On what is useful: not that which is advantageous, but that which is just and virtuous. It is to be found in losses, and is divided into what is useful for the body, and what is useful unto godliness.
22. In the first book we made our division in such a way as to set in the first place what is virtuous and what is seemly; for all duties are derived from these. In the second place we set what is useful. But as at the start we said that there was a difference between what is virtuous and what is seemly— which one can comprehend more easily than one can explain— so also when we are thinking of what is useful, we have to give considerable thought to what is the more useful.
23. But we do not reckon usefulness by the value of any gain in money, but in acquiring godliness, as the Apostle says: “But godliness is profitable unto all things, having promise of the life that now is, and of that which is to come.” Thus in the holy Scriptures, if we look carefully we shall often find that what is virtuous is called useful: “All things are lawful unto me, but all things are not profitable” [useful]. Before that he was speaking of vices, and so means: It is lawful to sin, but it is not seemly. Sins rest in one's own power, but they are not virtuous. To live wantonly is easy, but it is not right. For food serves not God but the belly.
24. Therefore, because what is useful is also just, it is just to serve Christ, Who redeemed us. They too are just who for His Name's sake have given themselves up to death, they are unjust who have avoided it. Of them it says: What profit is there in my blood? that is: what advance has my justice made? Wherefore they also say: “Let us bind the just, for he is useless to us,” that is: he is unjust, for he complains of us, condemns and rebukes us. This could also be referred to the greed of impious men, which closely resembles treachery; as we read in the case of the traitor Judas, who in his longing for gain and his desire for money put his head into the noose of treachery and fell.
25. We have then to speak of that usefulness which is full of what is virtuous, as the Apostle himself has laid it down in so many words, saying: “And this I speak for your own profit, not that I may cast a snare upon you, but for that which is comely.” It is plain, then, that what is virtuous is useful, and what is useful is virtuous; also that what is useful is just, and what is just is useful. I can say this, for I am speaking, not to merchants who are covetous from a desire to make gain, but to my children. And I am speaking of the duties which I wish to impress upon and impart to you, whom I have chosen for the service of the Lord; so that those things which have been already implanted and fixed in your minds and characters by habit and training may now be further unfolded to you by explanation and instruction.
26. Therefore as I am about to speak of what is useful, I will take up those words of the Prophet: “Incline my heart unto Your testimonies and not to covetousness,” that the sound of the word “useful” may not rouse in us the desire for money. Some indeed put it thus: “Incline my heart unto Your testimonies and not to what is useful,” that is, that kind of usefulness which is always on the watch for making gains in business, and has been bent and diverted by the habits of men to the pursuit of money. For as a rule most people call that only useful which is profitable, but we are speaking of that kind of usefulness which is sought in earthly loss “that we may gain Christ,” whose gain is “godliness with contentment.” Great, too, is the gain whereby we attain to godliness, which is rich with God, not indeed in fleeting wealth, but in eternal gifts, and in which rests no uncertain trial but grace constant and unending.
27. There is therefore a usefulness connected with the body, and also one that has to do with godliness, according to the Apostle's division: “Bodily exercise profits a little, but godliness is profitable unto all things.” And what is so virtuous as integrity? What so seemly as to preserve the body unspotted and undefiled, and its purity unsullied? What, again, is so seemly as that a widow should keep her plighted troth to her dead husband? What more useful than this whereby the heavenly kingdom is attained? For “there are some who have made themselves eunuchs for the kingdom of heaven's sake.”
Source: On the Duties of the Clergy (New Advent)