A diversity of ethical and religious beliefs hardly insures that they are all somehow true. A tribal culture may be scientifically wrong in thinking that the sun revolves around a flat earth. Ethical relativism eliminates the notion of a moral mistake. But this is just as fallacious as saying that every answer on a multiple-choice test is correct because there is a diversity of answers. There may also be less diversity between cultures than is often thought.
Every culture has taboos against stealing.
Yet a desert culture may penalize the theft of water much more highly than would a tropical culture. The diversity of moral codes does not rule out a basic agreement on deeper ethical principles . In an appendix to his excellent book against relativism, The Abolition of Man , C. S. Lewis listed common moral principles spanning thousands of years from diverse religions and civilizations. As Paul tells us in Romans 1-2, God has endowed with a conscience all those created in His own image, however much we efface or neglect it. Relativism also leads to absurd conclusions which undermine its credibility.