Parable of the Master and Unprofitable Servant, Luke 17:5-10

In a surprising passage in the Institutes III.14.14-15, John Calvin affirms supererogatory works (or superfluous works)! Is Calvin affirming the Roman Catholic merit system for justification after all? No. Calvin quotes the parable of the unprofitable servant (Luke 17:10) to remind us that all we do, even at our best, is still our duty and what is expected of us, such that if we do more than what was necessary and sufficient, it is truly a superfluous work, that has great value, but in no way does it merit for us or others a greater reward. Calvin also leans on Paul's example in 1 Cor 9:15, where Paul may have invoked his apostolic authority to receive what was due to him, but set that right aside for a greater gain at a self-cost. 

14. How can boasting in works of supererogation agree with the command given to us: “When ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do?” (Luke 17:10). To say or speak in the presence of God is not to feign or lie, but to declare what we hold as certain. Our Lord, therefore, enjoins us sincerely to feel and consider with ourselves that we do not perform gratuitous duties, but pay him service which is due. And truly. For the obligations of service under which we lie are so numerous that we cannot discharge them though all our thoughts and members were devoted to the observance of the Law; and, therefore, when he says “When ye shall have done all those things which are commanded you,” it is just as if he had said that all the righteousness of men would not amount to one of these things. Seeing, then, that every one is very far distant from that goal, how can we presume to boast of having accumulated more than is due? It cannot be objected that a person, though failing in some measure in what is necessary, may yet in intention go beyond what is necessary. For it must ever be held that in whatever pertains to the worship of God, or to charity, nothing can ever be thought of that is not comprehended under the Law. But if it is part of the Law, let us not boast of voluntary liberality in matters of necessary obligation.

15. On this subject, they ceaselessly allege the boast of Paul, that among the Corinthians he spontaneously renounced a right which, if he had otherwise chosen, he might have exercised (1 Cor 9:15); thus not only paying what he owed them in duty, but gratuitously bestowing upon them more than duty required. They ought to have attended to the reason there expressed, that his object was to avoid giving offense to the weak. For wicked and deceitful workmen employed this pretense of kindness that they might procure favor to their pernicious dogmas, and excite hatred against the Gospel, so that it was necessary for Paul either to peril the doctrine of Christ, or to thwart their schemes. Now, if it is a matter of indifference to a Christian man whether or not he cause a scandal when it is in his power to avoid it, then I admit that the Apostle performed a work of supererogation to his Master; but if the thing which he did was justly required in a prudent minister of the Gospel, then I say he did what he was bound to do. In short, even when no such reason appears, yet the saying of Chrysostom is always true, that everything which we have is held on the same condition as the private property of slaves; it is always due to our Master. Christ does not disguise this in the parable, for he asks in regard to the master who, on return from his labor, requires his servant to gird himself and serve him, “Does he thank that servant because he did the things that were commanded him? I trow not,” (Luke 17:9). But possibly the servant was more industrious than the master would have ventured to exact. Be it so: still he did nothing to which his condition as a servant did not bind him, because his utmost ability is his master’s. I say nothing as to the kind of supererogations on which these men would plume themselves before God. They are frivolities which he never commanded, which he approves not, and will not accept when they come to give in their account. The only sense in which we admit works of supererogation is that expressed by the prophet, when he says, “Who has required this at your hand?” (Isaiah 1:12). But let them remember what is elsewhere said of them: “Wherefore do ye spend money for that which is not bread? and your labour for that which satisfieth not?” (Isaiah 55:2). It is, indeed, an easy matter for these indolent Rabbis to carry on such discussions sitting in their soft chairs under the shade, but when the Supreme Judge shall sit on his tribunal, all these blustering dogmas will behave to disappear. This, this I say, was the true question: not what we can fable and talk in schools and corners, but what ground of defense we can produce at his judgment-seat.

- John Calvin, Institutes of The Christian Religion, Book 3, Chapter 14, Section 14-15, [http://www.ccel.org/ccel/calvin/institutes.v.xv.html#v.xv-p43]