The most popular form of atheism is a philosophy called naturalism. Naturalism is the view that science and science alone determines what exists. What exists is what our best scientific theories of the world require. If something is not required by our best scientific theories of the world, then it does not exist. But this is devastating for ethics because moral values are not required by science. Science is morally neutral. You cannot find moral values in a test tube. So it follows immediately, from the perspective of naturalism, that moral values do not really exist. They are just subjective illusions of human beings.
William Lane Craig
ethics, morality, philosophy