The now famous play Shadowlands, which is about to reopen in London's West End, is one of many pieces about my late stepfather, CS Lewis. There have been books (many books, of which I have perpetrated two), plays, movies and even songs written about him. They vary between good, bad and ugly, most of them written by people who either barely knew him or never knew him at all. They can tell you (with varying degrees of inaccuracy) what he was, where he was, when he was and what he did, but almost none of them are able to tell you who he was.

While he was alive I never knew "CS Lewis", the name on the spines of the books, for the living, breathing, delightfully talking man who filled my young life with his presence was "Jack". My first encounter with him was extra- ordinary. I was an eight-year-old American schoolboy, "straight off the boat", brought to Oxford a short while after arriving in this strange land of England, where the people dressed so oddly, spoke so oddly and ate strange and unlikely foods. I was being taken to meet the man who, as far as I was concerned, actually knew High King Peter of Narnia and the great lion Aslan; a man who, for all I knew, might be a member of King Arthur's court. I almost expected a tall, stalwart figure in armour, carrying a broadsword, but the reality was very different. In the kitchen of his house, The Kilns, we were greeted by a slightly stooped, balding, round-shouldered being with long nicotine-stained fingers and teeth, dressed in the shabbiest clothes I had ever seen. This was no knight, this was a don. An Oxford don at the time.

Despite my initial dismay, Jack soon emerged from my imaginary CS Lewis to become real. I lost an illusion and gained at first a friend and later a much-loved stepfather.

Within a short time Jack, "Warnie" (Jack's brother Major Warren H Lewis) and I were sawing up a heap of branches into firewood. Jack and Warnie, though academics both, were not above putting their hands to such menial tasks. Jack showed me the woods and the lake behind The Kilns, and taught me to look for fauns and dryads among the glowing sycamores and shimmering beeches. From him I learned a respect for plants, such as the giant horsetails in the marsh above the lake; I learned a love of fields, forests and animals; a delight in weather, from roaring winds to calm stillness, from pouring rain to bright sunshine; all have their place in my heart, and this I learned from Jack.

Jack taught me to read too. I don't mean to read as one learns in primary school, but to read for the love of reading and learning, for all the world's wisdom is to be found in books. Jack taught me that. The house was full of books, and none was barred to me.

At first I lived in London and visited The Kilns infrequently, but within a brief time we had moved to Headington a mile or so away, and Jack made it clear that I was a welcome guest. Some of the myths about Jack, and there are many, have come from his own pen. "Not good with children," he said of himself, and yet I rarely met a man who was better with children. I think perhaps he meant that he was never at ease with them, but then we are not meant to be entirely at ease with children other than our own. A misogynist some have libellously labelled him, and yet I never knew a man so considerate of women, nor one more charming and entertaining in their company.

I think that in today's sad and dark world many people will have difficulty in believing in the real Jack. He was a man who had grown up with the thinking of the 19th century. He believed in honesty, personal responsibility, commitment, duty, courtesy, courage, chivalry and all those great qualities that society in its wisdom dispensed with in the 20th century on the grounds that they were somehow outdated, and now needs so desperately to recall and recover. Jack also had come to understand a great deal about humanity and the nature of the species. He was no stranger to suffering: he lost his own mother at the age of nine, and experienced the horrors of one school that was Dickensian to say the least, and others of varying degrees of worth. Jack had fought in the first world war, and lived through the second one, losing friends and colleagues in both. He lived out his eve-of-battle commitment to fellow soldier Paddy Moore, taking care of that man's family for more than 30 years. Jack had learned to love and to lose, and had suffered the agonies of both. No one could have blamed him had he closed himself off and become (as he is often depicted) an isolated scholar surrounded by cloisters. Instead, released from his burden of caring for Mrs Moore after her death, he plunged once more into love and pain by marrying my mother, who was already dying at the time. He faced the pain of loving one whom he knew was unlikely to be with him very long, and also took on the responsibility for her sons, my brother and myself. Not an easy task under the best of circumstances, but under those with which he was faced, a task for a veritable hero.

My mother was an American, and had come to the truth of Christ through reading Jack's Mere Christianity and other of his works. She had written to him with her questions, for as with all highly intelligent people, she had questions. And she was delighted when he dealt with her questions and objections in masterful and economic style, and a lively correspondence soon grew up. Mother visited England in 1952, and she, Jack and Warnie were soon fast friends. Mother was intellectually Jack's equal, the only one I have ever met, and Jack was delighted when, as they debated and sparred, she would correct him in some minor error of quotation or detail. Mother was slightly more widely read than Jack, for she had read what he had read but also the more modern American writers. She also had travelled further, having been to America and returned to England with her progeny in 1953 when her marriage to my father ended. Jack and she married at her deathbed but the hand of God intervened and she rallied, going into a remission of several years, when they had the happiest years of their lives. It was during this time that the physical courage they both possessed was made evident to me. We were walking up the hill into the woods, my mother carrying her little "garden gun", which she used to scare pigeons off our vegetables and trespassers out of our woods, when the two of them, some distance ahead of me, were accosted by a young man with a bow and a quiver of arrows. "Excuse me," said Jack politely, "this is private land and you really shouldn't be here. Would you please leave?" The young man's response was to nock an arrow to the string and draw the bow, pointing it at them. Jack stepped in front of my mother to shield her, and stood there for a few seconds until he heard her say, in tones of chilled steel: "God damn it, Jack, get out of my line of fire!" Whereupon Jack stepped swiftly sideways, leaving the young man staring down the barrel of a gun. He took off rapidly. They were brave, and Jack thereafter needed to be, for Mother went on before him, leaving him alone to deal with her absence.

Was CS Lewis a great scholar? Undoubtedly. Was he a great writer? No honest scholar today can doubt that for a moment. Now some of his stories are moving into the medium of film, he is becoming more known throughout the world, greatly accelerating a trend that has been slowly happening over the 40 years since his death. Was CS Lewis a great teacher? That, I think, is also unquestionable: he taught at Oxford and Cambridge universities, and teaches on today through his books. Was he a great theologian? Many of today's finest Christian scholars strongly believe so. Although he would never have laid claim to any of those titles, nor perhaps even have accepted them from others, he was all of those things and a great deal more besides.

Shadowlands is a fictional retelling of a small part of Jack's story. In that play you will see a man in his hardest trial, bearing the heaviest of all burdens. You may get a glimpse of what great men go through. It's not 100% historically accurate, perhaps, and it's not supposed to be, but it is a fine play and richly insightful into the hearts of men.

You see, while CS Lewis was a great scholar, a great writer (in many genres), a great teacher and a great theologian, Jack was a great man.

· Shadowlands opens at the Wyndham's Theatre, London WC2, on Monday. Box office: 0870 950 0925.