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

p. 41

Christians are “in a different position from others who are trying to be good.  They hope, by being good, to please God if there is one; or—if they think there is not—at least they hope to win approval from good people.  But the Christian believes that any good he does comes from the Christ-life inside him.   He does not think that God will love us because we are good, but that God will make us good because he loves us; just as the roof of a greenhouse does not attract the sun because it is bright, but becomes bright because the sun shines on it.”