Please read Psalm 51 and Leviticus 14:1-7.
The priest would take two clean birds, cedar-wood, scarlet, and hyssop; one of the birds was killed; and the living bird, with the scarlet, cedar, and hyssop, dipped in the blood of the bird that had been killed, and then sprinkled over the person who had been infected.
But I need to mention that this ceremony was not performed until the plague of the leprosy had been healed in the leper; (Leviticus 14:3;) and the ceremony was for the purpose of declaring to the people that the man was healed, that he might be restored to his place in society, having been healed of a disease that the finger of God alone could remove.
This is what David seems to have in view; therefore he requests the Lord to make the sin-offering for him, and to show to the people that he had accepted him, and cleansed him from his sin.
1 Peter 1:18-19, Look it up.
Here we see THREE THINGS:
1. FAITH IN THE FACT OF ATONEMENT FOR SIN. “I shall be clean.”
2. FAITH IN THE FATHERS, APPLICATION FOR SIN. “Purge me.” Sprinkled as the blood of sacrifices.
3. FAITH IN THE FIRMNESS ACCOMPLISHED. “I shall be whiter than snow.”
What can wash away my sin, nothing but the blood of Jesus?