18 It does indeed make very much difference, for what cause, with what end, with what intention a thing be done: but those things which are clearly sins, are upon no plea of a good cause, with no seeming good end, no alleged good intention, to be done. Those works, namely of men, which are not in themselves sins, are now good, now evil, according as their causes are good or evil; as, to give food to a poor man is a good work, if it be done because of pity, with right faith; as to lie with a wife, when it is done for the sake of generation, if it be done with faith to beget subjects for regeneration.
These and the like works according to their causes are good or evil, because the self-same, if they have evil causes, are turned into sins: as, if for boasting sake a poor man is fed; or for lasciviousness a man lies with his wife; or children are begotten, not that they may be nurtured for God, but for the devil. When, however, the works in themselves are evil, such as thefts, fornications, blasphemies, or other such; who is there that will say, that upon good causes they may be done, so as either to be no sins, or, what is more absurd, just sins?
Who is there that would say, That we may have to give to the poor, let us commit thefts upon the rich: or, Let us sell false witness, especially if innocent men are not hurt thereby, but rather guilty men are rescued from the judges who would condemn them? For two good things are done by selling of this lie, that money may be taken wherewith a poor man may be fed, and a judge deceived that a man be not punished. Even in the matter of wills, if we can, why not suppress the true, and forge false wills that inheritances or legacies may not come to unworthy persons, who do no good with them; but rather to those by whom the hungry are fed, the naked clothed, strangers entertained, captives redeemed, Churches built?
For why should not those evil things be done for the sake of these good things, if, for the sake of these good things, those are not evil at all? Nay, further, if lewd and rich women are likely to enrich moreover their lovers and paramours, why should not even these parts and arts be undertaken by a man of merciful heart, to use them for so good a cause as that he may have whence to bestow upon the needy; and not hear the Apostle saying, “Let him that stole steal no more, but rather let him labor, working with his hands that which is good, that he may have to give to him that needs?” If indeed not only theft itself, but also false witness and adultery and every evil work will be not evil but good, if it be done for the sake of being the means of doing good.
Who can say these things, except one who endeavors to subvert human affairs and all manners and laws? For of what most heinous deed, what most foul crime, what most impious sacrilege, may it not be said that it is possible for it to be done rightly and justly; and not only with impunity, but even gloriously, that in perpetrating thereof not only no punishments should be feared, but there should be hope even of rewards: if once we shall concede in all evil works of men, that not what is done, but wherefore done, must be the question; and this, to the end that whatever are found to have been done for good causes, not even they should be judged to be evil?
But if justice deservedly punishes a thief, albeit he shall say and show that he therefore withdrew superfluities from a rich that he might afford necessaries to a poor man; if deservedly she punishes a forger, albeit he prove that he therefore corrupted another's will, that he might be heir, who should thence make large alms, not he who should make none; if deservedly she punishes an adulterer yea, though he shall demonstrate that of mercy he did commit adultery, that through her with whom he did it he might deliver a man from death; lastly, to draw nearer to the matter in question, if deservedly she punishment him who has with that intent mixed in adulterous embrace with some woman, privy to the turpitude of the Priscillianists, that he might enter into their concealments; I pray you, when the Apostle says, “Neither yield ye your members instruments of unrighteousness unto sin;” and therefore neither hands, nor members of generation, nor other members, can it be right to yield unto flagitious deeds with intent that we may be able to find out Priscillanists; what has our tongue, what our whole mouth, what the organ of the voice, offended us, that we should yield these as instruments to sin, and to so great a sin, in which, that we may apprehend and rescue Priscillianists from blaspheming in ignorance, we, without excuse of ignorance, are to blaspheme our God?
Source: To Consentius: Against Lying (New Advent)