The last three verses of Psalm 135 are devoted to blessing the Lord. There are several groups of people specifically mentioned that God desires to bless Him. To bless God means “to kneel; an act of adoration; praise; salute; thank.” (See Strong's Concordance)
Verse nineteen and the first part of verse twenty say, “19) Bless the LORD, O house of Israel: bless the LORD, O house of Aaron: 20) Bless the LORD, O house of Levi.” I find this order very interesting because Aaron is the descendant of Levi. Levi had three sons – Gershom, Kohath and Merari. Kohath’s son was Amram, and Amram was the father of Levi. However, Aaron was the first high priest of Israel and he and his descendants were of the priestly line. Levi was a son of Jacob, and the father of the tribe of Levites. The tribe of Levi served in the tabernacle. They also had various other duties and responsibilities toward the house of Israel. These three groups of people – house of Israel, house of Aaron, and house of Levi – were specifically mentioned by God to bless Him as they served Him.
There is one other group mentioned that God wants to bless Him. This group is found in the last part of verse twenty, “…ye that fear the LORD, bless the LORD.” All those who fear the Lord should bless Him. He is worthy of our praise and adoration and thanksgiving. We saw earlier in the Psalm that there is none greater than God. One day all the nations of the earth will come and worship before Him. He is in control of heaven and earth and all the powers of nature. Therefore, He is worthy of our praise and honor and adoration. “Blessed be the LORD out of Zion, which dwelleth at Jerusalem. Praise ye the LORD.” (verse 21)