(18) Do good.--The last two verses have occasioned much controversy. They do not fit in well with the theory of Davidic authorship, Theodoret long ago saying that they better suited the exiles in Babylon. They seem at first sight to contradict what has just been asserted of sacrifice. On both grounds they have been regarded as a liturgical addition, such as doubtless the compiler made, without any sense of infringement of the rights of authorship. On the other hand, it is not only these two verses which harmonise with the feelings of the restored exiles, but the whole psalm, and the contradiction in regard to the worth of sacrifices is only apparent. While vindicating spiritual religion, the psalmist no more abrogates ceremonies than the prophets do. As soon as their performance is possible they will be resumed.

Verses 18, 19. - That this is an addition made to the original psalm, during the time of the Babylonian exile, or later, for liturgical purposes, has been maintained by a large number of the commentators who ascribe the rest of the psalm to David. The chief ground for the supposition is the prayer in ver. 18, "Build thou the walls of Jerusalem," which has been supposed to imply that the walls were in ruins, whereas under David they should have been, it is thought, in good condition. But it has been pointed out, very justly, that the fortifications of Jerusalem were not complete in David's time, and that both he and Solomon added considerably to them (2 Samuel 5:9; 1 Kings 3:1; 1 Kings 9:15, 19). David may well have thought that, as a punishment for his sin, God might interfere with the work which he was doing for the benefit of his people, and hence have felt it needful to pray, "Do good unto Zion: build thou the wails of Jerusalem." Verse 18. - Do good in thy good pleasure unto Zion. It is characteristic of David to pass from prayer for himself to prayer for the people committed to him, and especially to do so at or near the end of a psalm (see Psalm 5:11, 12; Psalm 25:22; Psalm 28:9; Psalm 40:16). And he closely connects - nay, identifies - the people with their capital city (see Psalm 46:4; Psalm 48:11: 69:35, etc.). Build thou the walls of Jerusalem. Josephus says that David encompassed the whole city of Jerusalem with walls ('Ant. Jud.,' 7:3, § 2); and we are told, in the Second Book of Samuel, that he "built round about from Mille and inward." It has been argued that his walls were just approaching their completion at the time of his great sin (Christian Observer, No. 333).

51:16-19 Those who are thoroughly convinced of their misery and danger by sin, would spare no cost to obtain the remission of it. But as they cannot make satisfaction for sin, so God cannot take any satisfaction in them, otherwise than as expressing love and duty to him. The good work wrought in every true penitent, is a broken spirit, a broken and a contrite heart, and sorrow for sin. It is a heart that is tender, and pliable to God's word. Oh that there were such a heart in every one of us! God is graciously pleased to accept this; it is instead of all burnt-offering and sacrifice. The broken heart is acceptable to God only through Jesus Christ; there is no true repentance without faith in him. Men despise that which is broken, but God will not. He will not overlook it, he will not refuse or reject it; though it makes God no satisfaction for the wrong done to him by sin. Those who have been in spiritual troubles, know how to pity and pray for others afflicted in like manner. David was afraid lest his sin should bring judgements upon the city and kingdom. No personal fears or troubles of conscience can make the soul, which has received grace, careless about the interests of the church of God. And let this be the continued joy of all the redeemed, that they have redemption through the blood of Christ, the forgiveness of sins according to the riches of his grace.Do good in thy good pleasure unto Zion,.... This verse, and Psalm 51:19, are thought, by a Spanish Rabbi mentioned by Aben Ezra, to have been added by one of the holy men that lived in the time of the Babylonish captivity; though rather it is thought, by the latter, to be written by David, under a spirit of prophecy, concerning, times to come; and so Kimchi thinks they are prophetic of future things; of the destruction of the first and second temple, and of the acceptableness of sacrifices in the times of the Messiah: and by Zion is meant the church, under the Gospel dispensation, Hebrews 12:22; and the "good" prayed for includes all the good and glorious things spoken of the church of Christ in the latter day; such as an increase of its numbers, the bringing in the fulness of the Gentiles, the conversion of the Jews, and the kingdoms of this world becoming the kingdoms of our Lord and of his Christ; the spread of the Gospel all over the world, the purity of Gospel doctrine, worship, and ordinances, the spirituality of religion, the power of godliness, and an abounding of brotherly love, and the like. The "good pleasure" of God, in which this is desired to be done, may denote either , "the acceptable time"; or "time of good pleasure"; the Gospel dispensation, so called Isaiah 49:8, in which it has been foretold, and may be expected these things shall be done; or else the cause, source, and spring of them, which is the sovereign good will and pleasure of God, from whence flow all the blessings of grace and goodness;

build thou the walls of Jerusalem; not literally taken; for these do not appear to have stood in need of being repaired or rebuilt in David's time; but the church of God, which is a spiritual house, built up of lively stones, true believers; which may be said to be more and more built up by an addition of such unto it: it is as a city compact together, whose walls are salvation, and its gates praise, Isaiah 26:1; of the wall of the new Jerusalem, see Revelation 21:12.