37 The Hebrew participles provide the subject matter in this contrast. On the one hand is the person who covers over (מְכַסֶּה, mékhasseh) his sins. This means refusing to acknowledge them in confession, and perhaps rationalizing them away. On the other hand there is the one who both “confesses” (מוֹדֶה, modeh) and “forsakes” (עֹזֵב, ’ozev) the sin. To “confess” sins means to acknowledge them, to say the same thing about them that God does.
38 The verse contrasts the consequences of each. The person who refuses to confess will not prosper. This is an understatement (a figure of speech known as tapeinosis); the opposite is the truth, that eventually such a person will be undone and ruined. On the other hand, the penitent will find mercy. This expression is a metonymy of cause for the effect—although “mercy” is mentioned, what mercy provides is intended, i.e., forgiveness. In other passages the verb “conceal” is used of God’s forgiveness—he covers over the iniquity (Ps 32:1). Whoever acknowledges sin, God will cover it; whoever covers it, God will lay it open.
39 This verse is unique in the book of Proverbs; it captures the theology of forgiveness (e.g., Pss 32 and 51). Every part of the passage is essential to the point: Confession of sins as opposed to concealing them, coupled with a turning away from them, results in mercy.