Once again, from “The Weight of Glory”:
“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 rewards. There is the reward which has no natural connection 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. … The proper rewards are not simply tacked on to the activity for which they are given, but are the activity itself in consummation.
There is also a third case, which is more complicated. An enjoyment of Greek poetry is certainly a proper, and not a mercenary, reward for learning Greek; but only those who have reached the stage of enjoying Greek poetry can tell from their own experience that this is so. The schoolboy beginning Greek grammar cannot look forward to his adult enjoyment of Sophocles as a lover looks forward to marriage or a general to victory. He has to begin by working for marks, or to escape punishment, or to please his parents, or, at best, in the hope of a future good which he cannot at present imagine or desire. His position, therefore, bears a certain resemblance to that of the mercenary; the reward he is going to get will, in actual fact, be a natural or proper reward, but he will not know that till he has got it. Of course, he gets it gradually; enjoyment creeps in upon the mere drudgery, and nobody could point to a day or an hour when the one ceased and the other began. But it is just insofar as he approaches the reward that he becomes able to desire it for its own sake; indeed, the power of so desiring it is itself a preliminary reward.
The Christian, in relation to heaven, is in much the same position as this schoolboy. Those who have attained everlasting life in the vision of God doubtless know very well that it is no mere bribe, but the very consummation of their early discipleship; but we who have not yet attained it cannot know this in the same way, and cannot even begin to know it all except by continuing to obey and finding the first reward of our obedience in our increasing power to desire the ultimate reward. Just in proportion as the desire grows, our fear lest it should be a mercenary desire will die away and finally be recognized as an absurdity. But probably this will not, for most of us, happen in a day; poetry replaces grammar, gospel replaces law, longing transforms obedience, as gradually as the tide lifts a grounded ship.
But there is one other important similarity between the schoolboy and ourselves. If he is an imaginative boy, he will, quite probably, be reveling in the English poets and romancers suitable to his age some time before he begins to suspect that Greek grammar is going to lead him to more and more enjoyments of this same sort. He may even be neglecting his Greek to read Shelley and Swinburne in secret. In other words, the desire which Greek is really going to gratify already exists in him and is attached to objects which seem to him quite unconnected with Xenophon and verbs in [Greek]. Now, if we are made for heaven, the desire for our proper place will be already in us, but not yet attached to the true object, and will even appear as the rival of that object. And this, I think, is just what we find. No doubt there is one point in which my analogy of the schoolboy breaks down. The English poetry which he reads when he ought to be doing Greek exercises may be just as good as the Greek poetry to which the exercises are leading him, so that in fixing on Milton instead of journeying on to Aeschylus his desire is not embracing a false object. But our case is very different. If a transtemporal, transfinite good is our real destiny, then any other good on which our desire fixes must be in some degree fallacious, must bear at best only a symbolical relation to what will truly satisfy.” (pp. 25-28 in 1996 Touchstone edition)
You might want to go read this whole essay. You won’t regret it.