There is a strange idea abroad that in every subject the ancient books should be read only by the professionals, and that the amateur should content himself with the modern books. Thus I have found as a tutor in English Literature that if the average student wants to find out something about Platonism, the very last thing he thinks of doing is to take a translation of Plato off the library shelf and read the Symposium. He would rather read some dreary modern book ten times as long, all about “isms” and influences and only once in twelve pages telling him what Plato actually said. The error is rather an amiable one, for it springs from humility. The student is half afraid to meet one of the great philosophers face to face. He feels himself inadequate and thinks he will not understand him. But if he only knew, the great man, just because of his greatness, is much more intelligible than his modern commentator. The simplest student will be able to understand, if not all, yet a very great deal of what Plato said; but hardly anyone can understand some modern books on Platonism. It has always therefore been one of my main endeavours as a teacher to persuade the young that firsthand knowledge is not only more worth acquiring than secondhand knowledge, but is usually much easier and more delightful to acquire.
This mistaken preference for the modern books and this shyness of the old ones is nowhere more rampant than in theology. Wherever you find a little study circle of Christian laity you can be almost certain that they are studying not St. Luke or St. Paul or St. Augustine or Thomas Aquinas or Hooker or Butler, but M. Berdyaev or M. Maritain or M. Niebuhr or Miss Sayers or even myself.
Now this seems to me topsy-turvy. Naturally, since I myself am a writer, I do not wish the ordinary reader to read no modern books. But if he must read only the new or only the old, I would advise him to read the old. And I would give him this advice precisely because he is an amateur and therefore much less protected than the expert against the dangers of an exclusive contemporary diet. 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.
C.S Lewis Introduction to Athanasius: On the Incarnation