sons of Adam and two Daughters of Eve sit on the four thrones at Cair Paravel? Is Narnia really the realm of the mysterious Aslan, whose return is expected at any time?

Gradually, one narrative emerges as supremely plausible - the story of Aslan. Each individual story of Narnia turns out to be part of this greater narrative. The Lion, the Witch and the Wardrobe hints at (and partially discloses) the "big picture" which is expanded in the remainder of the Narnia series. This "grand narrative" of interlocking stories makes sense of the riddles of what the children see and experience around them. It allows the children to understand their experiences with a new clarity and depth, like a camera lens bringing a landscape into sharp focus.

Yet Lewis did not invent this Narnian narrative. He borrowed and adapted one that he already knew well, and had found to be true and trustworthy - the Christian narrative of creation, fall, redemption and final consummation. Following his late-evening conversation with Tolkien about Christianity as the "true myth" in September 1931, Lewis began to grasp the explanatory and imaginative power of an incarnational faith. Lewis came to believe in Christianity partly because of the quality of its literary vision - its ability to give a faithful and realistic account of life. Lewis was thus drawn to Christianity, not so much by the arguments in its favour, but by grasping its compelling vision of reality, which he could not ignore - and, as events proved, could not resist.

The "Chronicles of Narnia" are an imaginative re-telling of the Christian "grand narrative," fleshed out with ideas Lewis absorbed from the Christian literary tradition. The basic theological themes that Lewis set out in Mere Christianity are transposed to their original narrative forms, allowing the deep structure of the world to be seen with clarity and brilliance. A good and beautiful creation is spoiled and ruined by a Fall, in which the creator's power is denied and usurped. The creator then enters into the creation to break the power of the usurper, and restore things through a redemptive sacrifice. Yet even after the coming of the redeemer, the struggle against sin and evil continues, and will not be ended until the final restoration and transformation of all things. This Christian metanarrative - which early Christian writers called the "economy of salvation" - provides both a narrative framework and a theological underpinning to the multiple narratives woven together in Lewis's "Chronicles of Narnia."

In one sense, the "Chronicles of Narnia" are just a story. Yet to the initiated, they are a retelling of the greatest story of all, which no human story can ever articulate adequately. Lewis's remarkable achievement in the "Chronicles of Narnia" is to allow his readers to inhabit this metanarrative - to get inside the story, and feel what it is like to be part of it. Mere Christianity allows us to understand Christian ideas; the Narnia stories allow us to step inside and experience the Christian story, and judge it by its ability to make sense of things, and "chime in" with our deepest intuitions about truth, beauty and goodness.

Like Lewis's wardrobe, they throw open an imaginative gateway to discovering and embracing the "Great Story," for which this life is but a "title page."

Alister McGrath is Professor of Theology, Ministry and Education at King's College London, Senior Research Fellow at Harris Manchester College, Oxford University, and President of the Oxford Center for Christian Apologetics. His most recent books, published to mark the fiftieth anniversary of the death of C.S. Lewis, are C.S. Lewis - A Life. Eccentric Genius, Reluctant Prophet and The Intellectual World of C.S. Lewis