Lewis, an Anglican, intended to describe the Christian common ground. In Mere Christianity, he aims at avoiding controversies to explain fundamental teachings of Christianity, for the sake of those basically educated as well as the intellectuals of his generation, for whom the jargon of formal Christian theology did not retain its original meaning.
The Case for Christianity
Lewis spends most of his defense of the Christian faith on an argument from morality, a point which persuaded him from atheism to Christianity. He bases his case on a moral law, a "rule about right and wrong" commonly known to all human beings, citing the example of Nazism; both Christians and atheists believed that Hitler's actions were morally wrong. On a more mundane level, it is generally accepted that stealing is violating this moral law. Lewis argues that the moral law is like the law of nature in that it was not contrived by humans. However, it is unlike natural laws in that it can be broken or ignored, and it is known intuitively, rather than through observation. After introducing the moral law, Lewis argues that thirst reflects the fact that people naturally need water, and there is no other substance which satisfies that need. Lewis points out that earthly experience does not satisfy the human craving for "joy" and that only God could fit the bill; humans cannot know to yearn for something if it does not exist.
After providing reasons for his conversion to theism, Lewis goes over rival conceptions of God to Christianity. Pantheism, he argues, is incoherent, and atheism too simple. Eventually he arrives at Jesus Christ, and invokes a well-known argument now known as the "Lewis trilemma". Lewis, arguing that Jesus was claiming to be God, uses logic to advance three possibilities: either he really was God, was deliberately lying, or was not God but thought himself to be (which would make him delusional and likely insane). The book goes on to say that the latter two possibilities are not consistent with Jesus' character and it was most likely that he was being truthful.
Lewis claims that to understand Christianity, one must understand the moral law, which is the underlying structure of the universe and is "hard as nails." Unless one grasps the dismay which comes from humanity's failure to keep the moral law, one cannot understand the coming of Christ and his work. The eternal God who is the law's source takes primacy over the created Satan whose rebellion undergirds all evil. The death and resurrection of Christ is introduced as the only way in which our inadequate human attempts to redeem humanity's sins could be made adequate in God's eyes.
God "became a man" in Christ, Lewis says, so that mankind could be "amalgamated with God's nature" and make full atonement possible. Lewis offers several analogies to explain this abstract concept: that of Jesus "paying the penalty" for a crime, "paying a debt," or helping humanity out of a hole. His main point, however, is that redemption is so incomprehensible that it cannot be fully appreciated, and he attempts to explain that the method by which God atones for the sins of humanity is not nearly as important as the fact that he does so.
The next third of the book explores the ethics resulting from Christian belief. He cites the four cardinal virtues: prudence, justice, temperance, and fortitude. After touching on these, he goes into the three theological virtues: hope, faith, and charity. Lewis also explains morality as being composed of three layers: relationships between man and man, the motivations and attitudes of the man himself, and contrasting worldviews.
Lewis also covers such topics as social relations and forgiveness, sexual ethics and the tenets of Christian marriage, and the relationship between morality and psychoanalysis. He also writes about the great sin: pride, which he argues to be the root cause of all evil and rebellion.
His most important point is that Christianity mandates that one "love your neighbor as yourself." He points out that all persons unconditionally love themselves. Even if one does not like oneself, one would still love oneself. Christians, he writes, must also apply this attitude to others, even if they do not like them. Lewis calls this one of the great secrets: when one acts as if he loves others, he will presently come to love them.