David begged God to blot out his transgressions, wash him thoroughly from his iniquity, cleanse him from his sin, and purge his sin with hyssop (vv. 1-2, 7, 9). The verbs used by David—"blot out," "wash," "cleanse," and "purge"—indicate that his repentance and confession were thorough and that his asking for forgiveness was genuine.
In contrast, suppose someone would pray, saying, "God, I know that You are merciful. No matter how many sins I have committed, I know that You will forgive me." This kind of confession does not mean anything. Like David, we need to stay in the presence of God, confessing that we were born in sin and pleading with Him to wash us and cleanse us, to blot out our transgressions, and to purge our sin. To pray in this way indicates that we have no trust in ourselves. Realizing that we are sinful and that God is holy, we trust only in Him. Also, we realize that we need Christ to be our Mediator and our sacrifice.
In verse 7a David prayed, "Purge my sin with hyssop, and I will be clean." Hyssop typifies Christ in His humble and humiliated human nature (1 Kings 4:33a; Exo. 12:22a). In Psalm 51:7a hyssop implies Christ as the Mediator and the sacrifice.
In verse 10 David prayed, "Create in me a clean heart, O God,/And renew a steadfast spirit within me." The Hebrew word for "clean" here may also be translated "pure." David begged God not only to forgive him and purge him but also to renew him.
By sinning we become old, but after we are forgiven by God we may be renewed. Thus, after we enjoy God's forgiveness, we need to ask Him for His renewing. We need to pray that He will give us a pure heart and a steadfast spirit.
In verse 11 David continued, "Do not cast me from Your presence,/And do not take the Spirit of Your holiness away from me." We need a new heart and a steadfast spirit, and we also need God's presence. If we lose God's presence, we lose everything. God's presence is actually the Spirit. When the Spirit is away, God's presence also is gone.