Because we are human, Muslims and Christians (and everyone else) have a common and serious problem: we routinely underestimate the seriousness of our sin, both in its offense to God and in its power over us. I found the following post from John Piper a helpful reminder of our great problem.
In Psalm 51, as he laments and repents of his adultery with Bathsheba and his murder of Uriah, David confesses at least five ways that his sin is extremely serious.
1. He says that he can’t get the sin out of his mind.
It is blazoned on his conscience. Verse 3:
For I know my transgressions, and my sin is ever before me.
Ever before him. The tape keeps playing. And he can’t stop it.
2. He says that his exceeding sinfulness is only against God.
Nathan had said David despised God and scorned his word. So David says in verse 4,
Against you, you only, have I sinned and done what is evil in your sight.
This doesn’t mean Bathsheba and Uriah and the baby weren’t hurt. It means that what makes sin sin is that it is against God. Hurting man is bad. It is horribly bad. But that’s not the horror of sin. Sin is an attack on God—a belittling of God. David admits this in striking terms: “Against you, you only, have I sinned.”
3. He doesn’t justify himself.
David vindicates God, not himself. There is no self-justification. No defense. No escape. Verse 4:
…so that you may be justified in your words and blameless in your judgment.
God is justified. God is blameless. If God casts David into hell, God will be innocent.
This is radical God-centered repentance. This is the way saved people think and feel. God would be just to damn me. And that I am still breathing is sheer mercy. And that I am forgiven is sheer blood-bought mercy. David vindicates the righteousness of God, not himself.
4. He intensifies his guilt by drawing attention to his inborn corruption.
Behold, I was brought forth in iniquity, and in sin did my mother conceive me.
Some people use their inborn corruption to diminish their personal guilt. David does the opposite. For him the fact that he committed adultery and murdered and lied are expressions of something worse: He is by nature that way.
If God does not rescue him, he will do more and more evil.
5. He admits that he sinned not just against external law but against God’s merciful light in his heart.
Behold, you delight in truth in the inward being, and you teach me wisdom in the secret heart.
God had been his teacher. God had made him wise. David had done so many wise things. And then sin got the upper hand. For David, this made it all the worse. “I have been blessed with so much knowledge and so much wisdom. O how deep must be my depravity that it could sin against so much light.”
So in those five ways at least David joins the prophet Nathan and God in condemning his sin and confessing the depths of his corruption.