In the first chapter of Mere Christianity, Lewis discusses the "Law of Human Nature." By this he means the standard of morals that every civilization has adhered to for the most part since the beginning of time. There are certainly differences in the rights and wrongs of different civilizations but for the most part they have adhered to this moral law. And I would say to those that have strayed significantly had just become numb to conscience over a long period of time due to "little sins" becoming "big sins" against the moral law. As C.S. Lewis says, this is the only law of nature that man has a choice in. For example, we cannot choose whether or not we fall to earth due to gravity; that law we must follow. In the Law of Human Nature, we have a say in the matter. Our conscience will keep us from doing certain unspeakable acts, but we still have a lot of small decisions to make that our conscience will have little interference. It is the wrong choices in these situations that lead to the numbing of our conscience.
Lewis continues his argument for the moral law by addressing the arguments against it. Some have written to him saying that what he is talking about is just "herd instinct." Lewis's argument against this is that we always have instincts: to survive, to help people, etc. It is what helps us to judge between instincts that he would call the moral law. Since what tells us which instincts to suppress and which ones to follow cannot be an instinct in itself, it must be some type of natural law.
C.S. Lewis's ability to rationalize himself to truth never ceases to amaze me. Lewis, more than anyone it seems, has relied on reason to find God. In Mere Christianity especially, he lays out a concise argument for why God must exist. By laying out a series of proofs he seems to have proven the existence of God that is infinite. It is simply amazing to me how anyone could do this simply by putting together rational thoughts.