Charles H. Spurgeon on Psalms 149:6

Let the high praises of God be in their mouth, and a two edged sword in their hand. It seems they are not always on their beds, but are ready for deeds of prowess. When called to fight, the meek are very hard to overcome; they are just as steady in conflict as they are steadfast in patience. Besides, their way of fighting is of an extraordinary sort, for they sing to God but keep their swords in their hands. They can do two things at a time: if they do not wield the trowel and the sword, at least they sing and strike. In this Israel was not an example, but a type: we will not copy the chosen people in making literal war, but we will fulfil the emblem by carrying on spiritual war. We praise God and contend with our corruptions; we sing joyfully and war earnestly with evil of every kind. Our weapons are not carnal, but they are mighty, and wound with both back and edge. The word of God is all edge; whichever way we turn it, it strikes deadly blows at falsehood and wickedness. If we do not praise we shall grow sad in our conflict; and if we do not fight we shall become presumptuous in our song. The verse indicates a happy blending of the chorister and the crusader. Note how each thing in the believer is emphatic: if he sings, it is high praises, and praises deep down in his throat, as the original hath it; and if he fights, it is with the sword, and the sword is two edged. The living God imparts vigorous life to those who trust him. They are not of a neutral tint: men both hear them and feel them. Quiet is their spirit, but in that very quietude abides the thunder of an irresistible force. When godly men give battle to the powers of evil each conflict is high praise unto the God of goodness. Even the tumult of our holy war is a part of the music of our lives.