The heading tells where this psalm was written from: ^Title^"when Nathan the prophet went to him, after he had gone in to Bathsheba." The story is in 2 Samuel 11-12, a chain of desire, adultery, deception, and the murder of Uriah to close the account. When Nathan said, "You are the man" (2 Samuel 12:7), David's answer was six words: "I have sinned against the LORD" (2 Samuel 12:13). Psalm 51 is those words with the door left open behind them.
It begins with a pile of words. 1"Have mercy on me, O God, according to your steadfast love." Three words for sin appear in the first verses: transgression, which is rebellion, iniquity, which is a twist in the thing itself, and sin, which is missing the mark. Three verbs for cleaning answer them: blot out, wash, cleanse. To blot out is to erase a record. David's whole case rests on one phrase, mercy asked for plainly with nothing offered in trade.
4"Against you, you only, have I sinned." It seems odd, since Bathsheba and Uriah were plainly wronged. But David is speaking of who holds the final claim. God is the offended party and the avenger of the vulnerable, and only God has the right to pass sentence on a king. Paul uses the rest of that verse to show that God is just in whatever he judges (Romans 3:4). David leaves himself no excuses: 3"I know my transgressions, and my sin is ever before me."
Adultery and murder were what Numbers 15:30 calls sins done "with a high hand," defiant and deliberate, and the law provided no sacrifice for them. When David says, 16"You will not delight in sacrifice, or I would give it," he is saying that no offered sacrifice covers what he did, and that he would bring one if it did. What remains is a heart: 17"A broken and contrite heart, O God, you will not despise." This is the only door left, and it turns out to open. His plea 7"Purge me with hyssop" reaches for the plant used to sprinkle blood in cleansings. He asks for a cleansing no priest can perform.
Then comes the request that gives the psalm its title, for a clean heart. The verb is *bara*, the word that opens the Bible, and in the Old Testament only God is its subject. David asks for a new creation. The plea 11"Take not your Holy Spirit from me" has a face behind it, since David knew what had happened to Saul, from whom the Spirit of the LORD departed (1 Samuel 16:14).
The offering David could not find was coming. Ezekiel would promise clean water and a new heart (Ezekiel 36:25-26). Hebrews asks how much more the blood of Christ will purify our conscience (Hebrews 9:14), and John says it plainly: "the blood of Jesus his Son cleanses us from all sin" (1 John 1:7).
The psalm ends with joy. 12"Restore to me the joy of your salvation." It has done what David promised, teaching transgressors God's ways, and people have prayed it for three thousand years in the words of a man who knew what he had done and where to turn.
Ask what you have been afraid to bring to God, and pray verse 10 over it.
Prayer
Father, you know what we have done and hidden, and none of it is news to you. Have mercy on us according to your steadfast love. Wash us where we cannot wash ourselves, and do not take your Holy Spirit from us. Thank you that the blood of your Son cleanses us from all sin. Create in us clean hearts. In Jesus' name, amen.