Now, about these rewards: I remember a conversation as an example of the problem of rewards: If we are serving for rewards, does not that make us mercenary? Isn’t it beneath a Christian to serve Jesus to receive a reward?
C.S. Lewis can help here. In “The Pursuit of Happiness: C. S. Lewis’s Eudaimonistic Understanding of Ethic”, David Horner writes of the matter raised by Lewis’ sermon “The Weight of Glory”:
Although Lewis’s subject in this sermon concerns Christian discipleship more generally, he begins with a point about ethics. With characteristic awareness, Lewis knows that the legitimacy of being motivated by the promise of Heaven’s rewards will at first appear to be morally out of bounds for the Christian. The view in most “modern minds” of Christian ethics, and of Christian discipleship more generally, is that doing the right thing is most essentially a matter of self-denial, sacrifice, and “disinterested” fulfillment of obligation. Any positive relation that morality has to our own happiness or well-being-any essential connection between “doing good” and “my good”-is ruled out. Put differently, the “pursuit of happiness,” for us, is not a specifically moral pursuit. At best it is nonmoral, a matter of prudential self-interest: something in which we should perhaps be legally free to engage, in view of the Declaration of Independence, but only as long as our pursuit stays within the bounds of moral obligation. All too often, the pursuit of happiness represents to us something actually immoral: “because I want to be happy” is probably the most common reason we hear-or give-for justifying morally wrong behavior. This way of thinking about ethics, especially Christian ethics, has attained an almost self-evident status among Christians and critics of Christianity (e.g. Friedrich Nietzsche and Ayn Rand) alike.
But Lewis disagrees, as does the weight of ancient and medieval thought, both pagan and Christian, up until the late Middle Ages. Classical thinkers viewed happiness as intrinsically connected to ethics; indeed, they considered happiness to be the starting point of all moral thought. Moral action, in their view, is grounded rationally and normatively in the pursuit of happiness. These thinkers were, in other words, “ethical eudaimonists”: they understood moral action to be grounded in the pursuit of eudaimonia (Greek: well-being or flourishing – traditionally translated as “happiness”).
This gets to a point: We are so perverted by our thinking that we can believe that happiness is somehow separate from pursuit of God; that the right must somehow be painful and medicine to be good must be bitter.
It is true that the call of God can pinch our flesh. Peter’s comments discloses the pinch, “We have left all.” But Jesus responds with: You have completely misunderstood: You have left a lesser to gain a greater: You have left the temporal to gain the eternal. What does Lewis write:
If you asked twenty good men today what they thought the highest of the virtues, nineteen of them would reply, Unselfishness. But if you had asked almost any of the great Christians of old, he would have replied, Love. You see what has happened? A negative term has been substituted for a positive, and this is of more than philological importance. The negative idea of Unselfishness carries with it the suggestion not primarily of securing good things for others, but of going without them ourselves, as if our abstinence and not their happiness was the important point. I do not thik this is the Christian virtue of Love. The New Testament has lots to say about self-denial, but not about self-denial as an end in itself. We are told to deny ourselves and to take up our crosses in order that we may follow Christ; and nearly every description of what we shall ultimately find if we do so contains an appeal to desire. If there lurks in most modern minds the notion that to desire our own good and earnestly to hope for the enjoyment of it is a bad thing, I submit that this notion has crept in from Kant and the Stoics and is no part of the Christian faith. Indeed, if we consider the unblushing promises of reward and the staggering nature of the rewards promised in the Gospels, it would seem that Our Lord finds our desires not too strong, but too weak. We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by an offer of a holiday at the sea. We are far too easily pleased.
Lewis goes onto explain that a reward which is naturally connected to the activity is no mean and base desire, but rather exults the action:
We must not be troubled by unbelievers when they say that this promise of reward makes the Christian life a mercenary affair. There are different kinds of reward. There is the reward which has no natural connexion with the 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. A general who fights well in order to get a peerage is mercenary; a general who fights for victory is not, victory being the proper reward of battle as marriage is the proper reward of love. The proper rewards are not simply tacked on to the activity for which they are given, but the activity is itself in consummation.
Jesus in offering reward does not offer some secondary matter, mere “filthy lucre”. Rather, the promised reward does not take one away from God – rather, as shown in Revelation 21, the ultimate offer is of God himself:
1 Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. 2 And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. 3 And I heard a loud voice from the throne saying, “Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God. 4 He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.” 5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.” 6 And he said to me, “It is done! I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give from the spring of the water of life without payment. 7 The one who conquers will have this heritage, and I will be his God and he will be my son” Revelation 21:1–7 (ESV).
 A copy of the text is available here: http://www.verber.com/mark/xian/weight-of-glory.pdf