David was a man after God’s own heart (1 Samuel 13:14) But did you realize that despite all the things David did which are written in the Bible, God never said of David, “from today you’ve lost the special place in my heart.” Or God would have said to David, “you will have to work hard to earn my trust again.” Mercy is our seed we sow to the undeserving to reap their trust and loyalty. We see this reality in the New Testament, the story of the prostitute woman who anointed the feet of Jesus. She had committed many sins. But the Bible says while Jesus was sitting in Simons house, this immoral woman came with a beautiful alabaster jar filled with expensive perfume. “Then she knelt behind him at his feet, weeping. Her tears fell on his feet, and she wiped them off with her hair. Then she kept kissing his feet and putting perfume on them. When the Pharisee who had invited him saw this, he said to himself, “If this man were a prophet, he would know what kind of woman is touching him. She’s a sinner!” (Luke 7:38-39)
Religious people don’t understand the principles of mercy. To them because you have always done it, you will always do it and so your case is bad because you will never change. No, but that’s not how mercy looks at you. The more a person receives mercy, the more they love. Jesus’ response to the religious people was this: “I tell you, her sins—and they are many—have been forgiven, so she has shown me much love. But a person who is forgiven little shows only little love.” (Luke 7:47) NLT
Mercy makes you love more; that is what David is communicating in Psalm 103:9. Through his own personal dealings with God, he could testify that the Lord is merciful, gracious, slow to anger, and abounding in steadfast love. David also adds that God does not always "chide." The Hebrew word here is yā'rib, which implies a legal argument, accusation, or debate. God does not perpetually bring a court case against us. He certainly has enough evidence to condemn us, but He refuses to do so. He has a right to be angry with us, but He forgoes that right.
How do we, living so long after the book of Psalms was written, know that God is loving and merciful, even to sinners? The answer is Jesus -the mercy of God took the punishment for our sins. On the cross He was made sin for us "so that in him we might become the righteousness of God" (2 Corinthians 5:21). The prophet Isaiah predicted the sufferings of Jesus on our behalf. He wrote in Isaiah 53:6, "All we like sheep have gone astray; we have turned—every one—to his own way; and the LORD has laid on him the iniquity of us all."