Wash me, O Lord, from my iniquity and cleanse me from my sin. Ps 51:2This prayer has been reworded to match the Latin more closely. It, like the prayer before it, is inspired by David's words in Psalm 51.
The purpose of this rite is to impress upon the priest and congregation the need for “interior purification” (GIRM 76) for the proper celebration of these sacred mysteries. Just as the holy sacrifice about to be offered is pure and spotless, so too the priest and people offering that sacrifice should be holy and without blemish. Of course, this state of sanctity does not come from ourselves, but from God.