Albert Barnes Commentary Psalms 51:12

"Restore unto me the joy of thy salvation; And uphold me with a willing spirit." — Psalms 51:12 (ASV)

Restore unto me the joy of thy salvation - literally, “Cause the joy of thy salvation to return.” This implies that he had formerly known what the happiness was of being a friend of God and of having a hope of salvation. That joy had been taken from him by his sin. He had lost his peace of mind. His soul was sad and cheerless.

Sin always produces this effect. The only way to enjoy religion is to do that which is right; the only way to secure the favor of God is to obey his commands; the only way in which we can have comforting evidence that we are his children is by doing that which shall be pleasing to him (1 John 2:29; 1 John 3:7; 1 John 3:10). The path of sin is a dark path, and in that path neither hope nor comfort can be found.

And uphold me with thy free spirit - That is, sustain me; keep me from falling. The words “with thy” are not in the original, and there is nothing there to indicate that by the word “spirit” the psalmist refers to the Spirit of God, though it should be observed that there is nothing against such a supposition. The word rendered “free”—נדיב nâdı̂yb—means properly “willing, voluntary, ready, prompt” (1 Chronicles 28:21; Exodus 35:5).

Then the word means liberal, generous, noble-minded (Isaiah 32:5, Isaiah 32:8; Proverbs 17:7, Proverbs 17:26). It would seem here to mean “a ‘willing’ spirit,” referring to David’s own mind or spirit; and the prayer is, that God would uphold or sustain him in a willing spirit or state of mind; that is, a state of mind in which he would be willing and ready to obey all the commands of God, and to serve him faithfully.

What he prayed for was grace and strength that he might be kept in a state of mind which would be constant and firm (Psalms 51:10), and a state in which he would always be found willing and ready to keep the commandments of God. It is a proper object of prayer for all people that they may always be kept in a state of mind in which they will be willing to do all that God requires of them, and to bear all that may be laid on them.