Lewis discusses Christian behavior and morality in Book 3. He begins by clarifying three unique aspects of morality: how we treat one another, what is going on inside us, and the general purpose of humanity. Christian morality addresses all three aspects. There is broad agreement on the first aspect, but significant disagreement on the other two and whether they are even important for morality.
By Dr. Gordon Pettit; subscribe @gordonpettit; see more at gordonpettit.org