This psalm purports to be a psalm of David, and the contents of it accord with this supposition, and with the statement in the title in regard to the occasion on which it was composed. There would be no difficulty on the subject, and no ground for hesitation, in regard to the author and the occasion on which it was composed, if it were not for the prayer in Psalm 51:18, "Do good in thy good pleasure unto Zion; build thou the walls of Jerusalem," which, it has been maintained by DeWette, Rosenmuller, Venema, and others, must have been written in the time of the Babylonian cxile. Except this, it is admitted on all hands that the psalm in its composition accords entirely with the statement in the title, that it was composed by David. It has, in fact, been generally admitted that the psalm "was" composed by David, though it is the opinion of Rosenmuller, Venema, and Doederlein, that the last two verses were added by a later hand.
According to the title, the psalm was composed on occasion of the great fault and crime in the life of David, and as an expression of his penitence in view of his sin. On the phrase "To the chief Musician," see the notes at the title to Psalm 4:1-8. We are not to suppose that this title was prefixed to the psalm by David himself, but the use to be made of it by committing it to the "chief Musician," or to the overseer of the music in the public worship of God shows that the psalm was considered as designed to be used in public, and was not a mere expression of the private feelings of the author. It was, doubtless, commonly understood (and was probably so intended by David himself) that it was to be used as a "public" expression of his penitence in view of his crane; and both the fact of its composition, and the manner in which it was to be used, were to be interpreted as indicating his willingness that the widest publicity should be given to his confession, and that the memory of the crime and of his penitence should be perpetuated in all ages of the world. The phrase in the title, "A "Psalm" of David," denotes that it was to be used for public worship, or as connected with praise. It was designed not merely to express his private feelings, but was intended to be employed in the solemn services of public devotion. See introduction to Psalm 3:1-8.
The phrase "when Nathan the prophet came unto him," refers to the fact recorded in 2-Samuel 12:1-13. It means that the psalm was the "result" of the visit of Nathan to him; or that it records the feelings of the author, when the sense of his sin had been brought to his mind by the faithful message of the prophet. We may suppose that the record of his feelings was made without delay, for the psalm bears all the marks of having been composed under the deepest feeling, and not of being the result of calm reflection. On the phrase "after he had gone in to Bath-sheba," see the sad record in 2-Samuel 11:1-5.
DeWette, however, maintains that psalm could not have been composed David, but that it must have been in the time of the Babylonian exile. The only argument which he adduces in favor of this opinion is the prayer in Psalm 51:18, "Build thou the walls of Jerusalem," which, he says, could not have been a prayer offered by David, as there was in his time nothing which would make this prayer proper. Jerusalem was not then in ruins. It had been strongly fortified by David himself, and required no particular interposition of God as if to "restore" walls that had been thrown down; whereas, in the time of the exile, such a prayer would have been eminently proper, and would be a natural petition for one who loved his country, and who, as an expression of his own penitence, was desirous of doing all he could for the cause of religion. The difficulty will be more appropriately met in the notes at those verses.
It may be observed here, however, that possibly the expression "Build thou the walls of Jerusalem," "may" be used in a figurative or spiritual sense, expressive of a desire that God would bless his people; that he would interpose in their behalf; that he would be their protector and friend; that he would do for them what would be well expressed by building strong and secure wails around a city. But it may be asked, also, Is it absolutely certain that when the psalm was composed the work of enclosing the city of Jerusalem with walls had been completed? May it not have been, in fact, that at that very time David was engaged in "carrying out" his design of rendering the city impregnable by walls and towers, and that in the midst of his intense sorrow for his own sin, though so heinous and aggravated, his heart may have trurned to that which was so dear to him as an object to be accomplished, and that even then, in connection with his bitter repentance for his sin, he may have prayed that God would favor that great design?
It is no evidence that our sorrow for sin is not deep and genuine, that, even in our expressions of penitence, our heart turns to Zion - to the Church - to the great work which the Church is accomplishing - and that, though our prayers "began" with a reference to our own sin, they should "close" with a petition that God would bless his people, and fulfill the great purposes so near to the heart of piety in reference to the progress of true religion in the world. Indeed, from the very narrative in 2 Sam. 6-12. it would seem probable that the work of fortifying the city of Jerusalem, contemplated by David, was not yet completed, when he committed the crime for which this psalm is the expression of penitence. It was a work of years to do this: and it is not improbable that the guilty transaction to which this psalm refers occurred in the very midst of his design for the defense and protection of the capital of his kingdom.
The psalm consists of two parts:
I. In the first Psalm 51:1-12, the psalmist confesses his guilt, and prays for pardon. He begins with an earnest plea for mercy Psalm 51:1-2; he humbly acknowledges his offence, without any attempt to vindicate himself, or to apologise for it Psalm 51:3-6; he pleads with God to cleanse him, to pardon him, to create in him a new heart, and not to cast him off or to take his Holy Spirit from him Psalm 51:7-12.
II. In the second part Psalm 51:13-19 he shows how he would manifest his sense of the divine mercy if he was forgiven: expressing the purpose to lead a new life; to devote himself to the duties of religion; to do all in his power to repair the evils of his conduct, and especially to induce others to avoid the way of sin, warning them by his example. He says that he would teach transgressors the true ways of God, and that sinners would be converted to Him, Psalm 51:13; that he would sing aloud the praise of God, Psalm 51:14-15; that he would offer to God the sacrifice of a broken heart and a contrite spirit, Psalm 51:16-17; and he then pleads Psalm 51:18-19, that God would interpose and bless Zion, that the great work might be completed in which he had been engaged in defending the city, and in preparing a place which would be secure, where God might be worshipped, and where sacrifices and offerings might perpetually ascend on his altar.

The psalmist, with a deeply penitent heart, prays for remission of sins, Psalm 51:1-4; which he confesses, and deeply deplores, Psalm 51:5-14; states his willingness to offer sacrifice, but is convinced that God prefers a broken heart to all kinds of oblations, Psalm 51:15-17; prays for the restoration of the walls of Jerusalem, and promises that then the Lord's sacrifice shall be properly performed, Psalm 51:18, Psalm 51:19.
The title is long: "To the chief Musician, A Psalm of David, when Nathan the prophet came unto him, after he had gone in to Bath-sheba." The propriety of this title has been greatly suspected, says Bishop Horsley: "That this Psalm was not written on the occasion to which the title refers, is evident from the Psalm 51:4 and Psalm 51:18. The Psalm 51:4 ill suits the case of David, who laid a successful plot against Uriah's life, after he had defiled his bed: and the Psalm 51:18 refers the Psalm to the time of the captivity, when Jerusalem lay in ruins." Dr. Kennicott is of the same mind. He says: "The title is misplaced; that it was written during the captivity, and the cessation of the temple worship; the author under great depression of mind, arising from the guilt of some crime, probably some compliance with heathen idolatry, not murder nor adultery; is plain from the Psalm 51:4, "Against Thee Only have I sinned."
The crime mentioned in the title was not only against God, but against the whole order of civil society; against the life of the noble and valiant captain whose wife Bath-sheba was, and against every thing sacred in friendship and hospitality. It was a congeries of sins against God and society. Were it not for the Psalm 51:4, Psalm 51:18, and Psalm 51:19, the rest of the Psalm would accord well enough with the title, and the deep penitence it expresses would be suitable enough to David's state. But see on Psalm 51:4 (note), Psalm 51:18-19 (note).

INTRODUCTION TO PSALM 51
To the chief Musician, A Psalm of David, when Nathan the prophet came unto him, after he had gone in to Bathsheba. The occasion of this psalm was the sin of David with Bathsheba, signified by "going in to her"; an euphemism for "lying with her"; which sin was a very aggravated one, she being another man's wife, and the wife of a servant and soldier of his, who was at the same time exposing his life for his king and country's good; and David besides had many wives, and was also king of Israel, and should have set a better example to his subjects; and it was followed with other sins, as the murder of Uriah, and the death of several others; with scandal to religion, and with security and impenitence in him for a long time, until Nathan the prophet was sent to him of God, to awaken him to a sense of his sin; which he immediately acknowledged, and showed true repentance for it: upon which, either while Nathan was present, or after he was gone, he penned this psalm; that it might remain on record, as a testification of his repentance, and for the instruction of such as should fall into sin, how to behave, where to apply, and for their comfort. The history of all this may be seen in the eleventh and twelfth chapters of the second book of Samuel.

(Psalm 51:1-6) The psalmist prays for mercy, humbly confessing and lamenting his sins.
(Psalm 51:7-15) He pleads for pardon, that he may promote the glory of God and the conversion of sinners.
(Psalm 51:16-19) God is pleased with a contrite heart, A prayer for the prosperity of Zion.

Penitential Prayer and intercession for Restoration to Favour
The same depreciation of the external sacrifice that is expressed in Ps 50 finds utterance in Ps 51, which supplements the former, according as it extends the spiritualizing of the sacrifice to the offering for sin (cf. Psalm 40:7). This Psalm is the first of the Davidic Elohim-Psalm. The inscription runs: To the Precentor, a Psalm by David, when Nathan the prophet came to him, after he had gone in to Bathsheba. The carelessness of the Hebrew style shows itself in the fact that one and the same phrase is used of Nathan's coming in an official capacity to David (cf. 2-Samuel 12:1) and of David's going in unto Bathsheba (בּוא אל, as in Genesis 6:4; Psalm 16:2, cf. 2-Samuel 11:4). The comparative כּאשׁר, as a particle of time in the whole compass of the Latin quum, holds together that which precedes and that which subsequently takes place. Followed by the perfect (2-Samuel 12:21; 1-Samuel 12:8), it has the sense of postquam (cf. the confusing of this כאשׁר with אחרי אשׁר, Joshua 2:7). By בּבוא the period within which the composition of the Psalm falls is merely indicated in a general way. The Psalm shows us how David struggles to gain an inward and conscious certainty of the forgiveness of sin, which was announced to him by Nathan (2-Samuel 12:13). In Psalm 6:1-10 and Psalm 38:1 we have already heard David, sick in soul and body, praying for forgiveness; in Ps 51 he has even become calmer and more cheerful in his soul, and there is nothing wanting to him except the rapturous realization of the favour within the range of which he already finds himself. On the other hand, Psalm 32:1-11 lies even beyond Ps 51. For what David promises in Psalm 51:15, viz., that, if favour is again shown to him, he will teach the apostate ones the ways of God, that he will teach sinners how they are to turn to God, we heard him fulfil in the sententious didactic Psalm 32:1-11.
Hitzig assigns Ps 51, like Ps 50, to the writer of Isaiah 40:1. But the manifold coincidences of matter and of style only prove that this prophet was familiar with the two Psalm. We discern in Ps 51 four parts of decreasing length. The first part, Psalm 51:3, contains the prayer for remission of sin; the second, Psalm 51:12, the prayer for renewal; the third, Psalm 51:16, the vow of spiritual sacrifices; the fourth, vv. 20, 21, the intercession for all Jerusalem. The divine name Elohim occurs five times, and is appropriately distributed throughout the Psalm.