Thomas Aquinas, on Ps 50:2

“When Nathan the prophet came to him after he had sinned with Bethsabee.”

“"Wash me yet more." Here he asks for the removal of the uncleanness of guilt. A person who has a well-disposed mind abhors the uncleanness of guilt more than the severity of punishment; and therefore he says, "wash me yet more"; as if to say: I ask that you blot out the punishment; but more than that, I ask that you cleanse the stain. Or, "wash me more" than I understand. Rom. 8: "For we do not know what we should pray for as we ought." Eph. 3: "He who is able to do all things more abundantly than we ask or understand." Two things are necessary for removing the stain: namely, a preceding washing and a following cleanliness. In bodies, washing is done through water; and so, according to the Gloss, through water the Psalm prefigures the power of Baptism, by which God was going to forgive sin. Ezek. 36: "I will pour upon you clean water," etc. Zech. 13: "There shall be a fountain open to the house of David for the washing of the sinner and of the unclean woman." And although Baptism had not yet been instituted, nevertheless the power of God that washes was present in Baptism. Therefore, "wash me from my iniquity." Jer. 4: "Wash your heart from wickedness, O Jerusalem, that you may be saved." I also ask that "you cleanse me from my sin," because no one is cleansed except by you. Job 14: "Who can make clean that which is conceived of unclean seed?" Sir. 34: "Who shall be cleansed by the unclean?" And he says two things: namely, "iniquity" and "sin." Iniquity is contrary to justice; but sin is contrary to purity; and this is the adultery. And so there was iniquity insofar as he harmed another through homicide; but sin was through the adultery by which he defiled himself.”

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