From Lewis's The Poison of Subjectivism
If "good" means only the local ideology, how can those who invent the local ideology be guided by any idea of good themselves? The very idea of freedom presupposes some objective moral law which overarches rulers and ruled alike. Subjectivism about values is eternally incompatible with democracy. We and our rulers are of one kind only so long as we are subject to one law. But if there is no Law of Nature, the ethos of any society is the creation of its rulers, educators and conditioners; and every creator stands above and outside his creation.
You can read the entire essay on the link.