Psalms 51:18

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. There is nothing in the history which forbids this supposition, and the language is such as would be used by David on the occasion, if he had been actually engaged in completing the walls of the city, and rendering it impregnable, and if his heart was intensely fixed on the completion of the work.

(b) The other supposition is, that this is figurative language - a prayer that God would favor and bless his people as if the city was to be protected by walls, and thus rendered safe from an attack by the enemy. Such language is, in fact, often used in cases where it could not be pretended that it was designed to be literal. See Jude 1:20; Rom 15:20; 1 Corinthians 3:12; Galatians 2:18; Ephesians 2:22; Colossians 2:7.