A new book is still on trial, and the amateur is not in a position to judge it.... The only safety is to have a standard of plain, central Christianity ("mere Christianity" as [Richard] Baxter [the 17th Century pastor] called it), which puts the controversies of the moment in their proper perspective. Such a standard can only be acquired from 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.
-C.S. Lewis, God is in the Dock, ed. Walter Hooper (Grand Rapids, MI: Eerdmans, 1970), pp. 201-20s, as quoted in Foster and Smith, Devotional Classics: Selected Readings for Individuals & Groups (San Francisco, CA: Harper Collins, 1993) p. 2.
As to "mere Christianity" and C.S. Lewis, go here.