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. For example, on the one hand it is inconceivable that something like genocide would ever be considered moral, and on the other hand that something like genuine politeness could be considered immoral; but there will always be a middle ground between those extremes for things like birth control, divorce, diet, self defense, or patriotism, which will depend on the situation.
Any morality which is based on an unyielding structure above and beyond humanity is dangerous to human beings. History is filled with examples of what religious "morality" has done to worsen our lot. Whole cities can be gleefully exterminated in God's name. Society's "witches" can be eliminated. Free thought can be suppressed, squelching any hope for progress. (Why else were the Christian-dominated centuries called the "Dark" Ages?) Under Christian morality, anything goes if it furthers God's plan. In place of CS Lewis's Law of Morality, more enlightened people would champion reason and kindness: principles that are pliable and human, not rigid and cold.
Taken from the article Mere Assertions.