C. S. Lewis explains the background of Mere Christianity's title in the Preface—he takes the term from the English Puritan Richard Baxter, who refers to the basic aspects of "meer" Christianity in his seventeenth-century text Church History of the Government of Bishops ("I am a CHRISTIAN, a MEER CHRISTIAN, of no other Religion").

Mere Christianity is organized into four sections, based on several sets of talks C. S. Lewis gave on BBC radio in the early 1940s. In "Book 1: Right and Wrong as a Clue to the Meaning of the Universe" Lewis argues for the truth of Christianity based not on Scripture but on reason and logic. He states at the end of the first chapter,

First, . . . human beings, all over the earth, have this curious idea that they ought to behave in a certain way, and cannot really get rid of it. Secondly, . . . they do not in fact behave in that way. They know the Law of Nature; they break it. These two facts are the foundation of all clear thinking about ourselves and the universe we live in.

He goes on to posit that the “religious view” of the universe, as opposed to the “materialistic view,” is the only one that can account for the existence of the Law of Right and Wrong. Further, he says (1) that humanity is going the wrong way and needs to turn back; (2) that the Being behind the universe is intensely interested in right conduct—a terrifying prospect for man; and (3) that Christianity makes no sense until you accept points 1 and 2.

In “Book 2: What Christians Believe” Lewis discusses various views of Good and Evil and of God, defending, of course, the Christian position. He does not shy away from difficult subjects such as why a good God would allow evil in the world or whether Jesus was truly God incarnate.

A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on a level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse.

“Book 3: Christian Behaviour” covers topics such as morality (including social and sexual morality), virtue, marriage, forgiveness, love, and hope. And “Book 4: Beyond Personality: Or First Steps in the Doctrine of the Trinity” gets into the deepest theological waters in the work. Here Lewis wrestles with complex issues such as the three-in-one nature of God, His relationship to time, and how humans can become sons of God (the nature of salvation). The book ends with Lewis’s controversial views on Christianity and evolution—he asks whether spiritual redemption might not be the next step in man’s evolution.

If Christianity was something we were making up, of course we could make it easier. But it is not. We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with Fact. Of course anyone can be simple if he has no facts to bother about.