Psalms 51:18

18. Do good… unto Zion As a king he feels he has exposed his people and kingdom, no less than himself, to judgments, and these public calamities had been sternly foretold by Nathan. 2 Samuel 12:10-12. They must suffer with him; yea through their suffering the king more profoundly suffers.

Build… the walls of Jerusalem A figurative expression for the prosperity and strength of the nation, and as a proof or symbol of the divine protection. See Psalms 69:35. Or, it may be an allusion to the unfinished walls and fortifications in David’s time, which were completed in Solomon’s reign. 1 Kings 3:1; 1Ki 9:15 ; 1 Kings 9:19. It is not necessary to suppose these last two verses of the psalm to have been added by the returned exiles, much less that they are proof that the entire psalm was written at that date and by another hand than David’s. The word rendered “build” is never, in our version, translated rebuild, and seldom bears that sense. Hitherto David’s wars had been carried on in the enemy’s country, and it would be but natural that he should anticipate the possibility of a recoil of his foreign victories by an invasion that would test the strength of the fortifications of his capital. Having finished his penitential prayer for himself, and with his eye on the fiery denunciation of the prophet above alluded to, “the sword shall never depart from thy house,” his kingly heart turns to his people in prayer for their safety and the stability of his kingdom.