Update:

Here are 2 more things that Lewis wrote on the subject of the gap between modern and ancient societies. These are from letters that he wrote. The first is before making the BBC radio broadcasts that were later collected together into the book “Mere Christianity”.

1. I mainly want to talk about .. the Law of Nature, or objective right and wrong. It seems to me that the New Testament, by preaching repentance and forgiveness, always assumes an audience who already believes in the Law of Nature and know they have disobeyed it. In modern England we cannot at present assume this….(10 Feb. 1941, letter to Dr. James Welch, Director of Religious Broadcasting for the BBC).
2. The greatest barrier I have met is the almost total absence from the mind of my audience of any sense of sin… The early Christian preachers could assume in their hearers whether Jews, Metuentes or Pagans, a sense of guilt… Thus the Christain message was in those days unmistakably the … Good News. It promised healing to those who knew they were sick. We have to convince our hearers of the unwelcome diagnosis before we can expect them to welcome the news of the remedy. The ancient man approached God (or even the gods) as the accused person approaches his judge. For the modern man the roles are reversed. He is the judge: God is in the dock.”  (“God in the Dock”, God in the Dock, 1979, Fount 1998., p. 93) [ 『被告席の神』God in the Dock (1970年)　ウォルター・フーパー編　エッセー集。偉大なる奇跡 ―C.S.ルイス宗教著作集別巻1』（第1部） 本多峰子 訳　新教出版社　1998年8月.  『偉大なる奇跡『被告席に立つ神―C.S.ルイス宗教著作集別巻2』（第2部,第3部）本多峰子 訳　新教出版社　1998年11月

Here are some quotes from C.S. Lewis himself about the gap between modern man and man of antiquity (the past). The first 5 are from “De Descriptione Temporum”, Lewis’ first lecture at Cambridge University, 1954.

1. Christians and Pagans had much more in common with each other than either has with a post-Christian. The gap between those who worship different gods is not so wide as that between those who worship and those who do not…
2. I have come to regard as the greatest of all divisions in the history of the West that which divides the present from, say, the age of Jane Austen and Scott…
3. somewhere between us and the Waverley Novels, somewhere between us and “Persuasion”, the chasm runs.
4. I do not think that any previous age produced work which was, in its own time, as shatteringly and bewilderingly new as that of the Cubists, the Dadaists, the Surrealists, and Picasso has been in ours.
5. In [Jane Austen’s] days some kind and degree of religious belief and practice were the norm: now… they are the exception.
6. Between Jane Austen and us, but not between her and Shakespeare, Chaucer, Alfred, Virgil, Homer, or the Pharaohs, comes the birth of the machines… It alters Man’s place in nature.

The second set of quotes comes from Book 1 of “Mere Christianity”.

1. Now this Law or Rule about Right and Wrong used to be called the Law of Nature. Nowadays, when we talk of the “laws of nature” we usually mean things like gravitation, or heredity, or the laws of chemistry. But when the older thinkers called the Law of Right and Wrong “the Law of Nature,” they really meant the Law of Human Nature. The idea was that, just as all bodies are governed by the law of gravitation and organisms by biological laws, so the creature called man also had his law
2. This law was called the Law of Nature because people thought that every one knew it by nature and did not need to be taught it…taking the race as a whole, they thought that the human idea of decent behaviour was obvious to every one.
3. some people say …different civilisations and different ages have had quite different moralities. But this is not true. There have been differences between their moralities, but these have never amounted to anything like a total difference. If anyone will take the trouble to compare the moral teaching of, say, the ancient Egyptians, Babylonians, Hindus, Chinese, Greeks and Romans, what will really strike him will be how very like they are to each other and to our own.
4. Men have differed as regards what people you ought to be unselfish to – whether it was only your own family, or your fellow countrymen, or everyone. But they have always agreed that you ought not to put yourself first. Selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they have always agreed that you must not simply have any woman you liked.
5. the idea of a sort of behaviour [human beings] ought to practise, what you might call fair play, or decency, or morality, or the Law of Nature.
6. a Something which is directing the universe, and which appears in me as a law urging me to do right and making me feel responsible and uncomfortable when I do wrong.

And finally, from chapter 14 of “Surprised by Joy”:

Had something really dropped out of our lives? Was the archaic simply the civilized, and the modern simply the barbaric?