19 Come ye, hear, and I will tell, all you that fear God. Let us come, let us hear, what he is going to tell, “Come ye, hear, and I will tell.” But to whom, “Come ye, and hear”? “All you that fear God.” If God ye fear not, I will not tell. It is not possible that it be told to any where the fear of God is not. Let the fear of God open the ears, that there may be something to enter in, and a way whereby may enter in that which I am going to tell. But what is he going to tell? “How great things He has done to my soul.” Behold, he would tell: but what is he going to tell? Is it perchance how widely the earth is spread, how much the sky is extended, and how many are the stars, and what are the changes of sun and of moon? This creation fulfills its course: but they have very curiously sought it out, the Creator thereof have not known. This thing hear, this thing receive, “O you that fear God, how great things He has done to my soul:” if you will, to yours also. “How great things He has done to my soul.” “To Him with my mouth I have cried”. And this very thing, he says, has been done to his soul; that to Him with his mouth he should cry, has been done, he says, to his soul. Behold, brethren, Gentiles we were, even if not in ourselves, in our parents. And what says the Apostle? “You know, when Gentiles you were, to idols without speech how ye went up, being led.” Let the Church now say, “how great things He has done to my soul.” “To Him with my mouth I have cried.” I a man to a stone was crying, to a deaf stock I was crying, to idols deaf and dumb I was speaking: now the image of God has been turned to the Creator thereof. I that was “saying to a stock, My father you are; and to a stone, You have begotten me:” now say, “Our Father, which art in Heaven.”...“To Him with my mouth I have cried, and I have exalted Him under my tongue.” See how in secret He would be uncorrupt that offers marrowed holocausts. This do ye, brethren, this imitate, so that you may say, “Come ye, see how great things He has done to my soul.” For all those things of which he tells, by His Grace are done in our soul. See the other things of which he speaks.
20. “If I have beheld iniquity in my heart, may not the Lord hearken”. Consider now, brethren, how easily, how daily men blushing for fear of men do censure iniquities; He has done ill, He has done basely, a villain the fellow is: this perchance for man's sake he says. See whether you behold no iniquity in your heart, whether perchance that which you censure in another, you are meditating to do, and therefore against him dost exclaim, not because he has done it, but because he has been found out. Return to yourself, within be to yourself a judge. Behold in your hid chamber, in the very inmost recess of the heart, where thou and He that sees are alone, there let iniquity be displeasing to you, in order that you may be pleasing to God. Do not regard it, that is, do not love it, but rather despise it, that is, contemn it, and turn away from it. Whatever pleasing thing it has promised to allure you to sin; whatever grievous thing it has threatened, to drive you on to evil doing; all is nought, all passes away: it is worthy to be despised, in order that it may be trampled upon; not to be eyed lest it be accepted....
21. “Therefore God has hearkened to me”. Because I have not beheld iniquity in my heart. “And He has listened to the voice of my prayer.” “Blessed be my God, that has not thrust away my supplication and His mercy from me”. Gather the sense from that place, where he says, “Come ye, hear, and I will tell you, all you that fear God, how great things He has done to my soul:” he has both said the words which you have heard, and at the end thus he has concluded: “Blessed be my God, that has not thrust away my supplication and His mercy from me.” For thus there arrives at the Resurrection he that speaks, where already we also are by hope: yea both it is we ourselves, and this voice is ours. So long therefore as here we are, this let us ask of God, that He thrust not from us our supplication, and His mercy, that is, that we pray continually, and He continually pity. For many become feeble in praying, and in the newness of their own conversion pray fervently, afterwards feebly, afterwards coldly, afterwards negligently: as if they have become secure. The foe watches: you sleep. The Lord Himself has given commandment in the Gospel, how “it behooves men always to pray and not to faint.” And he gives a comparison from that unjust judge, who neither feared God, nor regarded man, whom that widow daily importuned to hear her; and he yielded for weariness, that was not influenced by pity: and the naughty judge says to himself, “Though neither God I fear, nor men I regard, even because of the weariness which this widow daily puts upon me, I will hear her cause, and will avenge her.” And the Lord says, “If a naughty judge has done this, shall not your Father avenge His chosen, that to Him do cry day and night? Yea, I say unto you, He shall make judgment of them speedily.” Therefore let us not faint in prayer. Though He puts off what He is going to grant, He puts it not away: being secure of His promise, let us not faint in praying, and this is by His goodness. Therefore he has said, “Blessed is my God, that has not thrust away my supplication and His mercy from me.” When you have seen your supplication “not thrust away from you,” be secure, that His mercy has not been thrust away from you.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)