What does Psalm 30:8 mean?

David cries out to the Lord for mercy. This may have been the way he begged for God's mercy after making a sinful choice as king (1 Chronicles 21:7–17). While David was by no means sinless, he was keenly aware of his limitations. He acknowledged his need to be forgiven. This helps define the biblical concept of "mercy:" withholding what every sinner deserves. Grace, on the other hand, is when God provides something good which sinners do not deserve.

Every sinner—meaning every person (Romans 3:10, 23)—deserves divine judgment. Romans 6:23 assures us, "The wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." If God were to mete out what every sinner deserves, hell would be everyone's destiny. Yet, in mercy, God withholds from believers what everyone deserves, and graciously imparts them with salvation which no one deserves (Titus 3:5). Jesus bore the punishment of sin on the cross and rose to life victorious; all who put their faith in Him are saved and given eternal life (Ephesians 2:1–10). Those who refuse God's gracious offer of salvation remain condemned (John 3:16–18, 36).

Jesus told a story about two men who entered the temple to pray (Luke 18:9–14). One was a tax collector; the other was a Pharisee. The Pharisee was consumed with a false sense of self-righteousness. His "thanks" to God was just a form of self-praise. God saw through the Pharisee's hypocrisy. There was no awareness of sin, so God did not forgive him. The tax collector, on the other hand prayed, "God, be merciful to me, a sinner," and God justified him. David was wise to plead for mercy.