The peoples of this world are hostile to God's rule. But they need to be taught that God is King in spite of their rejection of him, since it is only in this way that people can begin to understand that sin is rebellion against God and begin to realize how serious sin is. The people who must teach them this are those who have accepted God's rule and praise him as King, which is what the writer of Psalm 149 does. 

I have been emphasizing in these final studies of Psalms that worship is a serious mental activity. It involves hard thinking, and it is possible only because of God's prior revelation in the Bible, which means that we must begin by studying that book. In order to praise God we must know who God is, and the only way we can know who he is and what he has done is by God's own disclosure of himself in Scripture. 

Psalm 149 encourages us to think about singing and what it means for the people of God. “Sing” is the psalm's first word after "Hallelujah," and what we are told to sing is "a new song" (v. 1). 

Here is where we need to see ourselves in the picture, if we have come to know God in Jesus Christ. For this is the importance of Israel, that it was through this people, providentially preserved by God, that the divine drama of redemption was unfolded. It was through Israel that the Messiah Savior came. Therefore, it is in the company of those who believe on that Savior that what was begun in the past and is referred to by the psalmist here was brought to fulfillment. It is through the church and her message that God may be seen and worshiped now. 

In the final analysis, that is what we all do apart from God's grace in drawing us to faith in Jesus Christ; we put ourselves in God's place. Adam and Eve did it in Eden. Nebuchadnezzar did it in his prideful boast over Babylon (Dan. 4:30). We do it too, often subtly—we put our own interests before God's or other people's—but also blatantly sometimes.