What does Psalm 51:11 mean?

Undoubtedly David recalled that the Lord had removed His Spirit from King Saul, David's predecessor (1 Samuel 16:14). He did not want the same thing to happen to him, so he prayed: "Cast me not away from your presence." As the writer of Psalm 23, the Shepherd Psalm, David had been keenly aware of the Lord's constant presence. He wrote that he would fear no evil even in the valley of the shadow of death because the Lord was with him. However, sin had destroyed David's steadfast assurance of the Lord's presence, so he asked the Lord not to cast him away from His presence.

David also asked the Lord not to remove His Holy Spirit from him. In Old Testament times the Spirit often came upon believers and left them later on. New Testament believers, in contrast, have the assurance that the Holy Spirit lives in them forever (John 14:15–18; Ephesians 1:13–14). Nevertheless, New Testament believers may grieve the Holy Spirit (Ephesians 4:30–32), lie to him (Acts 5:1–3), and even quench his influence (1 Thessalonians 5:19).