First, Psalm 51 is for those who have never come to grips with the horror of human sin and the magnitude of divine grace. Often grace becomes meaningless, and certainly less than “amazing”, because we lose sight of the depths of our depravity. David helps us on both counts by describing in graphic detail the reality of his sin and the breath-taking glory of forgiving grace.
Second, this psalm is for those who think some people are too high or too holy to fall. Let us never forget that this psalm describes the experience of David, King of Israel, the “man after God’s own heart” (1 Sam. 13:14)!
Third, this psalm is also for those who think that once you have fallen, you can never get back up again. It is for those who think it’s possible to fall beyond the reach of God’s grace and forgiveness or that there is a quantifiable limit to divine mercy. But no one is so holy that he/she can’t fall, or so fallen that he/she can’t be forgiven.
Fourth, Psalm 51 is for those who think that if you have fallen and have actually gotten back up, perhaps even forgiven, you are still useless from that point on both to God and the church. David’s experience will prove otherwise.