Albert Barnes Commentary Psalms 51:18

"Do good in thy good pleasure unto Zion: Build thou the walls of Jerusalem." — Psalms 51:18 (ASV)

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.

He would also be more naturally led to offer such a prayer from remembering the dishonor he had brought on the cause of religion. It was natural for him to pray that his own misconduct would not hinder God's cause in the world.

The psalms often take this turn. When they begin with a personal reference to the author himself, the thoughts often end with a reference to Zion and to the promotion of God's cause in the world.

Build thou the walls of Jerusalem - De Wette, Doederlein, and Rosenmuller rely on this expression as 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 accept this supposition. There are two other solutions to the difficulty, either of which would address all that is implied in the language.

1. One is that the walls of Jerusalem, which David had undertaken to build, were not yet complete, or that the public works he had commenced 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, rendering it impregnable, and if his heart was intensely fixed on the completion of the work.
2. The other supposition is that this is figurative language—a prayer that God would favor and bless His people as if the city were to be protected by walls, and thus made safe from an enemy attack. Such language is, in fact, often used in cases where it could not be claimed that it was intended to be literal. (Romans 15:20; 1 Corinthians 3:12; Galatians 2:18; Ephesians 2:22; Colossians 2:7).