Purge me with hyssop, and I shall be clean - On the word hyssop, see the notes at John 19:29; notes at Hebrews 9:19. The plant or herb was much used by the Hebrews in their sacred purifications and sprinklings: Exodus 12:22; Leviticus 14:4, Leviticus 14:6,Leviticus 14:49, Lev 14:51; 1 Kings 4:33. Under this name the Hebrews seem to have comprised not only the common “hyssop” of the shops, but also other aromatic plants, as mint, wild marjoram, etc. - Gesenius, “Lexicon” The idea of the psalmist here evidently is not that the mere sprinkling with hyssop would make him clean; but he prays for that cleansing of which the sprinkling with hyssop was an emblem, or which was designed to be represented by that. The whole structure of the psalm implies that he was seeking an “internal” change, and that he did not depend on any mere outward ordinance or rite. The word rendered “purge” is from the word חטא châṭâ' - which means “to sin.” In the Piel form it means to bear the blame (or “loss”) for anything; and then to “atone for, to make atonement, to expiate:” Genesis 31:39; Leviticus 6:26; Numbers 19:19. Here it conveys the notion of cleansing from sin “by” a sacred rite, or by that which was signified by a sacred rite. The idea was that the sin was to be removed or taken away, so that he might be free from it, or that “that” might be accomplished which was represented by the sprinkling with hyssop, and that the soul might be made pure. Luther has rendered it with great force - Entsundige mich mit Ysop - “Unsin me with hyssop.”
Wash me - That is, cleanse me. Sin is represented as “defiling,” and the idea of “washing” it away is often employed in the Scriptures. See the notes at Isaiah 1:16.
And I shall be whiter than snow - See the notes at Isaiah 1:18. The prayer is, that he might be made “entirely” clean; that there might be no remaining pollution in his soul.