Charles H. Spurgeon on Psalms 138:4

All the kings of the earth shall praise thee, O Lord, when they hear the words of thy mouth. Kings have usually small care to hear the word of the Lord; but King David feels assured that if they do hear it they will feel its power. A little piety goes a long way in courts; but brighter days are coming, in which rulers will become hearers and worshippers: may the advent of such happy times be hastened. What an assembly! —"all the kings of the earth!" What a purpose! Gathered to hear the words of Jehovah's mouth. What a preacher! David himself rehearses the words of Jehovah. What praise! when they all in happy union lift up their songs unto the Lord. Kings are as gods below, and they do well when they worship the God above. The way of conversion for kings is the same as for ourselves: faith to them also cometh by hearing, and hearing by the word of God. Happy are those who can cause the word of the Lord to penetrate palaces; for the occupants of thrones are usually the last to know the joyful sounds of the gospel. David, the king, cared for kings' souls, and it will be wise for each man to look first after those who are of his own order. He went to his work of testimony with fall assurance of success: he meant to speak only the words of Jehovah's mouth, and he felt sure that the kings would hear and praise Jehovah.