Back when I reviewed Edward Feser’s Scholastic Metaphysics, I noted that most the disagreements between Catholics and Protestants during the Reformation and in the scholastic period afterward were focused on matters of “grace”, largely connected with what God has done in history. This would be distinct from how the post-Barthian/Van-Tillian Reformed world sees the matter. Yet, there was at least one place in which Protestants and Roman Catholics disagreed on something that would properly speaking be a matter of nature. This is the question of supererogatory works in ethics.
Roman Catholics both in the past and still today argue that people can be in a situation where they would be morally permitted to do something not intrinsically evil, but also not the most good they could do. If the agent chooses to do the most good, they go beyond duty, and can accrue merit. This could potentially be given to others by means of the Treasury of Merit. Correlated with this position was a certain approach to the ethical teaching of Jesus. That is, the Sermon on the Mount is read along the lines of the commands/counsels distinction, wherein things like non-resistance are taken not as a duty for all Christians (let alone all people), but rather as a supererogatory way, which is largely the province of the religious or the clergy in this case.
Protestants, on the other hand, have taken a different perspective on both nature and the meaning of scripture. Richard Hooker, for example, states that the first principle of ethics in way that in fact denies the possibility of supererogation:
In every kind of knowledge some such grounds there are, as that being proposed the mind doth presently embrace them as free from all possibility of error, clear and manifest without proof. In which kind axioms or principles more general are such as this, “that the greater good is to be chosen before the less.” If therefore it should be demanded what reason there is, why the Will of Man, which doth necessarily shun harm and covet whatsoever is pleasant and sweet, should be commanded to count the pleasures of sin gall, and notwithstanding the bitter accidents wherewith virtuous actions are compassed, yet still to rejoice and delight in them: surely this could never stand with Reason, but that wisdom thus prescribing groundeth her laws upon an infallible rule of comparison; which is, “That small difficulties, when exceeding great good is sure to ensue, and on the other side momentany benefits, when the hurt which they draw after them is unspeakable, are not at all to be respected.” This rule is the ground whereupon the wisdom of the Apostle buildeth a law, enjoining patience unto himself; “The present lightness of our affliction worketh unto us even with abundance upon abundance an eternal weight of glory; while we look not on the things which are seen, but on the things which are not seen: for the things which are seen are temporal, but the things which are not seen are eternal:” therefore Christianity to be embraced, whatsoever calamities in those times it was accompanied withal. Upon the same ground our Saviour proveth the law most reasonable, that doth forbid those crimes which men for gain’s sake fall into. “For a man to win the world if it be with the loss of his soul, what benefit or good is it?”
Similarly, an exegete like John Calvin expresses the Protestant perspective on Jesus’ ethics:
There appear to have been chiefly two reasons, which induced him to declare this agreement between the law and the Gospel. As soon as any new method of teaching makes its appearance, the body of the people immediately look upon it, as if everything were to be overturned. Now the preaching of the Gospel, as I mentioned a little ago, tended to raise the expectation, that the Church would assume a totally different form from what had previously belonged to it. They thought that the ancient and accustomed government was to be abolished. This opinion, in many respects, was very dangerous. Devout worshippers of God would never have embraced the Gospel, if it had been a revolt from the law; while light and turbulent spirits would eagerly have seized on an occasion offered to them for entirely overthrowing the state of religion: for we know in what insolent freaks rash people are ready to indulge when there is any thing new.
Besides, Christ saw that the greater part of the Jews, though they professed to believe the Law, were profane and degenerate. The condition of the people was so decayed, every thing was filled with so many corruptions, and the negligence or malice of the priests had so completely extinguished the pure light of doctrine, that there no longer remained any reverence for the Law. But if a new kind of doctrine had been introduced, which would destroy the authority of the Law and the Prophets, religion would have sustained a dreadful injury. This appears to be the first reason, why Christ declared that he had not come to destroy the Law. Indeed, the context makes this abundantly clear: for he immediately adds, by way of confirmation, that it is impossible for even one point of the Law to fail, — and pronounces a curse on those teachers who do not faithfully labor to maintain its authority.
The second reason was, to refute the wicked slander which, he knew was brought against him by the ignorant and unlearned. This charge, it is evident, had been fastened on his doctrine by the scribes: for he proceeds immediately to direct his discourse against them. We must keep in mind the object which Christ had in view. While he invites and exhorts the Jews to receive the Gospel, he still retains them in obedience to the Law; and, on the other hand, he boldly refutes the base reproaches and slanders, by which his enemies labored to make his preaching infamous or suspected.
Calvin’s reading of Jesus’ preface to his sermon has him declaring continuity between his teaching and the Law. The Law, of course, declares the duties of the people of God, and Jesus still “retains them in obedience” to it. This point from Calvin and the previous one from Hooker are very closely related, insofar as both Reformers would have considered the moral law of scripture (the part that Calvin teaches is retained) and the natural law to be identical in content.
For Protestants, there is no gap between duty and perfection that would allow for merit, or for a lesser good option. They took seriously the words of the Lord to his disciples (Luke 17:10): “So you also, when you have done all that you were commanded, say, ‘We are unworthy servants; we have only done what was our duty.’”
The Reformed and Lutheran approach to ethics have implications still relevant to political philosophy today. It entails, among other things, that what some pundits and thinkers on the Christian Right think of as the purview of voluntary charity is actually a matter of justice.
Consider the following: What God demands in his law is what is good for people to do, and there is no “lesser good” option. But among the many requirements of the law, preservation of the poor from death by destitution is certainly not the least significant. Now, if the good required by God is for one neighbour to take care for the well-being of another, then a failure to do so, a sin of omission, is actually wrong. And moreover, it is a wrong to the neighbour as well as to God, insofar as the good that God obligates us to do is a good that we are obligated to do for our neighbour. In effect, then, to do wrong in this way is to wrong our neighbour.
Now, I will take it for granted in this post that punishment is a justifiable act. Given that assumption, the essence of punishment is to effectually communicate the wrong done to the wrongdoer, and this is done by returning a natural evil upon the offender who has committed a moral evil. To put it more bluntly, punishments hurt, by definition. As Oliver O’Donovan puts it in The Ways of Judgment: “punishment is bested understood as a judgment enacted on the person, property, or liberty of the condemned party.”  He elaborates on what it means for punishment to be a kind of judgment:
Punishment is judgment, in saying which we presuppose all that has been said about judgment up to this point: it is an act of moral discrimination, that pronounces upon a preceding act or existing state of affairs to establish a new public context. A rational act of condemnation, it is neither irrational, like impulsive revenge, nor inactive, like reflective disapproval, but an “expressive act” or “communication.” 
A little later he clarifies in what sense punishment “gives back” (re-tributes) to the offender:
The core of the retributive idea is the thought that in punishment something which the offender has put forth comes back. This thought is true, as far as it goes. But it comes back as a representation. Judgment brings an old act back by a new act, an act that corresponds to the old an so expresses it truthfully. What the offender gets back in being punished is different from what was put forth, not an echo but an answer. More precisely, it is a “judgment.” 
A punishment communicates the wrongness of the wrong that the offender has committed by returning a proportionate natural evil upon him.
Bringing all that we have seen together, the Protestant perspective on ethics entails a vision of social justice (if we want to use that term) very different from that of the common Christian Right take today. If God’s moral law requires care for the poor, and if failure to keep the law is wrong, and if that wrong is done to the poor, and if the concept of punishment is justified as a communicative act that adequately declares the evil of the offense by returning a kind of evil on the offender, then poor people neglected by those with means could have some justification for demanding government coercion in their favour.
When writers on the Right object that the redistribution of wealth for the benefit of the poor amounts to theft, they would have to assume one of two things. Either that the wealthy of this world have no obligation to help the poor, or else that the obligation is something more like a counsel of perfection than a demand of justice. Neither of these options, it seems to me, is open to those who follow the ethics of the Reformation. It is probably not coincidental that the states which promoted Reformed religion also in practice rejected extreme libertarian approaches to poverty relief. Justice was for them, as it should be for their theological heirs, not simply about exchange, but also about final distribution.