Albert Barnes Commentary Psalms 89:5

"And the heavens shall praise thy wonders, O Jehovah; Thy faithfulness also in the assembly of the holy ones." — Psalms 89:5 (ASV)

And the heavens shall praise thy wonders, O Lord — That is, the inhabitants of heaven will find new occasion for praise in the faithfulness demonstrated in carrying out the promise to David, and in the marvelous things that will occur under that promise, and in its accomplishment. If we suppose that this promise embraced the Messiah and his reign, then we will see what new occasions the angels would find for praise: in the incarnation of the Redeemer, and in all that would be accomplished by him.

Thy faithfulness also in the congregation of the saints — In the assembly of the holy ones; that is, the angels. In their songs of praise, this will be among the things that will fill them with joy. The idea is that the inhabitants of the heavens—the holy angels—would take a deep interest in the fulfillment of this promise, as it would provide new manifestations of the character of God. (1 Peter 1:12.)