In a letter to a pastor, John Wesley, cautions him about his lack of reading: (link)

What has exceedingly hurt you in time past, nay, and I fear to this day, is want of reading. I scarce ever knew a preacher read so little. And perhaps, by neglecting it, you have lost the taste for it. [Your preaching] is lively, but not deep; there is little variety; there is no compass of thought. Reading only can supply this …

In the introduction to Athanasius’ On the Incarnation (amazon, online), C.S. Lewis talks about the importance of what we are reading (introduction). Particularly, he warns us against reading that is comprised of an “exclusive contemporary diet”, instead encouraging us to read books from the past (emphasis added).

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. … this mistaken preference for the modern books and this shyness of the old ones is nowhere more rampant than in theology.

While the reader may be “half afraid to meet one of the great philosophers face to face”, Lewis reminds us, reading the original is not nearly as difficult as we might have thought.

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.

The same can be said of the books of Scripture as well as those early sources such as Justin Martyr, Irenaeus, Origen, Tertullian among others.

Lewis goes on to point out what can be summarized as four dangers for those who are prone to reading only (or primarily) modern works of theology.

The original sources are often minimally represented and made more confusing.

Humorously, Lewis notes that the modern book is often “ten times as long … and only once in twelve pages” does it ever quote the ancient sources.

one of my main endeavors as a teacher is 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.

Modern books and controversies must be tested against orthodox Christian thought.

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 of its hidden implications … have to be brought to light.

Every age has blind spots and being well read helps us understand what they are.

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. Nothing strikes me more when I read the controversies of past ages that the fact that both sides were … united … by a great mass of common assumptions.

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.

It would be an interesting exercise to wrestle with what the common and faulty assumptions of our age might be.

We miss the unity that ties Christianity together through the ages.

We are all rightly distressed, and ashamed also, at the divisions of Christendom. But those who have always lived within the Christian fold may be too easily dispirited by them. They are bad, but such people do not know what it looks like from without. Seen from there, what is left intact despite all the divisions, still appears (as it truly is) an immensely formidable unity.  … That unity any of us can find by going out of his own age.

The rest of the essay explores Athanasius and his work. But in doing so Lewis wrote something that struck me. Perhaps because I am a reader of more “doctrinal” than “devotional” works. Here is what he wrote:

I believe that many who find that “nothing happens” when they sit down, or kneel down, to a book of devotion, would find that the heart sings unbidden while they are working their way through a tough bit of theology with a pipe in their teeth and a pencil in their hand.