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... 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.
From the introduction to Athanasius's On the Incarnation, an old book worth reading.
(I nabbed the quote from here, so direct any grammatical errors to Goodreads, please.)