God is holy and just. God is worthy of full devotion and unyielding allegiance from His creation. Nothing short of perfection is right in the eyes of the Lord and is worthy of ultimate and infinate wrath. David understood this, or was reminded of this, and now he faces the greatest circumstance of sin: being in the presence of God and facing the consequences. This is our same fate whenever we sin against God. We must face the consequences that such actions of rebellion bring.
Yet, there is something else that David understood. There is something else about God that rushed to the forefront of David's entire being as he contemplated this reckoning from the Lord. God is a God of love. We should not confuse this with the notion that His love is a "it does not matter what you do" kind of love. No, God is holy and holiness is the expectation of all mankind. Yet, what David knew from the history of God with the Jews was that God's love sprang, not from who and what man is, but who He, the Lord, is.
God's love is like the love a father shows when teaching his child to ride a bike. The child wants to ride the bike but in learning how to use his muscles properly to hold up and navigate the bike, he falls often. The dad knows the son will fall so when he does, the father is not upset or frustrated. Instead, he walks alongside the son as he learns and until he is able to ride on his own. I expected my son to be able to ride the bike but I knew he would need my help in doing so. That's an expression of what God's love is like. He knows we want to serve Him fully but He also knows we need help to do so because we, like riding a bike, will fall. The difference is that the child will, one day, ride on his own. For us, life is like learning to ride a bike which we will never learn in this life and, therefore, will be in constant need of God's help - His love.
In this period of his life David has fallen off the bike, as it were, with this series of defections of deception, murder and adultery. He knows that God is holy but he also knows about God's hesed (love). This love of God is the covenantal love that He promised to His people and is a love based on Himself alone and not our righteousness. This love is what is expressed by the Lord in Genesis 15 when God passed between the sacrifices - showing that His covenant to man was different from the covenants man makes with man.
When man makes a covenant with man the fulfillment of it depends on both parties fulfilling their duties within the covenant. However, with the Lord, His covenant promise to us was established and fulfilled by His righteousness. He promised to fulfill His part and our part! David, before this holy Lord, appeals to His covenant love. The first words from his pen (mouth) were "Have mercy on me, O God, according to your steadfast love... (Psalm 51:1). David calls for God's mercy as an expression of His covenant love. In essence David is saying, "Lord, I know you could rightly condemn me, but I am coming to you in my failure pleading that you would deal with me according to Your covenant promise of love."
This is a powerful reality when we consider what is going on in David's mind and heart at this moment. As one man wrote, Hesed (covenant love),
"is not merely an emotion or feeling but involves action on behalf of someone who is in need.
Hesed describes a sense of love and loyalty that inspires merciful and compassionate behavior
toward another person."
David was pleading with the Lord to act on his behalf. He was admitting that he was in desperate need, not of his own righteousness (see Psalm 51:17) but the righteousness of God! He was showing that he was empty and worthy of damnation unless or until the Lord acted on his behalf by extending that which he did not deserve - God's mercy. That is what repentance is. It is the response of a broken man or woman to a God who would be right to judge, but appealing to His love.
The Septuagint (the Greek translation of the Old Testament) uses the word agape for love in verse 1 of Psalm 51. That is instructive because agape is the word the Bible uses to describe God's love revealed in Jesus' sacrificial death on the cross. That death was love because it was God acting on our behalf. It was for us. Jesus' life, suffering and death was God's hesed - covenant love acting on the behalf of humanity without which there could be no salvation. In this love Jesus came and lived a perfect life because we could not. Jesus came and, to complete the bike illustration, He rode the bike for us because knew we could never ride it for ourselves. Then, when He rose, He gave us the bike back but sent the Holy Spirit to walk alongside us so that when we fall He would be present in us to cleanse us (1st John 1:9). That is love.
So, when we fall into sin and are overtaken by it remember this: God not only established His relationship with those who are in Christ but He will also fulfill His relationship with us - in spite of our many failings (Philippians 1:6). So, when we fall, we can repent on the basis of God's remarkable, incredible and unending love for us that is based totally on His righteousness and not our ability to never fall off the bike.