This is from Lewis's essay on the Humanitarian Theory of Punishment.
A great many popular blue prints for a Christian society are merely what the Elizabethans called ‘eggs in moonshine’ because they assume that the whole society is Christian or that the Christians are in control. This is not so in most contemporary States. Even if it were, our rulers would still be fallen men, and, therefore neither very wise nor very good. As it is, they will usually be unbelievers. And since wisdom and virtue are not the only or the commonest qualifications for a place in the government, they will not often be even the best unbelievers.