Mere Christianity, in The Complete C. S. Lewis Signature Classics

pp. 48-49

“God is no fonder of intellectual slackers than of any other slackers.  If you are thinking of becoming a Christian, I warn you, you are embarking on something that is going to take the whole of you, brains and all. But, fortunately, it works the other way around.  Anyone who is honestly trying to be a Christian will soon find his intelligence sharpened: one of the reasons why it needs no special education to be a Christian is that Christianity is an education itself.  That is why an uneducated believer like John Bunyan was able to write a book that has astonished the whole world [The Pilgrim’s Promise].