18 Therefore, my Brethren, those that in the very congregation of these walls ye see to be rebellious men, proud, seeking their own, lifted up; not having a zeal for God that is chaste, sound, quiet, but ascribing to themselves much; ready for dissension, but not finding opportunity; are the very chaff of the Lord's floor. From hence these few men the wind of pride has dislodged: the whole floor will not fly, save when He at the last shall winnow. But what shall we do, save with this man sing, with this man pray, with this man mourn and say securely, “He shall redeem in peace my soul”. Against them that love not peace: “in peace He shall redeem my soul.” “Because with those that hated peace I was peace-making.” “He shall redeem in peace my soul, from those that draw near to me.” For from those that are afar from me, it is an easy case: not so soon does he deceive me that says, Come, pray to an idol: he is very far from me. Are you a Christian? A Christian, he says. Out of a neighbouring place he is my adversary, he is at hand. “He shall redeem in peace my soul, from those that draw near to me: for in many things they were with me.” Wherefore have I said, “draw near to me”? Because “in many things they were with me.” In this verse two propositions occur. “In many things they were with me.” Baptism we had both of us, in that they were with me: the Gospel we both read, they were in that with me: the festivals of martyrs we celebrated, they were there with me: Easter's solemnity we attended, they were there with me. But not entirely with me: in schism not with me, in heresy not with me. In many things with me, in few things not with me. But in these few things wherein not with me, there is no profit to them of the many things wherein they were with me. For see, brethren, how many things has recounted the Apostle Paul: one thing, he has said, if it shall have been wanting, in vain are those things. “If with the tongues of men and of angels I shall speak,” he says, “if I have all prophecy, and all faith, and all knowledge; if mountains I shall remove, if I shall bestow all my goods upon the poor, if I shall deliver my body even so that it be burned.” How many things he has enumerated! To all these many things let there be wanting one thing, charity; the former in number are more, the latter in weight is greater. Therefore in all Sacraments they are with me, in one charity not with me: “In many things they were with me.” Again, by a different expression: “For in many things they were with me.” They that themselves have separated from me, with me they were, not in few things, but in many things. For throughout the whole world few are the grains, many are the chaffs. Therefore he says what? In chaff with me they were, in wheat with me they were not. And the chaff is nearly related to the wheat, from one seed it goes forth, in one field is rooted, with one rain is nourished, the same reaper it suffers, the same threshing sustains, the same winnowing awaits, but not into one barn enters.
19. “God will hear me, and He shall humble them That is before ages”. For they rely on some leader or other of theirs that has begun but yesterday. “He shall humble them That is before ages.” For even if with reference to time Christ is of Mary the Virgin, nevertheless before ages: “In the beginning He is the Word and the Word with God, and the Word God.” “He shall humble them That is before ages. For to them is no changing:” of them I “speak to whom is no changing.” He knew of some to persevere, and in the perseverance of their own wickedness to die. For we see them, and to them is no changing: they that die in that same perverseness, in that same schism, to them is no changing. God shall humble them, shall humble them in damnation, because they are exalted in dissension. To them is no changing, because they are not changed for the better, but for the worse: neither while they are here, nor in the resurrection. For all we shall rise again, but not all shall be changed. Wherefore? Because “'To them is no changing: and they have not feared God.”...
20. “He stretches forth His hand in requiting”. “They have polluted His Testament.” Read the testament which they have polluted: “In your seed shall be blessed all nations.” Thou against these words of the Testator sayest what? The Africa of holy Donatus has alone deserved this grace, in him has remained the Church of Christ. Say at least the Church of Donatus. Wherefore do you add, of Christ? Of whom it is said, “In your seed shall be blessed all nations.” After Donatus will you go? Set aside Christ, and then secede. See therefore what follows: “They have polluted His Testament.” What Testament? To Abraham have been spoken the promises, and to his seed. The Apostle says, “Nevertheless, a man's testament confirmed no one makes void, or super-adds to: to Abraham have been spoken the promises, and to his seed. He says not, And to seeds, as if in many; but as if in one, And to your Seed, which is Christ.” In this Christ, therefore, what Testament has been promised? “In your seed shall be blessed all nations.” Thou that hast given up the unity of all nations, and in a part hast remained, hast polluted His Testament....
21. “And His heart has drawn near”. Of whom do we understand it, except of Him, by the anger of whom they have been divided? How “has his heart drawn near”? In such sort, that we may understand His will. For by heretics has been vindicated the Catholic Church, and by those that think evil have been proved those that think well. For many things lay hid in the Scriptures: and when heretics had been cut off, with questions they troubled the Church of God: then those things were opened which lay hid, and the will of God was understood. Thence is said in another Psalm, “In order that they might be excluded that have been proved with silver.” For let them be excluded, He has said, let them come forth, let them appear. Whence even in silver-working men are called “excluders,” that is, pressers out of form from the sort of confusion of the lump. Therefore many men that could understand and expound the Scriptures very excellently, were hidden among the people of God: but they did not declare the solution of difficult questions, when no reviler again urged them. For was the Trinity perfectly treated of before the Arians snarled thereat? Was repentance perfectly treated of before the Novatians opposed? So not perfectly of Baptism was it treated, before rebaptizers removed outside contradicted; nor of the very oneness of Christ were the doctrines clearly stated which have been stated, save after that this separation began to press upon the weak: in order that they that knew how to treat of and solve these questions (lest the weak should perish vexed with the questions of the ungodly), by their discourses and disputations should bring out unto open day the dark things of the Law....This obscure sense see in what manner the Apostle brings out into light; “It is needful,” he says, “that also heresies there be, in order that men proved may be made manifest among you.” What is “men proved”? Proved with silver, proved with the word. What is “may be made manifest”? May be brought out. Wherefore this? Because of heretics. So therefore these also “have been divided because of the anger of His countenance, and His heart has drawn near.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)