Lewis was a high-powered Oxford professor and perhaps the 20th century's most famous convert to Christianity. An atheist from boyhood, he converted at age 33 and devoted much of the rest of his life to writing about faith. His series of allegorical books known as The Chronicles of Narnia remains especially popular with children. Lewis and his close friend J.R.R. Tolkien were part of the casual Oxford literary group known as The Inklings.

Besides teaching generations of undergraduates at Oxford and Cambridge Universities, C.S. Lewis (1898-1963) wrote more than 40 books. Indeed, his published output sometimes appears to be the work of at least three different authors.

One was a serious scholar who, as a young man, won top honors for his study of classics, philosophy, and English. His book The Allegory of Love (1936) remains a landmark in the criticism of medieval and Renaissance literature. This was also the Lewis who, according to academic legend, had read every book published in the English language during the 16th century.

A second C.S. Lewis may have been the greatest 20th-century practitioner of apologetics -- the branch of Christian theology arguing for the soundness of its doctrines against the objections of unbelievers. Lewis had become an atheist in his teens, but underwent a dramatic conversion in 1931, largely under the influence of discussions with a fellow Oxford medievalist named J.R.R. Tolkien. Lewis told the story of his conversion in Surprised by Joy (1955), and delivered numerous lectures, such as the BBC radio addresses gathered in Mere Christianity (1952).

Finally, there was Lewis the author of science fiction and fantasy. Out of the Silent Planet (1938) was the first volume of what became known as the Space trilogy. After publishing The Lion, the Witch, and the Wardrobe (1950), Lewis found that it required six more novels to chronicle the land of Narnia -- with literary and theological echoes never quite drowning out his voice as a teller of adventure stories.

This third Lewis also wrote, in a more cynical vein, The Screwtape Letters (1942). The narrator, Screwtape, is a wise old member of the infernal bureaucracy; he offers advice on human psychology to a young demon to help him lure his first victim to eternal damnation. Lewis reveals a dry, donnish wit, enjoyable even to readers who don't share his religious beliefs. As a satirist, the Christian author becomes, almost literally, the devil's advocate.