Albert Barnes Commentary Psalms 132:9

"Let thy priest be clothed with righteousness; And let thy saints shout for joy." — Psalms 132:9 (ASV)

Let your priests be clothed with righteousness - This is also substantially the same language that was used by Solomon at the dedication of the temple. See again (2 Chronicles 6:41).

The idea is, that in the service of such a God, the priests, the ministers of religion, should be holy. The honor of religion demanded it. It was the first qualification of those who “served the altar”: a qualification without which all other endowments would be valueless.

On the word clothed, see the notes at (Psalms 35:26); compare (Psalms 65:13); (Psalms 93:1); (Psalms 104:1); (Isaiah 61:10); (1 Peter 5:5).

And let your saints shout for joy - Your holy ones; all who truly worship and honor you. Let them be happy in such a God; in your presence; in your service. The fact that there is a God, and such a God, and that this God is ours - that we may serve him, glorify him, enjoy him - is suited to fill the mind with joy.