Sin is a theme of the psalm. There are three (maybe four, depending on one's interp.) distinct words for sin in the text. First, there is a word for "transgression" which specifically refers to rebellion against God in a sort of legal setting. Second, there is the most common term for sin denoting "missing the mark," or "deviating off of the correct/right path." Finally, a word most commonly translated "iniquity" in our English translations signifies that which is totally corrupt and twisted; almost a sort of criminality.
Forgiveness is another prominent theme. Again, three different words or phrases are found. First, the idea of a sin being "lifted up;" or "carried away" is one way David refers to God's forgiveness. Second, he speaks of God "covering" our sin as if our sin could be hidden from God's sight (amazing thought to try to comprehend--if you know what I mean). Third, he speaks of God not crediting our sins to us (this is the idea that Paul picks up in Romans 4 as he quotes Ps 32 in his masterful argumentation for imputed righteousness by faith alone apart from works).
Repentance proves to be another peak of the psalm as David speaks of his desire to: first, make known his sins to the Lord. Second, he does not cover his sins. Third, he confesses them to God. And fourth, he beckons that all who are Godly pray to God while He may be found.
Joy is the fourth theme of the psalm. Verse 11 offers three different verbs (commands!) to rejoice, be glad (exult!), and shout for joy!
I'm arguing that repentance must be a constant discipline in every Christian's life; and if it's not, then one has good reason to question his salvation and see whether he is truly in the faith (cf. 2 Cor 7:10; 13:5).
I call upon Thomas Watson--a good and helpful, though dead, friend of mine--to help solidify this argument:
Carnal Protestants [refuse to repent], who are strangers to godly sorrow. They cannot endure a serious thought, nor do they love to trouble their heads about sin. Paracelsus [a Swiss physician from the 16th c.] spoke of a frenzy some have which will make them die dancing. Likewise sinners spend their days in mirth; tehy fling away sorrow and go dancing to damnation. Some have lived many years, yet never put a drop in God's bottle, nor do they know what a broken heart means. They weep and wring their hands as if they were undone when their estates are gone, but have no agony of soul for sin (Doctrine of Repentance, 26).
And again:
One may leave sin for fear, as in a storm the plate and jewels are cast overboard, but the nauseating and loathing of sin argues a detestation of it. Christ is never loved till sin be loathed. Heaven is never longed for till sin be loathed. When the soul sees an issue of blood running, he cries out, Lord, when shall I be freed from this body of death? When shall I put off these filthy garments of sin and have the fair mitre of glory set upon my head? Let all my self-love be turned into self-loathing (Zech 3:4-5). We are never more precious in God's eyes than when we are lepers in our own (Ibid., 45).
