Part 1: "Right and Wrong as a Clue to the Meaning of the Universe"
Everyone is aware of basic moral standards, knowing security and happiness depend on fair play, yet we fail to comply perfectly. An objective observer would never guess a Moral Law existed.
The Moral Law encourages individuals to choose between impulses, but is neither impulse nor instinct. Though we teach it to children, we can't alter its content. Its universality is indicated by our quarrels over questions of fairness.
Virtues are universal despite different cultural expression. When a moral standard appears to change significantly over time (e.g. executing witches), it is usually because factual knowledge has changed (e.g. witches donÍt exist).
The "religious" view holds that humans were created by an intelligent being that prefers our good behavior, while the contrasting "materialist" view holds that the world was created through a series of impossibly slim chances.
Christianity speaks only to people who already recognize the Moral Law and think they need forgiveness for failing to live up to it.
Part 2: "What Christians Believe"
If the mind was produced by evolution, it is untrustworthy. "Unless I believe in God," Lewis wrote, "I can't believe in thought: so I can never use thought to disbelieve in God."
Injustice does not disprove God's existence because the concept of injustice derives either from God's Moral Law or from personal preference.
Abrahamic religions believe God made the universe and takes moral positions. Christians may think other religions are partly correct, whereas atheists must believe they're all wrong.
Jesus talked as if he were God: forgiving sins, judging the world, always existing. He couldn't have spoken pantheistically (believing God animates the world and is beyond good and evil) because he was Jewish. He either told the truth about his divinity, or was crazy or evil; calling him merely a great teacher is not an option.
The belief in a dualistic battle between good and evil is incoherent because no one is evil for evil's sake and because a third party would have to judge which side is "good." Christianity instead maintains there is "a civil war, a rebellion." The Devil gave Adam and Eve the desire to usurp God.
It is more important to accept what Christ did--dying to cleanse us and conquer death--than to understand theories about it. Punishing the innocent Christ would have been unfair; rather, he paid our debt of repentance. He was the only one who could repent perfectly because was both God and man.
We are given a choice to join God willingly before he invades the world. "Christians are Christ's body"; to have "Christ-life" means to be animated by Christ and to be helped by him to do good works. Accepting God is the only path to true happiness.
C. S. Lewis. The Case for Christianity. 1942) New York: MacMillan Co., 1960.
This summary was written in 2005, along with a series of other 500-word summaries of philosophy books, as an exercise in brevity.