A new book is still on its trial and the amateur is not in a position to judge it. It has to be tested against the great body of Christian thought down the ages, and all its hidden implications (often unsuspected by the author himself) have to be brought to light. Often it cannot be fully understood without the knowledge of a good many other modern books. If you join at eleven o'clock a conversation which began at eight you will often not see the real bearing of what is said. Remarks which seem to you very ordinary will produce laughter or irritation and you will not see why—the reason, of course, being that the earlier stages of the conversation have given them a special point. In the same way sentences in a modern book which look quite ordinary may be directed "at" some other book; in this way you may be led to accept what you would have indignantly rejected if you knew its real significance. The only safety is to have a standard of plain, central Christianity ("mere Christianity" as Baxter called it) which puts the controversies of the moment in their proper perspective. Such a standard can be acquired only from the old books. It is a good rule, after reading a new book, never to allow yourself another new one till you have read an old one in between. If that is too much for you, you should at least read one old one to every three new ones.
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books. All contemporary writers share to some extent the contemporary outlook—even those, like myself, who seem most opposed to it. Nothing strikes me more when I read the controversies of past ages than the fact that both sides were usually assuming without question a good deal which we should now absolutely deny. They thought that they were a completely opposed as two sides could be, but in fact they were all the time secretly united—united with each other and against earlier and later ages—by a great mass of common assumptions. We may be sure that the characteristic blindness of the twentieth century—the blindness about which posterity will ask, "But how could they have thought that?"—lies where we have never suspected it, and concerns something about which there is untroubled agreement between Hitler and President Roosevelt or between Mr. H.G. Wells and Karl Barth. None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books. Where they are true they will give us truths which we half knew already. Where they are false they will aggravate the error with which we are already dangerously ill. The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books. Not, of course, that there is any magic about the past. People were no cleverer then than they are now; they made as many mistakes as we. But not the same mistakes. They will not flatter us in the errors we are already committing; and their own errors, being now open and palpable, will not endanger us. Two heads are better than one, not because either is infallible, but because they are unlikely to go wrong in the same direction. To be sure, the books of the future would be just as good a corrective as the books of the past, but unfortunately we cannot get at them.
From God in the Dock: Essays on Theology and Ethics by C.S. Lewis.