51:7-15 Purge me with hyssop, with the blood of Christ applied to my soul by a lively faith, as the water of purification was sprinkled with a bunch of hyssop. The blood of Christ is called the blood of sprinkling, Heb 12:24. If this blood of Christ, which cleanses from all sin, cleanse us from our sin, then we shall be clean indeed, Heb 10:2. He asks not to be comforted, till he is first cleansed; if sin, the bitter root of sorrow, be taken away, he can pray in faith, Let me have a well-grounded peace, of thy creating, so that the bones broken by convictions may rejoice, may be comforted. Hide thy face from my sins; blot out all mine iniquities out of thy book; blot them out, as a cloud is blotted out and dispelled by the beams of the sun. And the believer desires renewal to holiness as much as the joy of salvation. David now saw, more than ever, what an unclean heart he had, and sadly laments it; but he sees it is not in his own power to amend it, and therefore begs God would create in him a clean heart. When the sinner feels this change is necessary, and reads the promise of God to that purpose, he begins to ask it. He knew he had by his sin grieved the Holy Spirit, and provoked him to withdraw. This he dreads more than anything. He prays that Divine comforts may be restored to him. When we give ourselves cause to doubt our interest in salvation, how can we expect the joy of it? This had made him weak; he prays, I am ready to fall, either into sin or into despair, therefore uphold me with thy Spirit. Thy Spirit is a free Spirit, a free Agent himself, working freely. And the more cheerful we are in our duty, the more constant we shall be to it. What is this but the liberty wherewith Christ makes his people free, which is contrasted with the yoke of bondage? Ga 5:1. It is the Spirit of adoption spoken to the heart. Those to whom God is the God of salvation, he will deliver from guilt; for the salvation he is the God of, is salvation from sin. We may therefore plead with him, Lord, thou art the God of my salvation, therefore deliver me from the dominion of sin. And when the lips are opened, what should they speak but the praises of God for his forgiving mercy?

7-12. A series of prayers for forgiveness and purifying.

Purge … hyssop—The use of this plant in the ritual (Ex 12:22; Nu 19:6, 18) suggests the idea of atonement as prominent here; "purge" refers to vicarious satisfaction (Nu 19:17-20).