David is guilty of the death of Uriah (2 Samuel 11:15). Although he hadn’t physically touched Uriah’s dead body, there is a sense of uncleanness because of his involvement in Uriah’s death. So when David cries out to the Lord to be purged with hyssop, he is calling on God to cleanse him from his sin. David could have offered more sacrifices than nearly anyone who has ever lived. He was a rich man! But he knew that God delights in “a broken spirit; a broken and contrite heart,” rather than animal sacrifices (Psalm 51:16-17), and certainly hyssop is similarly a prefigurement of something…or Someone.