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

p. 51

“Christ did not come to preach any brand new morality.  The Golden Rule of the New Testament (Do as you would be done by) is a summing up of what everyone, at bottom, had always known to be right.  Really great moral teachers never do introduce new moralities: it is quacks and cranks who do that.  As Dr. Johnson said, ‘People need to be reminded more often than they need to be instructed.’  The real job every moral teacher is to keep on bringing us back, time after time, to the old simple principles which we are all so anxious not to see; like bringing a horse back and back to the fence it has refused to jump.”