From this there follows a corollary about the Apologist’s private reading. There are two questions he will naturally ask himself. (1) Have I been ‘keeping up’, keeping abreast of recent movements in theology? (2) Have I stood firm (super monstratas vias)[Jeremiah 6:16] amidst all these ‘winds of doctrine’?[Ephesians 4:14] I want to say emphatically that the second question is far the more important of the two. Our upbringing and the whole atmosphere of the world we live in make it certain that our main temptation will be that of yielding to winds of doctrine, not that of ignoring them. We are not at all likely to be hide- bound: we are very likely indeed to be the slaves of fashion. If one has to choose between reading the new books and reading the old, one must choose the old: not because they are necessarily better but because they contain precisely those truths of which our own age is neglectful. The standard of permanent Christianity must be kept clear in our minds and it against that standard that we must test all contemporary thought. In fact, we must at all costs not move with the times. We serve One who said ‘Heaven and Earth shall move with the times, but my words shall not move with the times.’ [Matthew 24:35]
C.S. Lewis, "Christian Apologetics" (1945) included in God in the Dock (Eerdmans, 1970) 91-92.