C. S. Lewis, in Liberating Ministry From The Success Syndrome, K Hughes, Tyndale, 1988, p. 158
We must not be troubled by unbelievers when they say that this promise of rewards makes the Christian's life a mercenary affair. There are different kinds of reward. There is the reward which has no natural connection with things you do to earn it, and is quite foreign to the desires that ought to accompany those things. Money is not the natural reward of love; that is why we call a man mercenary if he marries a woman for the sake of her money. But marriage is the proper reward for a real lover, and he is not mercenary for desiring it.