C. S. Lewis (1898-1963), who lectured in English literature at Oxford for most of his life, was a prolific writer in many areas and a man who powerfully and eloquently defended Christianity. Half a century after his death many of his books remain bestsellers: one, Mere Christianity, sells a quarter of a million copies a year.
Why have Lewis's books endured? There are several reasons. For a start, he was a brilliant writer who used English to maximum effect. He was also an enormously intelligent and creative man capable of analysing problems from different angles, courageous enough to tackle difficult topics (for example, two of his books are called Miracles and The Problem of Pain) and creative enough to branch out into children's fantasy (the Narnia Chronicles). Yet although these are all important in explaining the lasting popularity of C. S. Lewis, I think there are other factors and they are all to do with how he saw the world.
First, Lewis was always intensely aware of the past. There is a tendency in our culture to dismiss dead authors as 'irrelevant'. Such views were alien to Lewis, a remarkably well-read man, even by the standards of his contemporaries at Oxford and Cambridge. So when he grappled with an issue such as miracles he was able to bring to bear not just his own thinking but the accumulated wisdom of the past. He saw his own Christian faith not as some recent invention but as merely the latest addition to a nearly two-thousand-year-old tradition. His grasp of the 'big picture' of Christian history and philosophy meant that he was able to keep contemporary issues in perspective. So although The Screwtape Letters, his brilliantly imagined fantasy of the correspondence from a senior to a junior devil, was written in the darkest days of the Second World War, Hitler and the Nazis are barely mentioned. It was a wise move; Hitler has gone but the devil remains. The same awareness of history encouraged Lewis to step back from many of the debates within the Christianity of his day. He called his great defence of belief Mere Christianity precisely because he believed many of the church divisions and controversies of his time were irrelevant in the long term. Here too he was right. His priorities as a Christian writer are summarised in a haunting quote of his: 'All that is not eternal is eternally out of date.'
Second, Lewis was always aware of the real world. Given that for most of his life, he was an unmarried man working in relative seclusion at the very highest level of education you might well assume that his writings were 'academic' in the worst sense of the word, being irrelevant and incomprehensible to all but a few. The reality is the exact opposite. Lewis was a gifted communicator and went out of his way to include, rather than exclude, people in his writings, something he managed without being condescending. He gave talks on Christianity to ordinary soldiers in the Second World War, wrote books for children and painstakingly replied to every letter he received with thought and concern. (Someone has estimated that he personally answered over twelve thousand letters during his lifetime.) People in Lewis's profession are often alleged to live in an 'ivory tower', a place excluded from the trials and tribulations of day-to-day life. You will find no sense of that in Lewis's writings. His books, full of realism and overflowing with a sense of life's joys, hurts and griefs, contain profound insights into the peculiarities of human nature.
Finally, and perhaps most importantly, Lewis was aware of the spiritual world. From the moment of his somewhat surprising conversion to Christianity in 1930 he was careful to think and write about everything that he did in the light of his faith. Quietly, sensibly, in a very matter-of-fact way, he wrote about God, Christ, angels, heaven and hell. While some religious writers display an awkwardness when they switch between 'ordinary life' and 'spiritual matters' there is no trace of any such gulf in Lewis's writings; both worlds were real to him.
Lewis was a great writer. In all that he stood for he is an antidote to our age, with its haste, shallowness and materialism. Reading Lewis encourages believers and challenges sceptics. If you have not read him, read him. And if you have read Lewis in the past, reread him. It will do you good.