Albert Barnes Commentary Psalms 135:19-21

"O house of Israel, bless ye Jehovah: O house of Aaron, bless ye Jehovah: O house of Levi, bless ye Jehovah: Ye that fear Jehovah, bless ye Jehovah. Blessed be Jehovah out of Zion, Who dwelleth at Jerusalem. Praise ye Jehovah." — Psalms 135:19-21 (ASV)

Bless the Lord, O house of Israel ... - This passage, also, is evidently an imitation of the passage in (Psalms 115:9–13). The form in (Psalms 115:0), however, is rather an exhortation to trust in the Lord, and an assurance that God would bless the classes spoken of, than a call on them to bless the Lord. Still the same classes of persons are referred to; the house of Israel; the house of Aaron; and those who feared the Lord. The passage needs no further illustration than what is found in the notes at (Psalms 115:9–13). It is an earnest call on all classes of the people to bless and praise the Lord.

It is language expressive of overflowing joy; the utterance of a heart full of exalted conceptions of the majesty, the glory, and the mercy of God; of a heart which feels to the utmost the fitness of praise, and desires that all classes of people—priests and people—that all created things should unite in the praise of Yahweh. Who, in reading the psalm, can fail to catch the feelings of the psalmist, and to say Amen and amen!