In fact, Lewis is confident that his readers will be tacitly convinced of the correctness of this line of thinking. And Lewis can afford to relax, I think, because most of his readers are Christians who buy the book because they are looking for substantiation. They are not skeptical searchers of truth. Humanistic morality is a code of ethics based on the value and quality of human life. It is not derived from absolute engravings on a cosmic stone tablet.
Morality is relative to human things like happiness, health, peace, beauty, love, joy, and justice.
It is the preferring of those actions and ideas which enhance the human condition over those which threaten it. The Nazis, who were mostly Catholics and Lutherans, were wrong not because they broke an absolute law, but because they desecrated human life. Even though humanistic morality does assert some rights and wrongs relative to the human condition, it is flexible and free to improve. Any morality which is based on an unyielding structure above and beyond humanity is dangerous to human beings. Whole cities can be gleefully exterminated in God's name.