The petitions for cleansing are, in ver. 3, urged on the ground of the psalmist's consciousness of sin. Penitent confession is a condition of forgiveness. There is no need to take this verse as giving the reason why the psalmist offered his prayer, rather than as presenting a plea why it should be answered. Some commentators have adopted the former explanation, from a fear lest the other should give countenance to the notion that repentance is a meritorious cause of forgiveness; but that is unnecessary scrupulousness. "Sin is always sin, and deserving of punishment, whether it is confessed or not. Still, confession of sin is of importance on this account--that God will be gracious to none but to those who confess their sin" (Luther, quoted by Perowne).
Ver. 4 sounds the depths in both its clauses. In the first the psalmist shuts out all other aspects of his guilt, and is absorbed in its solemnity as viewed in relation to God. It is asked, How could David have thought of his sin, which had in so many ways been "against" others, as having been "against Thee, Thee only"? As has been noted above, this confession has been taken to demonstrate conclusively the impossibility of the Davidic authorship. But surely it argues a strange ignorance of the language of a penitent soul, to suppose that such words as the psalmist's could be spoken only in regard to sins which had no bearing at all on other men. David's deed had been a crime against Bathsheba, against Uriah, against his family and his realm; but these were not its blackest characteristics. Every crime against man is sin against God. "Inasmuch as ye have done it unto one of the least of these ... ye have done it unto Me" is the spirit of the Decalogue as well as the language of Jesus. And it is only when considered as having relation to God that crimes are darkened into sins. The psalmist is stating a strictly true and profound thought when he declares that he has sinned "against Thee only." Further, that thought has, for the time being, filled his whole horizon. Other aspects of his shameful deed will torture him enough in coming days, even when he has fully entered into the blessedness of forgiveness; but they are not present to his mind now, when the one awful thought of his perverted relation to God swallows up all others. A man who has never felt that all-engrossing sense of his sin as against God only has much to learn.
The second clause of ver. 4 opens the question whether "in order that" is always used in the Old Testament in its full meaning as expressing intention, or sometimes in the looser signification of "so that," expressing result. Several passages usually referred to on this point (_e.g._, Psalm xxx. 12; Exod. xi. 9; Isa. xliv. 9; Hos. viii. 4) strongly favour the less stringent view, which is also in accordance with the genius of the Hebrew race, who were not metaphysicians. The other view, that the expression here means "in order that," insists on grammatical precision in the cries of a penitent heart, and clogs the words with difficulty. If their meaning is that the psalmist's sin was intended to show forth God's righteousness in judging, the intention must have been God's, not the sinner's; and such a thought not only ascribes man's sin directly to God, but is quite irrelevant to the psalmist's purpose in the words. For he is not palliating his transgression or throwing it on Divine predestination (as Cheyne takes him to be doing), but is submitting himself, in profoundest abasement of undivided guilt, to the just judgment of God. His prayer for forgiveness is accompanied with willingness to submit to chastisement, as all true desire for pardon is. He makes no excuses for his sin, but submits himself unconditionally to the just judgment of God. "Thou remainest the Holy One; I am the sinner; and therefore Thou mayest, with perfect justice, punish me and spurn me from Thy presence" (Stier).