Charles H. Spurgeon on Psalms 51:18

Do good in thy good pleasure unto Zion. Let blessings according to thy wont be poured upon thy holy hill and chosen city. Zion was David's favourite spot, whereon he had hoped to erect a temple. The ruling passion is so strong on him, that when he has discharged his conscience he must have a word for Zion. He felt he had hindered the project of honouring the Lord there as he desired, but he prayed God still to let the place of his ark be glorious, and to establish his worship and his worshipping people. Build thou the walls of Jerusalem. This had been one of David's schemes, to wall in the holy city, and he desires to see it completed; but we believe he had a more spiritual meaning, and prayed for the prosperity of the Lord's cause and people. He had done mischief by his sin, and had, as it were, pulled down her walls; he, therefore, implores the Lord to undo the evil, and establish his church. God can make his cause to prosper, and in answer to prayer he will do so. Without his building we labour in vain; therefore are we the more instant and constant in prayer. There is surely no grace in us if we do not feel for the church of God, and take a lasting interest in its welfare.