Charles H. Spurgeon on Psalms 138:5

Yea, they shall sing in the ways of the LORD. Here is a double wonder —kings in God's ways, and kings singing there. Let a man once know the ways of Jehovah, and he will find therein abundant reason for song; but the difficulty is to bring the great ones of the earth into ways so little attractive to the carnal mind. Perhaps when the Lord sends us a King David to preach, we shall yet see monarchs converted and hear their voices raised in devout adoration. For great is the glory of the LORD. This glory shall overshadow all the greatness and glory of all kings: they shall be stirred by a sight of it to obey and adore. O that Jehovah's glory were revealed even now! O that the blind eyes of men could once behold it, then their hearts would be subdued to joyful reverence. David, under a sense of Jehovah's glory, exclaimed, "I will sing" (Ps 138:1), and here he represents the kings as doing the same thing.