It is proved by the witness of Scripture that all duty is either “ordinary” or “perfect.” To which is added a word in praise of mercy, and an exhortation to practise it.
36. Every duty is either “ordinary” or “perfect,” a fact which we can also confirm by the authority of the Scriptures. For we read in the Gospel that the Lord said: “If you will enter into life, keep the commandments. He says: Which? Jesus said to him: You shall do no murder, You shall not commit adultery, You shall not steal, You shall not bear false witness, Honour your father and your mother, You shall love your neighbour as yourself.” These are ordinary duties, to which something is wanting.
37. Upon this the young man says to Him: “All these things have I kept from my youth up, what lack I yet? Jesus said unto him: If you will be perfect, go and sell all your goods and give to the poor, and you shall have treasure in heaven; and come and follow Me.” And earlier the same is written, where the Lord says that we must love our enemies, and pray for those that falsely accuse and persecute us, and bless those that curse us. This we are bound to do, if we would be perfect as our Father Who is in heaven; Who bids the sun to shed his rays over the evil and the good, and makes the lands of the whole universe fertile with rain and dew without any distinction. This, then, is a perfect duty (the Greeks call it κατόρθωμα), whereby all things are put right which could have any failings in them.
38. Mercy, also, is a good thing, for it makes men perfect, in that it imitates the perfect Father. Nothing graces the Christian soul so much as mercy; mercy as shown chiefly towards the poor, that you may treat them as sharers in common with you in the produce of nature, which brings forth the fruits of the earth for use to all. Thus you may freely give to a poor man what you have, and in this way help him who is your brother and companion. Thou bestowest silver; he receives life. You give money; he considers it his fortune. Your coin makes up all his property.
39. Further, he bestows more on you than thou on him, since he is your debtor in regard to your salvation. If you clothe the naked, you clothe yourself with righteousness; if you bring the stranger under your roof, if you support the needy, he procures for you the friendship of the saints and eternal habitations. That is no small recompense. You sow earthly things and receive heavenly. Do you wonder at the judgment of God in the case of holy Job? Wonder rather at his virtue, in that he could say: “I was an eye to the blind, and a foot to the lame. I was a father to the poor. Their shoulders were made warm with the skins of my lambs. The stranger dwelt not at my gates, but my door was open to every one that came.” Clearly blessed is he from whose house a poor man has never gone with empty hand. Nor again is any one more blessed than he who is sensible of the needs of the poor, and the hardships of the weak and helpless. In the day of judgment he will receive salvation from the Lord, Whom he will have as his debtor for the mercy he has shown.
Source: On the Duties of the Clergy (New Advent)