18 But see what he annexes: “With Principal Spirit,” he says, “confirm Thou me.” Wherein “confirm”? Because You have pardoned me, because I am secure, that what You have forgiven is not to be ascribed, on this being made secure and with this grace confirmed, therefore I am not ungrateful. But I shall do what? “I would teach unrighteous men Your ways”. Being myself of the unrighteous (that is, one that was myself an unrighteous man, now no longer unrighteous; the Holy Spirit not having been taken away from me, and I being confirmed with Principal Spirit). “I would teach unrighteous men Your ways.” What ways will you teach unrighteous men? “And ungodly men to You shall be converted.” If David's sin is counted for ungodliness, let not ungodly men despair of themselves, forasmuch as God has spared an ungodly man; but let them take heed that to Him they be converted, that His ways they learn. But if David's deed is not counted for ungodliness, but this is properly call ungodliness, namely, to apostatize from God, not to worship one God, or never to have worshipped, or to have forsaken, Him whom one did worship, then what he says has the force of superabundance, “And ungodly men shall to You be converted.” So full are you of the fatness of mercy, that for those converted to You, not only sinners of any sort, but even ungodly, there is no cause for despair. Wherefore? That believing on Him that justifies an ungodly man, their faith may be counted for righteousness.
19. “Deliver me from bloods, O God, God of my health”. The Latin translator has expressed, though by a word not Latin, yet an accuracy from the Greek. For we all know that in Latin, sanguines (bloods) are not spoken of, nor yet sanguina (bloods in the neuter), nevertheless because the Greek translator has thus used the plural number, not without reason, but because he found this in the original language the Hebrew, a godly translator has preferred to use a word not Latin, rather than one not exact. Wherefore then has he said in the plural number, “From bloods”? In many bloods, as in the origin of the sinful flesh, many sins he would have to be understood. The Apostle having regard to the very sins which come of the corruption of flesh and blood, says, “Flesh and blood shall not possess the kingdom of God.” For doubtless, after the true faith of the same Apostle, that flesh shall rise again and shall itself gain incorruption, as He says Himself, “This corruptible must put on incorruption, and this mortal put on immortality.” Because then this corruption is of sin, by the name thereof sins are called. In like manner as both that morsel of flesh and member which plays in the mouth when we articulate words is called a tongue, and that is called a tongue which by the tongue is made, so we call one tongue the Greek, another the Latin; for the flesh is not diverse, but the sound. In the same manner, then, as the speech which is made by the tongue is called a tongue; so also the iniquity which is made by blood is called blood. Heeding, then, his many iniquities, as in the expression above, “And all my iniquities blot out,” and ascribing them to the corruption of flesh and blood, “Free me,” he says, “from bloods:” that is, free me from iniquities, cleanse me from all corruption....Not yet is the substance, but certain hope. “And my tongue shall exult of Your righteousness.”
20. “O Lord, my lips You shall open, and my mouth shall tell of Your praise”. “Your praise,” because I have been created: “Your praise,” because sinning I have not been forsaken: “Your praise,” because I have been admonished to confess: “Your praise,” because in order that I might be secured I have been cleansed.
21. “Because if You had willed sacrifice, I would have given it surely”. David was living at that time when sacrifices of victim animals were offered to God, and he saw these times that were to be. Do we not perceive ourselves in these words? Those sacrifices were figurative, foretelling the One Saving Sacrifice. Not even we have been left without a Sacrifice to offer to God. For hear what he says, having a concern for his sin, and wishing the evil thing which he has done to be forgiven him: “If You had willed,” he says, “sacrifice, I would have given it surely. With holocausts You will not be delighted.” Nothing shall we therefore offer? So shall we come to God? And whence shall we propitiate Him? Offer; certainly in yourself you have what you may offer. Do not from without fetch frankincense, but say, “In me are, O God, Your vows, which I will render of praise to You.” Do not from without seek cattle to slay, you have in yourself what you may kill. “Sacrifice to God is a spirit troubled, a heart contrite and humbled God despises not”. Utterly he despises bull, he-goat, ram: now is not the time that these should be offered. They were offered when they indicated something, when they promised something; when the things promised come, the promises are taken away. “A heart contrite and humbled God despises not.” You know that God is high: if you shall have made yourself high, He will be from you; if you shall have humbled yourself, He will draw near to you.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)