Psalms 51:4

David, being convinced of his sin, poured out his soul to God in prayer for mercy and grace. Whither should backsliding children return, but to the Lord their God, who alone can heal them? he drew up, by Divine teaching, an account of the workings of his heart toward God. Those that truly repent of their sins, will not be ashamed to own their repentance. Also, he instructs others what to do, and what to say. David had not only done much, but suffered much in the cause of God; yet he flees to God's infinite mercy, and depends upon that alone for pardon and peace. He begs the pardon of sin. The blood of Christ, sprinkled upon the conscience, blots out the transgression, and, having reconciled us to God, reconciles us to ourselves. The believer longs to have the whole debt of his sins blotted out, and every stain cleansed; he would be thoroughly washed from all his sins; but the hypocrite always has some secret reserve, and would have some favorite lust spared. David had such a deep sense of his sin, that he was continually thinking of it, with sorrow and shame. His sin was committed against God, whose truth we deny by wilful sin; with him we deal deceitfully. And the truly penitent will ever trace back the streams of actual sin to the fountain of original depravity. He confesses his original corruption. This is that foolishness which is bound in the heart of a child, that proneness to evil, and that backwardness to good, which is the burden of the regenerate, and the ruin of the unregenerate. He is encouraged, in his repentance, to hope that God would graciously accept him. Thou desirest truth in the inward part; to this God looks, in a returning sinner. Where there is truth, God will give wisdom. Those who sincerely endeavour to do their duty shall be taught their duty; but they will expect good only from Divine grace overcoming their corrupt nature.

Against thee, thee only have I sinned. The virus of sin lies in its opposition to God: the psalmist's sense of sin towards others rather tended to increase the force of this feeling of sin against God. All his wrong doing centred, culminated, and came to a climax, at the foot of the divine throne. To injure our fellow men is sin, mainly because in so doing we violate the law of God. The penitent's heart was so filled with a sense of the wrong done to the Lord himself, that all other confession was swallowed up in a broken hearted acknowledgment of offence against him. And done this evil in thy sight. To commit treason in the very court of the king and before his eye is impudence indeed: David felt that his sin was committed in all its filthiness while Jehovah himself looked on. None but a child of God cares for the eye of God, but where there is grace in the soul it reflects a fearful guilt upon every evil act, when we remember that the God whom we offend was present when the trespass was committed. That thou mightest be justified when thou speakest, and be clear when thou judgest. He could not present any argument against divine justice, if it proceeded at once to condemn him and punish him for his crime. His own confession, and the judge's own witness of the whole transaction, places the transgression beyond all question or debate; the iniquity was indisputably committed, and was unquestionably a foul wrong, and therefore the course of justice was clear and beyond all controversy.