In Mere Christianity, C. S. Lewis argues for the logical validity of Christianity, defends the religion from its critics, and looks in detail at what the life of a Christian is like.
In the first part of the book, Lewis discusses the “law of human nature.” When studying human history, he claims, one is struck by how similar different societies’ moral codes are, at least at a fundamental level. Lewis argues that moral law isn’t just an arbitrary human invention—it’s actually a real, timeless thing—invented by an all-powerful being who stands outside the confines of material space and time, and reveals itself to humans through moral law.
In the second part of the book, Lewis discusses a few competing theories about the all-powerful being’s identity. Some religious groups, the Pantheists, believe that the all-powerful being, God, is neither good nor evil. Pantheists believe that God is the universe, meaning that everything in the material universe is divine. Other religious groups, such as Muslims, Jews, and Christians, believe that God created the universe, yet is distinct from it; thus, God is good, and wants humans to work hard to make the universe a better place. Christians also believe in the existence of an ultimate evil, the Devil. However, in Christianity, evil isn’t equal to good—evil is “spoiled good”; i.e., the perversion or corruption of goodness. Looking around the world, it is obvious that good has been corrupted into evil almost everywhere.
When Lewis was a much younger man, he found it impossible to believe in a just God who would allow Earth to become a sad, unjust place. However, the only way for an atheist to criticize the Christian model of God would be to appeal to some standard for “just” and “unjust”—i.e., the same standard that led us to accept the existence of God in Part One. Lewis then attempts to resolve the apparent contradiction in the existence of a good God who allows evil things to happen by arguing that God gives people the gift of free will: they face the challenge of behaving virtuously in spite of the temptations of evil—a challenge for which they will be richly rewarded in Heaven.
At the end of part two, Lewis introduces Jesus Christ, the cornerstone of the Christian religion. The only way to be truly virtuous, Lewis argues, is to worship Christ. While there are many different Christian sects that worship Christ in different ways, Lewis argues that they can agree on the basic facts about Christ’s existence, and therefore can all attain salvation.
In Part Three of the book, Lewis studies the life of a good Christian. To begin with, Lewis proposes that morality consists of three different parts: harmony between people, harmony within a person, and constant vigilance in achieving a state of salvation. All virtues uphold the three parts of morality, and all sins contradict at least one of these parts. Lewis briefly discusses the four “Cardinal virtues”—prudence, temperance, justice, and fortitude, and explains why they’re necessary for living morally.
Lewis spends time defending the most unpopular aspect of Christian morality—the notion of chastity. Modern society, he insists, is overrun with sex and sexuality, to the point where people think that having lots of sex is “normal” and “healthy.” Lewis, however, argues that the sexual instinct—like any instinct—must be controlled and subdued. Lewis also defends the institution of marriage. While it’s popular to say that “being in love” is the only reason to get married, Lewis argues that being married is actually much more beautiful and majestic than being in love—a married couple must remain together for a lifetime, demonstrating their loyalty and respect for one another as human beings, rather than just following their emotions.
Lewis argues that pride is the most dangerous of all sins, since it encourages humans to place themselves “above God.” He suggests that many people who consider themselves to be good Christians actually worship a “false God,” and secretly think themselves to be superior to everyone else—a state of mind that will lead them to damnation unless they’re careful.
At the end of Part Three, Lewis discusses the three Theological virtues: charity, hope and faith. Charity is a challenging virtue, because it requires humans to be gracious and generous to people they might not necessarily like. But one of the miracles of virtue, Lewis claims, is that when we pretend to respect other people, we eventually do respect them. One of the most challenging aspects of Christianity, Lewis writes, is faith, especially in the challenging sense of having faith in God’s salvation. After a Christian becomes familiar with obeying the moral law of God, they sometimes reach a point of despair, during which they realize their own sinful nature. But even in their despair, the good Christian will find the strength to carry on, cautiously optimistic—not certain—that God will help them find the way to Heaven. A good Christian must trust their fate to God, while also working hard to be good.
In the fourth and final part of the book, Lewis turns to theology, the “science” of God. He analyzes the Holy Trinity, and the strange-sounding idea that God is both one thing and three. Lewis compares the Holy Trinity to a die: just as a die “contains” six square sides, and yet is one three-dimensional object at the same time, so too does God “contain” three parts and yet remain one being. Lewis also tackles the apparent contradiction of an all-knowing God who gives humans the gift of free will—one would think that, if God knows everything, then humans don’t truly choose their fates at all. Lewis resolves this apparent contradiction by arguing that God exists outside of time, meaning that he experiences humans’ past, present, and future in the same instant, whereas humans have free will within time as they experience it.
Lewis goes on to write that, by worshipping Christ, humans can transcend their mortal nature and experience the divine life of Christ himself; put another way, by worshipping Christ, they, too, can become “sons of God.” Good Christians unite together in their love for God—and yet they don’t sacrifice their individuality in doing so. On the contrary, Lewis argues, the only way to truly be an individual and fulfill one’s potential is to worship Jesus Christ. In the act of prayer, a human being assumes the guise of Jesus Christ; with practice and faith, prayer can help human beings become divine by leading them toward salvation in Heaven. In Heaven, people lose their desires for earthly things, and thus, the basic components of their so-called “personalities” on Earth. But in place of their old personalities, the saved discover their true selves: unique, individual, and yet united in love for God, in much the same way that the different organs of the human body are different from one another, and yet united in the facilitation of life. In short, Lewis argues, one must sacrifice one’s earthly needs and desires in order to be a Christian—but as a reward, one will find Christ, and rejoice in Heaven for eternity.