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.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)