What does Psalm 51:18 mean?

In this verse David prays for Jerusalem. His disgraceful conduct had serious implications for Israel's security. Proverbs 14:34 declares, "Righteousness exalts a nation, but sin is a reproach to any people." When Achan sinned in the days of Israel's conquest of Canaan, the army of Israel could not conquer the tiny town of Ai until Achan's sin was judged. One man's sin had destroyed the nation's effectiveness (Joshua 7).

David refers to Jerusalem's walls in this verse. He asks God to build up the city's walls. Some Bible teachers believe verses 18 and 19 were added to Psalm 51 during the Babylonian exile. There is no compelling reason to think these verses were added later. It seems David was in the process of repairing and building the city's walls and longed for the successful completion of the project. Perhaps David's sins had distracted him from the task of rebuilding the walls, which would leave Jerusalem vulnerable to attacks. Before David's son Solomon built the temple, secure city walls had to be in place. We read in 1 Kings 3:1 that Solomon finished building the wall around Jerusalem.