In C.S. Lewis' essay, "The Poison of Subjectivism", he starts off by stating that practical reason is based solely upon a person's point of view on the difference between good and evil. If both sides have their view on "good", then who can say which is right? It's all based upon judgment. "Unless the measuring rod is independent of the things measured, we can do no measuring."
Humans cannot create this rod, which we can judge other things against, for we have no more power to create values than we have power to create a new sun. Lewis also explores the idea that humans can achieve a sense of improved morality. He disbelieves in improving our morality because, there is a set human nature that is constant in even very different cultures. "We do not hunt witches because we disbelieve in their existence. We do not kill men to avert pestilence because we do not think pestilence can thus be averted. We do 'sacrifice' men in war, and we do hunt spies and traitors."
As a result of the Fall, humans are now unable to fully understand the Law of Nature. So who then knows this Law and can create this measuring rod? God, in His divinity, created all values and morals and it is only He who has the authority to judge both the living and the dead.