This Hymn of Praise divides into two surprising parts that may be seen in this way:
(Vv 1–4) Let us praise God with song
(Vv 5–9) Let us praise God with a sword
The first half invites God’s people to “Sing to the LORD a new song!” because he is both our Maker and our King. This should lead to exuberant worship in which each worshiper plays a part in giving honor to God. In this exhilarating exercise even dance has a place (this is practiced among Orthodox Jews; the movie “Fiddler on the Roof” is a good example of this kind of praise).
The church is given a similar missionary charge, to extend the kingdom of Christ throughout the earth (Matthew 28.18–20). While our power is not military, it is by means of the same Word of God that we engage in this work. Military metaphors are used throughout the New Testament to picture this work:
“For though we walk in the flesh, we are not waging war according to the flesh. For the weapons of our warfare are not of the flesh but have divine power to destroy strongholds. We destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ, being ready to punish every disobedience, when your obedience is complete” (2 Corinthians 10.3-6; see also Ephesians 6.13–17).
It is important for us to remember that when Jesus announced his public ministry in Nazareth, he read in the synagogue meeting from the Isaiah scroll:
“The Spirit of the Lord God is upon me, because the Lord has anointed me to bring good news to the poor; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those who are bound; to proclaim the year of the Lord's favor” (Isaiah 61.1–2a).
He ended his reading in the middle of verse 2, not reading the remainder of the verse:”…and the day of vengeance of our God” (Isaiah 61.2b).
He didn’t finish the sentence not because vengeance is not a part of his commission as the Messiah-King, but because that is part of his second appearance, not his first. He came in humility the first time to “seek and save the lost” (Luke 19.10); he will appear a second time “in flaming fire, inflicting vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus” (2 Thessalonians 1.8).
It is an honor for all God’s people to be a part of extending his gracious rule into the lives of people all over the world (Psalm 149.9). We do this as our righteous lives and words shine the light of the gospel to both reveal the disfiguring effects of sin in people’s lives and to show them the way to God.
Collect for Psalm 149: Lord, let Israel rejoice in you and acknowledge you as creator and redeemer. In your loving-kindness embrace us now, that we may proclaim the wonderful truths of salvation with your saints in glory; through your Son, Jesus Christ our Lord. 
 Collect for Psalm 149, For All the Saints, A Prayer Book for and by the Church © 1995 The American Lutheran Publicity Bureau, Delhi, NY