13 But where is humility from hyssop? Hear what follows: “To my hearing You shall give exultation and gladness, and bones humbled shall exult”. I will rejoice in hearing You, not in speaking against You. You have sinned, why defendest you yourself? You will speak: suffer thou; hear, yield to divine words, lest you be put to confusion, and be still more wounded: sin has been committed, be it not defended: to confession let it come, not to defence. Thou engagest yourself as defender of your sin, you are conquered: no innocent patron have you engaged, your defence is not profitable to you. For who are you that defendest yourself? You are meet to accuse yourself. Say not, either, “I have done nothing;” or, “What great thing have I done?” or, “Other men as well have done.” If in doing sin you say you have done nothing, you will be nothing, you will receive nothing: God is ready to give indulgence, you close the door against yourself: He is ready to give, do not oppose the bar of defence, but open the bosom of confession. “To my hearing You shall give exultation and gladness.”...
14. “Turn Thou away Your face from my sins, and all mine iniquities blot out”. For now bones humbled exult, now with hyssop cleansed, humble I have become. “Turn Thou away Your face,” not from me, but “from my sins.” For in another place praying he says, “Turn not away Your face from me.” He that would not that God's face be turned away from himself, would that God's face be turned away from his sins. For to sin, when God turns not Himself away, he adverts: if he adverts, he animadverts. “And all mine iniquities blot out.” He is busied with that capital sin: he reckons on more, he would have all his iniquities to be blotted out: he relies on the Physician's hand, on that “great mercy,” upon which he has called in the beginning of the Psalm: “All mine iniquities blot out.” God turns away His face, and so blots out; by “turning away” His face, sins He blots out. By “turning towards,” He writes them. You have heard of Him blotting out by turning away, hear of Him by turning towards, doing what? “But the countenance of the Lord is upon men doing evil things, that He may destroy from the earth the remembrance of them:” He shall destroy the remembrance of them, not by “blotting out their sins.” But here he does ask what? “Turn away Your face from my sins.” Well he asks. For he himself does not turn away his face from his own sins, saying, “For my sin I acknowledge.” With reason you ask and well ask, that God turn away from your sin, if you from thence dost not turn away your face: but if you set your sin at your back, God does there set His face. Do thou turn sin before your face, if you will that God thence turn away His face; and then safely you ask, and He hears.
15. “A clean heart create in me, O God”. “Create”— he meant to say, “as it were begin something new.” But, because repentant he was praying (that had committed some sin, which before he had committed, he was more innocent), after what manner he has said “create” he shows. “And a right spirit renew in my inner parts.” By my doing, he says, the uprightness of my spirit has been made old and bowed. For he says in another Psalm, “They have bowed my soul.” And when a man does make himself stoop unto earthly lusts, he is “bowed” in a manner, but when he is made erect for things above, upright is his heart made, in order that God may be good to him. For, “How good is the God of Israel to the upright of heart!” Moreover, brethren, listen. Sometimes God in this world chastises for his sin him that He pardons in the world to come. For even to David himself, to whom it had been already said by the Prophet, “Your sin is put away,” there happened certain things which God had threatened for that very sin. For his son Abessalom against him waged bloody war, and many ways humbled his father. He was walking in grief, in the tribulation of his humiliation, so resigned to God, that, ascribing to Him all that was just, he confessed that he was suffering nothing undeservedly, having now an heart upright, to which God was not displeasing. A slanderous person and one throwing in his teeth harsh curses he patiently heard, one of the soldiers on the opposite side, that were with his unnatural son. And when he was heaping curses upon the king, one of the companions of David, enraged, would have gone and smitten him; but he is kept back by David. And he is kept back how? For that he said, God sent him to curse me. Acknowledging his guilt he embraced his penance, seeking glory not his own, praising the Lord in that good which he had, praising the Lord in that which he was suffering, “blessing the Lord always, ever His praise was in his mouth.” Such are all the upright in heart: not those crooked persons who think themselves upright and God crooked: who when they do any evil thing, rejoice; when they suffer any evil thing, blaspheme; nay, if set in tribulation and scourging, they say from their distorted heart, “O God, what have I done to You?” Truly it is because they have done nothing to God, for they have done all to themselves. “And an upright spirit, renew in my inner parts.”
16. “Cast me not forth from Your face”. Turn away Your face from my sins: and “cast me not forth from Your face.” Whose face he fears, upon the face of the Same he calls. “And Your Holy Spirit take not away from me.” For in one confessing there is the Holy Spirit. Even now, to the gift of the Holy Spirit it belongs, that what you have done displeases you. The unclean spirit sins do please; the Holy One they displease. Though then thou still implore pardon, yet you are joined to God on the other part, because the evil thing that you have committed displeases you: for the same thing displeases both you and Him. Now, to assail your fever, you are two, thou and the Physician. For the reason that there cannot be confession of sin and punishment of sin in a man of himself: when one is angry with himself, and is displeasing to himself, then it is not without the gift of the Holy Spirit, nor does he say, Your Holy Spirit give to me, but, “Take not away from me.”
17. “Give back to me the exultation of Your salvation”. “Give back” what I had; what by sinning I had lost: to wit, of Your Christ. For who without Him can be made whole? Because even before that He was Son of Mary, “In the beginning He was the Word, and the Word was with God, and the Word was God;” and so, by the holy fathers a future dispensation of flesh taken upon Him, was looked for; as is believed by us to have been done. Times are changed, not faith. “And with Principal Spirit confirm me.” Some have here understood the Trinity in God, Itself God; the dispensation of Flesh being excepted therefrom: since it is written, “God is a Spirit.” For that which is not body, and yet is, seems to exist in such sort as that it is spirit. Therefore some understand here the Trinity spoken of: “In upright Spirit,” the Son; in “Holy Spirit,” Holy Ghost; in “Principal Spirit,” Father. It is not any heretical opinion, therefore, whether this be so, or whether “upright Spirit” He would have to be taken of man himself (when He says, “An upright spirit renew in my inner parts”), which I have bowed and distorted by sinning, so that in that case the Holy Spirit be Himself the Principal Spirit: which also he would not have to be taken away from him, and thereby would have himself to be confirmed therein.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)