I. Looking at this triad of petitions, they teach us, first, how David thought of his sin. (1) Observe the reiteration of the same earnest cry in all these clauses. It is not a mere piece of Hebrew parallelism. It is much more the earnestness of a soul that cannot be content with once asking for the blessings and then passing on, but dwells upon them with repeated supplication, not because it thinks that it shall be heard for its much speaking, but because it longs for them so eagerly. (2) Notice, again, that he speaks of his evil as transgressions and as sin, using the plural and then the singular. He regards it first as being broken up into a multitude of isolated acts, and then as being all gathered into one knot, as it were, so that it is one thing. But he does not stop there. His sins are not merely a number of deeds, but they have, deep down below, a common root from which they all come, a centre in which they all inhere. And so he says, not only "Blot out my transgressions," but "Wash me from mine iniquity." (3) In all the petitions we see that the idea of his own single responsibility for the whole thing is uppermost in David's mind. It is " mytransgression," it is "mineiniquity," and it is "mysin." (4) The three words which the Psalmist employs for sin give prominence to different aspects of it. Transgression is not the same as iniquity, and iniquity is not the same as sin. The word rendered "transgression" literally means rebellion, a breaking away from, and setting one's self against, lawful authority. That translated "iniquity" literally means that which is twisted, bent. The word in the original for "sin" literally means missing a mark, an aim.
II. Those petitions show us how David thinks of forgiveness. (1) The first petition conceives of the Divine dealing with sin as being the erasure of a writing, perhaps of an indictment. Our past is a blurred manuscript, full of false things and bad things. We have to spread the writing before God and ask Him to remove the stained characters from the surface that was once fair and unsoiled. (2) The second prayer, "Wash me throughly from mine iniquity," does not need any explanation, except that the word expresses the antique way of cleansing garments by treading and beating. David then here uses the familiar symbol of a robe to express the "habit" of the soul, or, as we say, the character. That robe is all splashed and stained. He cries to God to make it a robe of righteousness and a garment of purity. (3) "Cleanse me from my sin." That is the technical word for the priestly act of declaring ceremonial cleanness, the cessation of ceremonial pollution, and for the other priestly act of making, as well as declaring, clean from the stains of leprosy. With reference to both of these uses the Psalmist employs it here.
III. These petitions likewise show us whence the Psalmist draws his confidence for such a prayer. His whole hope rests upon God's own character as revealed in the endless continuance of His acts of love. And for us who have the perfect love of God perfectly expressed in His Son, that same plea is incalculably strengthened, for we can say, "According to Thy tender mercies in Thy dear Son, blot out my transgressions."
A. Maclaren, Sermons Preached in Manchester,2nd series, p. 95.
References: Psalms 51:1-6. R. S. Candlish, The Gospel of Forgiveness,p. 376. Psalms 51:1-13. Preacher's Monthly,vol. ii., p. 25.