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"I acknowledged my sin unto thee, and mine iniquity have I not hid. I said, I will confess my transgressions unto the LORD; and thou forgavest the iniquity of my sin." – Psa. 32:5
I N Psalm 32, David has penned a psalm which is first and foremost a Maschil. That is, a teaching psalm; one giving instruction to all who read and sing the words. Its note is one of heartfelt confession of sin and the blessedness of forgiveness. The great apostle Paul takes up the psalm in Romans 4 as he unfolds the glorious doctrine of justification. Perhaps David still had in mind the great sin that marred his adult life and the experience of it poured forth Psalm 51. There are those who believe that Psalm 32 was written "for the great day of the national atonement, on the tenth day of the seventh month, Leviticus 16." 
The psalm's great theme is the need for confession and acknowledgement of sin, if we are to have any enjoyment of covenant mercies and blessings. Verse 5 says: "I acknowledged my sin unto thee, and mine iniquity have I not hid. I said, I will confess my transgressions unto the LORD; and thou forgavest the iniquity of my sin. Selah." Why is the sinner required to CONFESS sin? Even the child of God has this duty laid upon him. Why so?
(1) Because God demands it. On the Day of Atonement, two goats were brought before Aaron. One was offered up as a sin-offering; the other, to become the Scape-goat. Both are a type and spiritual representation of Christ and it is noteworthy that Aaron was to lay both hands upon the head of the live goat and confess over him all the iniquities of the children of Israel. This done, the goat was set free to wander into the wilderness, signifying the removal of sin's guilt from the believing sinner. All this can be seen in Leviticus 16. By this means, we have underlined for us a principle established even from earliest times, that no atonement is possible without confession of guilt. God requires it and has ever done so throughout biblical history. In this connection note but two other references by way of example: Psalm 51 and 1 John 1:6 to 10.
(2) Our need to be Humbled is the second reason why God requires confession of sin. The natural state of the heart is one of pride and self reliance. No-one likes to be corrected or told they have transgressed God's Law and are in a lost spiritual state. In confession, we acknowledge how destitute we are of God's grace and see him as our only salvation through Christ Jesus.
(3) Confession implies Imputation. By this we mean that in confession of sin, we are acknowledging that our guilt was transferred (or imputed) to Christ. This is the symbolism of Leviticus 16:21, where it says upon confession, the sins of the children of Israel were put upon the head of the Scape-goat. In confessing (and believing) we affirm Christ has borne our sin and the wrath of God for us. Allied to this, we enter into the blessedness of having the righteousness of Christ imputed to us in justification. John Calvin could write: "An acceptance, by which God receives us into his favour, and esteems us as righteous persons." Then he adds that it "consists in the remission of sins and the imputation of the righteousness of Christ." 
(4) Confession on our part also 'Justifies God'. By this we do not mean that there is sin in God. Rather, we acknowledge he is holy and that the divine assessment of us is correct (see Psalm 14). In all of this, God is glorified as we confess him to be righteous and the only source of our salvation. Luke 7:29 is also suggestive of this, as the publicans justified or glorified God in their turning to him in John's baptism.
(5) Confession is also an Uncovering. In some respects, it also helps to heal a troubled conscience. Gregory of Nazianzen: "a salve for a troubled soul."  If we but accused ourselves, then Satan would no longer accuse and disturb our consciences, but view us covered in the righteousness of Christ.
(6) Confession is also allied to Repentance. There cannot be one without the other. A godly sorrow will accompany our confession of sins, as well it does in repentance. Such acknowledgment of guilt should fill us with the resolve not to sin again.
Here then are the reasons why we are to confess our sins to God. As we ponder this matter, may we:–
(i) Confess our sins, not as a formality. Let it be with mourning over our transgressions. Such sorrow is given vent in confession. (See Neh. 9:22 & Hos. 5:15).
(ii) Ensure that we are sincere in the matter.
(iii) Confess particular sins. Hide nothing. Acknowledge also that the whole of our nature is polluted.
(iv) Never cease from this duty. One old christian lady I knew believed it was unnecessary to go on confessing our sins AFTER conversion. In this connection, remember 1 John 1:6 to 10, which was written to believers. Besides, if we know anything of our inner corruptions, we shall feel the need to confess our sinfulness again and again. As R.M. M'Cheyne said, we shall need hourly pardons.
(v) Confess to God alone, not to men or an earthly priest. In saying this, there may be times when it will be right to acknowledge particular sins to others, especially if we have sinned against them and the church of God. It may also be helpful on occasions, to speak with a close friend about the state of our hearts, so that we can pray for one another.
(vi) Christ alone is our righteousness. He has made full atonement for us, therefore we go to him for pardon and know that we shall not be refused.
"O blessed is the man to whom
is freely pardoned
All the transgression he hath done,
whose sin is covered.
Bless'd is the man to whom the Lord
imputeth not his sin,
And in whose sp'rit there is no guile,
nor fraud is found therein."
– Psalm 32:1-2, Metrical Version.
[l] Brown, John, Notes upon the Metrical Psalms. See also Spurgeon, C.H., The Treasury of David.
 Calvin, J., Institutes, III.xi.2.
 As quoted in Watson, Thomas, The Doctrine of Repentance, p.28. This whole section on "Confession of Sin" is well worth the reading.