Psalms 51:18

Do good in thy good pleasure unto Zion. Let blessings according to thy wont be poured upon thy holy hill and chosen city. Zion was David's favourite spot, whereon he had hoped to erect a temple. The ruling passion is so strong on him, that when he has discharged his conscience he must have a word for Zion. He felt he had hindered the project of honouring the Lord there as he desired, but he prayed God still to let the place of his ark be glorious, and to establish his worship and his worshipping people. Build thou the walls of Jerusalem. This had been one of David's schemes, to wall in the holy city, and he desires to see it completed; but we believe he had a more spiritual meaning, and prayed for the prosperity of the Lord's cause and people. He had done mischief by his sin, and had, as it were, pulled down her walls; he, therefore, implores the Lord to undo the evil, and establish his church. God can make his cause to prosper, and in answer to prayer he will do so. Without his building we labour in vain; therefore are we the more instant and constant in prayer. There is surely no grace in us if we do not feel for the church of God, and take a lasting interest in its welfare.

Do good in thy good pleasure unto Zion - This and the following verse most evidently refer to the time of the captivity, when the walls of Jerusalem were broken down, and the temple service entirely discontinued; and, consequently, are long posterior to the times of David. Hence it has been concluded that the Psalm was not composed by David, nor in his time and that the title must be that of some other Psalm inadvertently affixed to this. The fourth verse has also been considered as decisive against this title: but the note on that verse has considerably weakened, if not destroyed, that objection. I have been long of opinion that, whether the title be properly or improperly affixed to this Psalm, these two verses make no part of it: the subject is totally dissimilar; and there is no rule of analogy by which it can be interpreted as belonging to the Psalm, to the subject, or to the person. I think they originally made a Psalm of themselves, a kind of ejaculatory prayer for the redemption of the captives from Babylon, the rebuilding of Jerusalem, and the restoration of the temple worship. And, taken in this light, they are very proper and very expressive. The Psa 117:1-2 contains only two verses; and is an ejaculation of praise from the captives who had just then returned from Babylon. And it is a fact that this Psalm is written as a part of the cxvith in no less than thirty-two of Kennicott’s and De Rossi’s MSS.; and in some early editions. Again, because of its smallness, it has been absorbed by the cxviiith, of which it makes the commencement, in twenty-eight of Kennicott’s and De Rossi’s MSS. In a similar way I suppose the two last verses of this Psalm to have been absorbed by the preceding, which originally made a complete Psalm of themselves; and this absorption was the more easy, because, like the cxviith it has no title. […]

Do good in thy good pleasure unto Zion - From himself - his deep sorrow, his conscious guilt, his earnest prayer for pardon and salvation - the psalmist turns to Zion, to the city of God, to the people of the Lord. These, after all, lay nearer to his heart than his own personal salvation; and to these his thoughts naturally turned even in connection with his own deep distress. Such a prayer as is here offered he would also be more naturally led to offer from the remembrance of the dishonor which he had brought on the cause of religion, and it was natural for him to pray that his own misconduct might not have the effect of hindering the cause of God in the world. The psalms often take this turn. Where they commence with a personal reference to the author himself, the thoughts often terminate in a reference to Zion, and to the promotion of the cause of religion in the world. Build thou the walls of Jerusalem - It is this expression on which De Wette, Doederlein, and Rosenmuller rely in proof that this psalm, or this portion of it, was composed at a later period than the time of David, and that it must have been written in the time of the captivity, when Jerusalem was in ruins. See the introduction to the psalm. But, as was remarked there, it is not necessary to adopt this supposition. There are two other solutions of the difficulty, either of which would meet all that is implied in the language. (a) One is, that the walls of Jerusalem, which David had undertaken to build, were not as yet complete, or that the public works commenced by him for the protection of the city had not been finished at the time of the fatal affair of Uriah. […]