Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Psa 149:1-5) Joy to all the people of God.
(Psa 149:6-9) Terror to their enemies.
New mercies continually demand new songs of praise, upon earth and in heaven. And the children of Zion have not only to bless the God who made them, but to rejoice in him, as having created them in Christ Jesus unto good works, and formed them saints as well as men. The Lord takes pleasure in his people; they should rejoice in Him. When the Lord has made sinners feel their wants and unworthiness, he will adorn them with the graces of his Spirit, and cause them to bear his image, and rejoice in his happiness for ever. Let his saints employ their waking hours upon their beds in songs of praise. Let them rejoice, even upon the bed of death, assured that they are going to eternal rest and glory.
Some of God's servants of old were appointed to execute vengeance according to his word. They did not do it from personal revenge or earthly politics, but in obedience to God's command. And the honour intended for all the saints of God, consists in their triumphs over the enemies of their salvation. Christ never intended his gospel should be spread by fire and sword, or his righteousness by the wrath of man. But let the high praises of God be in our mouths, while we wield the sword of the word of God, with the shield of faith, in warfare with the world, the flesh, and the devil. The saints shall be more than conquerors over the enemies of their souls, through the blood of the Lamb and the word of his testimony. The completing of this will be in the judgement of the great day. Then shall the judgement be executed. Behold Jesus, and his gospel church, chiefly in her millennial state. He and his people rejoice in each other; by their prayers and efforts they work with him, while he goes forth in the chariots of salvation, conquering sinners by grace, or in chariots of vengeance, to destroy his enemies.