Summary of C. S. Lewis's Mere Christianity (Unabridged‪)‬

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A few key insights from Chapter 1:

People quarrel over many things, but quarrels always involve one party trying to prove the other wrong. They almost always appeal to a set of inherent rules that everyone implicitly knows about human behavior. These rules make up the Law of Human Nature.
Similar to the law of gravity, the Law of Human Nature sets a precedent for how all humans should act and behave with others. However, unlike gravity, humans do not need to obey this law. While they know the law’s inherent principles, they can choose to ignore them.
The idea of correct behavior is neither universal nor obvious. Some argue that this Law of Human Nature applying to all men is unsound, due to different civilizations and cultures. But taking a look at every great civilization throughout history, a common morality can be traced.
Everyone at some point has failed to practice the type of behavior they expect from others. The need to make excuses for these lapses in judgment is proof that we believe in these expected behaviors. Why else do we reflect so anxiously over our own bad behavior?