Thomas Aquinas, on Ps 50:9

“Thou shalt sprinkle me with hyssop, and I shall be cleansed: thou shalt wash me, and I shall be made whiter than snow.”

“"Turn away." Here he asks for the recovery of innocence. And because he considers that the evil of guilt is in him and the good of grace, he first asks for the removal of evil or sin; second, he asks for the removal of the effect of sin, at "Create in me a clean heart, O God." For sin is removed not in the sense that the sin never existed, but in the sense that the committed sin is not imputed to him for punishment, according to Ps. 31: "Blessed is the man to whom the Lord has not imputed sin." And he speaks in the likeness of a judge who punishes, who first considers the magnitude of the guilt and then determines the punishment; therefore he asks that God not consider his sin but be unmindful of it; and therefore he says, "Turn away your face from my sins." Second, he asks that punishment not be inflicted; hence he says, "and blot out all my iniquities"; as if to say: I know that I have done evil before you, and therefore I ask that you turn away your face from my sins, that is, that you not consider my sins for punishment. Ezek. 18: "I will not remember all his iniquities." Also, I merited the punishment of damnation; but I ask that you blot it out, because God, even if he does not change his counsel, nevertheless changes the sentence.”

Imported from an open dataset — not yet checked against the printed edition.