In the poison of subjectivism, Lewis's overarching theme is objective morality. Lewis firmly believes in a moral law that is not subject to the time or place that it governs.
Lewis's main fear is that subjectivism is tainting thoughts of every individual and causing good intentions to have bad results.
Subjectivism is the idea that morals change throughout time and in different circumstances. Along with that idea comes the idea that we as humans can come up with new moral standards or evolve into beings with new moral standards. Even further, words like "better" and "good" get thrown around when talking about moral standards of the present. The question then becomes "better than what?" or "good compared to what?" Lewis's argument is that there must be some overarching standard that we hold our interpretations of morality in the present up against. What judges the validity or "betterness" of interpretations of morality cannot be the interpretations themselves.
Lewis has two main points. One is that we cannot invent a new moral to add to the natural law. Morality is stagnant; it was, is, and will be the same. The second point is that every attempt to create a new moral merely consists of taking one moral and exalting it above the rest. This has the consequence of tunnel vision and often times leads to racial superiority or an aristocratic ethic.