Depending on your translation, we might well run over Psalm 149 without much thought, but one word in a different translation makes a difference.
The NIV (1984) issues a call to sing praise in the assembly of the “saints.” But the word for “saint” there is not the normal one we think of. All of God’s people are “saints,” “holy ones” set apart by God for Himself and Him alone. But the word carries with it greater meaning than that.
The Old Testament speaks of God’s “faithfulness,” His devotion – even when it is undeserved. This is His “love,” His “grace.” The people addressed in Psalm 149 are not just the recipients of His faithfulness, they have adopted the faithfulness of God in their lives and are “devoted” to Him as He is to them. Thus newer translations speak of the assembly of the “faithful people” or “the committed” or “the devoted.”
We want to be careful with this psalm. The writer mentions a number of ways of praising the Lord – including the use of dancing. While David danced in praise before the Lord, it does not appear that dancing was a part of the temple ritual. Note also the committed ones sing for joy “on their beds.” Likewise, it doesn’t have reference to some kind of bed ceremony in the temple. Rather, all these mentions of ways of worship and praise have to do with the normal course of life, and the call to praise is the call to all those committed to the Lord to praise God in every station and period of their lives. This includes, when God calls to war, marching out in the confidence of His calling.