Here’s an interesting one, to Mrs. Edward A. Allen, February 1, 1958:

I quite agree with the Archbishop that no sin, simply as such, should be made a crime. Who the deuce are our rulers to enforce their opinions about sin on us? — a lot of professional politicians, often venal time-servers, whose opinion on a moral problem in one’s life we shd attach very little value to. Of course many acts which are sins against God are also injuries to our fellow citizens, and must on that account, but only on that account, be made crimes. But of all the sins in the world I shd have thought homosexuality was the one that least concerns the State. We hear too much of the State. Government is at its best a necessary evil. Let’s keep it in its place.

I don’t agree with Lewis that homosexuality is a sin, but his view here is pretty progressive all things considered. Elsewhere he talks about defending gay people from the interference of “snoopers and busybodies” (letter to Delmar Banner, May 27, 1960). It’s clear that one of the things Lewis loathed the most was the moral busybody; he wrote in God in the Dock that

of all tyrannies, a tyranny exercised for the good of its victims may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron’s cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience.

I think Lewis was in many ways an old-fashioned liberal who wanted the State to mind its own business and leave him to mind his. Here he’s enunciating a version of J.S. Mill’s harm principle: that the State is only justified in using force against someone to prevent harm to others.

This is a kind of libertarianism, or at least classical liberalism, but one based more on man’s fallen nature than on his intrinsic goodness like you get with the techno-utopian brand of libertarianism. Liberty is important not because people can be trusted to always do the right thing, but because it creates a sphere that protect us from other people’s moral certainties.