Albert Barnes Commentary Psalms 138:5

"Yea, they shall sing of the ways of Jehovah; For great is the glory of Jehovah." — Psalms 138:5 (ASV)

Yes, they shall sing in the ways of the Lord - In the ways which God has appointed. They shall join with all that love him - with the humblest of the people - in acknowledging God. Kings and people shall thus bow before God in common acts of praise, and as being on the same level before him. As people, as sinners, as redeemed, as traveling to the grave, they are all alike before God.

For great is the glory of the Lord - Great is his character; great his dignity; great his honor; and all this will be seen to be so when those of most exalted rank thus worship and adore him. The most lofty on earth shall acknowledge that there is one who is more exalted than they are, and their own dignity and splendor shall thus contribute to deepen the impression of the honor and glory of God.