Charles H. Spurgeon on Psalms 138:4

All the kings of the earth shall praise thee. In a sense sufficiently striking this promise was fulfilled to David, and to the nation of Israel, as surrounding monarchs beheld the wonderful dispensations of divine providence which attended their steps (2Sa 5:11 8:10); but in its completest sense, it shall realize its accomplishment in the future conquests of Messiah, when the princes and potentates of the earth receive his word, learn by divine grace to celebrate the glorious methods of his love, and see in the light of faith the greatness of Jehovah's glory as the God of salvation. "All the kings of the earth" shall yet praise the Lord, and shall hasten with their numerous subjects to hail the triumphs of his grace. —John Morison.