Albert Barnes Commentary Psalms 149:9

"To execute upon them the judgment written: This honor have all his saints. Praise ye Jehovah." — Psalms 149:9 (ASV)

To execute upon them the judgment written - This means either what is written in the law in general as a threat to wicked men, or what was written for their particular case, or what they were specifically commanded to do. (Deuteronomy 32:41–43). Most probably, the reference is to some particular command in this case.

This honor have all his saints -

1. It is an honor to engage in executing or carrying out the purposes of God. As it is an honor to be a magistrate, a judge, a sheriff, a constable, a commander of an army, or an admiral in a navy, to execute the purposes of a government—an honor sought with great eagerness among people as one of the most valued distinctions of life—why should it be less honorable to execute the purposes of God? Are the objects He seeks in His administration less important than those sought among people? Are His laws of less importance? Are His aims less pure? Is there less justice, equity, and benevolence in His plans?
2. It is an honor that pertains to “all the saints”—to all who love and fear God—to be engaged in carrying out or executing His plans. In their own way and in their own sphere—it may, indeed, be a very humble sphere—each and all in their own sphere are engaged in executing the purposes of God. In the duties of a family; in kindness to the poor; in the office of a teacher or a magistrate; in clearing a farm; in cultivating the land; in building a schoolhouse; in founding a church, a college, an asylum for the blind, the mute, the lame, or the insane; in contributing to send the gospel abroad over our own land or among pagans, or in going to carry that gospel to a benighted world—in some of these ways, all who are truly the friends of God, or who are entitled to be enrolled among the “saints of the Lord,” are, in fact, carrying out the purposes of the Lord—the “judgments written” to guide humankind. Indeed, one’s highest honor here, as it will be in heaven, is to carry out the purposes of the Lord.

Praise ye the Lord - Hallelujah. It is a subject of praise and thanksgiving; it should lead us to shout Hallelujah that we are permitted to be employed in any way, however humble, in carrying out the divine plans or in accomplishing those great designs He contemplates toward humanity.