Psalms 51:18 - Joseph Benson’s Commentary on the Old and New Testaments

Do good in thy good pleasure unto Zion Hebrew, ברצונךְ, birtzonecha, for, or according to, thy grace, favour, or pleasure That is, thy free and rich mercy, and thy gracious purpose and promise, made to and concerning thy church and people, here termed Zion. Build the walls of Jerusalem Perfect the walls and buildings of that city, and especially let the temple be built and established in it, notwithstanding my great sins whereby I have polluted it, which I pray thee to purge away. But he may also be understood as speaking figuratively in these words, and praying for the enlargement and establishment of God's church, often meant by Jerusalem.