Holiness, grace, mercy
Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions. Wash me throughly from mine iniquity, and cleanse me from my sin. For I acknowledge my transgressions: and my sin is ever before me. Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest. Psalm 51:1-4
Think about what these verses, and indeed this entire psalm, says about God. First of all, it says something about his holiness. You don't cry out to God for mercy and grace and forgiveness if God doesn't care about holiness. But God does. There are consequences to sin: eternal consequences apart from Christ, but even for those who, like David, belong to him, there are severe disciplinary consequences. David didn't get off scot-free; to some extend he suffered the consequences of his sin with Bathsheba for the rest of his life. God cares about holiness because God is holy and we will never appreciate who God or approach him as we should until we come face to face with this reality, a reality reflected in his Law. We should be thankful for this, for an unholy God would be a terrifying and despicable being. God is holy: thank him for his holiness!
Second, this says something about God's grace. When David appeals to God's lovingkindness, he is appealing to the grace of God. For God's love of David did not rest ultimately in David himself; the foundation of God's saving love for anyone can only be found in God. David in fact didn't deserve anything, and he understood this, which is why he appeals to God the way he did. According to the terms of the Mosaic Law, he had committed at least two capital offenses. And yet God pardoned him and let him go. Why? Because God does not deal with his children according to their deserts but according to the terms of sovereign grace, grace that comes to us not on the basis of who we are or what we have done but according to who Christ is and what he has done. Thank God for his grace!
Third, this says something about God's mercy. We can tend to conflate grace and mercy, but they are distinct. Grace is free and unmerited favor, however it comes. But mercy is kindness and relief to the miserable. And David was miserable. His sin made him so, and it was the mercy of God that didn't allow him to remain hardened in his sin. But more than that, God granted David's request by restoring to him the joy of his salvation, by taking him out of the miry pit into which he had flung himself. Thank God for his mercy!