Apologist - a person who writes or speaks in defense or justification of a doctrine, faith, action, etc.
Clive Staples (Jack) Lewis (1898-1963) was a professor of medieval and Renaissance literature at Oxford and Cambridge Universities, as well as a writer of more than thirty books, among which are included what probably are his most famous and best beloved, The Chronicles of Narnia, a series of fantasy novels set in a fairy-tale world. It was by reading the Chronicles that I knew of him first, and probably best.
But the Narnian Chronicles have a dual purpose - they not only serve as a means of entertainment geared toward young people, but they tell the tale of Christianity through their stories, and even more, they reflect Lewis' own beliefs. Until I knew about Lewis, I did not know what was meant by a Christian Apologist, but without going too deeply into the definition and instead thinking back on what I know of the man through his writing, I can guess that a Christian Apologist is a person who tries to cut through the hypocrisies and cruelties and intolerance that have grown up around Christianity and strip it back to the basic beliefs.
Mere Christianity is C.S. Lewis' own doctrine of Christian belief. First heard as a series of informal radio broadcasts which were then published as three separate books (The Case for Christianity, Christian Behavior, and Beyond Personality), Mere Christianity brings together what its author sees as the fundamental truths of the religion.
Though rather short, this is definitely quite a "heavy" book to read; I had to take it in short bits in order to get through it. What makes it harder to read is that it wasn't written as a book, but as a broadcast, so the tone is decidedly conversational in a professorial type of way. But it is an excellent guide for anyone, Christian or not, seeking to learn the truth behind the faith - truth that has been clouded by centuries of added dogma and human intolerance and stupidity. If you have been hurt by fundamentalists in the past, you won't find such accusatory writings here - Lewis rejects no denominations as Christian or un-Christian, but rather rejects the boundaries that divide the faith's many denominations, finding a common ground on which all those who have Christian faith can stand together.
For the non-Christian reading this book, this is what Christianity should be. For the Christian reading this book, I say the same: it takes a great deal of courage to practice the faith the way Lewis believed it, with primary concern being aimed at the behavior of the self rather than obsessing over the behavior of others. If more practitioners of any religion held faith the way C.S. Lewis held it, it would definitely be an easier world to live in.