Psalms 51:2 - Matthew Poole's English Annotations on the Holy Bible

Wash me throughly, Heb. multiply to wash me; by which phrase he implies the greatness of his guilt, and the insufficiency of all legal washings, and the absolute necessity of some other and better thing to wash him, even of God's grace, and the blood of Christ; which as Abraham saw by faith, 1 Thessalonians 8:56, so did David, as is sufficiently evident (allowing for the darkness of the dispensation and expressions of the Old Testament) from divers passages of the Psalms, of which I have spoken in their proper places; and his earnest and passionate desire of pardon, which he desires above all other things; wherein he showeth himself to be a true penitent, because his chief care and desire was to obtain God's favour, and the forgiveness of his sins, and not the prevention of those external sore judgments which God by Nathan threatened to bring upon him and his house, 2 Samuel 12:10,11, about which here is not one word in this Psalm; whereas the cares and desires of hypocrites chiefly are bent towards worldly things, as we see in Cain, Genesis 4:13,16,17, and Saul, 1 Samuel 15:30, and others, Hosea 7:14.