6 Hear therefore these words, and say thou with him: “Have pity upon me, O God, after Your great mercy”. He that implores great mercy, confesses great misery. Let them seek a little mercy of You, that have sinned in ignorance: “Have pity,” he says, “upon me, after Your great mercy.” Relieve a deep wound after Your great healing. Deep is what I have, but in the Almighty I take refuge. Of my own so deadly wound I should despair, unless I could find so great a Physician. “Have pity upon me, O God, after Your great mercy: and after the multitude of Your pities, blot out my iniquity.” What he says, “Blot out my iniquity,” is this, “Have pity upon me, O God.” And what he says, “After the multitude of Your pities,” is this, “After Your great mercy.” Because great is the mercy, many are the mercies; and of Your great mercy, many are Your pitying. Thou dost regard mockers to amend them, dost regard ignorant men to teach them, dost regard men confessing to pardon. Did he this in ignorance? A certain man had done some, aye many evil things he had done; “Mercy,” he says, “I obtained, because ignorant I did it in unbelief.” This David could not say, “Ignorant I did it.” For he was not ignorant how very evil a thing was the touching of another's wife, and how very evil a thing was the killing of the husband, who knew not of it, and was not even angered. They obtain therefore the mercy of the Lord that have in ignorance done it; and they that have knowing done it, obtain not any mercy it may chance, but “great mercy.”
7. “More and more wash me from mine unrighteousness”. What is, “More and more wash”? One much stained. More and more wash the sins of one knowing. Thou that hast washed off the sins of one ignorant. Not even thus is it to be despaired of Your mercy. “And from my delinquency purge Thou me.” According to the manner in which He is physician, offer a recompense. He is God, offer sacrifice. What will you give that you may be purged? For see upon whom you call, upon a Just One you call. He hates sins, if He is just; He takes vengeance upon sins, if He is just; you will not be able to take away from the Lord God His justice: entreat mercy, but observe the justice: there is mercy to pardon the sinner, there is justice to punish the sin. What then? Thou ask mercy; shall sin unpunished abide? Let David answer, let those that have fallen answer, answer with David, and say, No, Lord, no sin of mine shall be unpunished; I know the justice of Him whose mercy I ask: it shall not be unpunished, but for this reason I will not that Thou punish me, because I punish my sin: for this reason I beg that Thou pardon, because I acknowledge.
8. “For mine iniquity I acknowledge, and my delinquency is before me ever”. I have not put behind my back what I have done, I look not at others, forgetful of myself, I pretend not to pull out a straw from my brother's eye, when there is a beam in my eye; my sin is before me, not behind me. For it was behind me when to me was sent the Prophet, and set before me the parable of the poor man's sheep. For says Nathan the Prophet to David, “There was a certain rich man having very many sheep; but a poor man his neighbour had one little ewe sheep, which in his bosom and of his own food he was feeding: there came a stranger to the rich man, nothing from his flock he took, for the lithe ewe sheep of the poor man his neighbour he lusted; her he slew for the stranger: what does he deserve?” But the other being angry does pronounce sentence: then the king, evidently knowing not wherein he had been taken, declared the rich man deserving of death, and that the sheep be restored fourfold. Most sternly and most justly. But his sin was not yet before him, behind his back was what he had done: his own iniquity he did not yet acknowledge, and therefore another's he did not pardon. But the Prophet, being for this purpose sent, took from his back the sin, and before his eyes placed it, so that he might see that sentence so stern to have been pronounced against himself. For cutting and healing his heart's wound, he made a lancet of his tongue....
9. “Against You alone have I sinned, and before You an evil thing have I done”. What is this? For before men was not another's wife debauched and husband slain? Did not all men know what David had done? What is, “Against You alone have I sinned, and before You an evil thing have I done.” Because You alone are without sin. He is a just punisher that has nothing in Him to be punished; He is a just reprover that has nothing in Him to be reproved. “That you may be justified in Your sayings, and conquer when You are judged.” To whom he speaks, brethren, to whom he speaks, is difficult to understand. To God surely he speaks, and it is evident that God the Father is not judged. What is, “And conquer when You are judged”? He sees the future Judge to be judged, one just by sinners to be judged, and therein conquering, because in Him was nothing to be judged. For alone among men could truly say the God-Man, “If you have found in Me sin, say.” But perchance there was what escaped men, and they found not what was really there, but was not manifest. In another place He says, “Behold there comes the Prince of the world,” being an acute observer of all sins; “Behold,” He says, “there comes the Prince of this world,” with death afflicting sinners, presiding over death: for, “By the malice of the devil death came into the world.” “Behold,” He says, “there comes the Prince of the world:”— He said these words close upon His Passion:— “and in Me he shall find nothing,” nothing of sin, nothing worthy of death, nothing worthy of condemnation. And as if it were said to Him, Why then do You die? He continues and says, “But that all men may know that I do the will of My Father; arise, let us go hence.” I suffer, He says, undeserving, for men deserving, in order that them I may make deserving of My Life, for whom I undeservedly suffer their death. To Him then, having no sin, says on the present occasion the Prophet David, “Against You only have I sinned, and before You an evil thing have I done, that You may be justified in Your sayings, and conquer when You are judged.” For Thou overcomest all men, all judges; and he that deems himself just, before You is unjust: Thou alone justly judgest, having been unjustly judged, That hast power to lay down Your life, and hast power again to take it. Thou conquerest, then, when You are judged. All men Thou overcomest, because You are more than men, and by You were men made.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)