1 Have mercy on me, O God,
according to your unfailing love;
according to your great compassion
blot out my transgressions.
2 Wash away all my iniquity
and cleanse me from my sin.
3 For I know my transgressions,
and my sin is always before me.
4 Against you, you only, have I sinned
and done what is evil in your sight,
so that you are proved right when you speak
and justified when you judge.
5 Surely I was sinful at birth,
sinful from the time my mother conceived me.
20 The LORD has dealt with me according to my righteousness;
according to the cleanness of my hands he has rewarded me.
21 For I have kept the ways of the LORD;
I have not done evil by turning from my God.
22 All his laws are before me;
I have not turned away from his decrees.
23 I have been blameless before him
and have kept myself from sin.
24 The LORD has rewarded me according to my righteousness,
according to the cleanness of my hands in his sight.
25 To the faithful you show yourself faithful,
to the blameless you show yourself blameless,
26 to the pure you show yourself pure,
but to the crooked you show yourself shrewd.
27 You save the humble
but bring low those whose eyes are haughty.
I have not done evil I have done what is evil in your sight
I have kept myself from sin Surely I was sinful at birth
I have been blameless I know my transgressions
I have not turned away My sin is always before me
Two different Psalms with two radically different messages. If we read them through a few times, it seems obvious that these are written by two completely different persons; one, a scoundrel and one, a saint. But when we find out who wrote them, we might be shocked. Both of these Psalms are written by the same person (King David), but show radically different experiences, one of deep repentance for sin (51) and one that displays a confidence in faithfulness and cleanness (18).
These sound like two completely different things, two completely different people; what in the world was wrong with David? Was he crazy? Addled in the head? Because we can be one or the other, but not both, can we?