John Calvin Commentary Psalms 149:5

"Let the saints exult in glory: Let them sing for joy upon their beds." — Psalms 149:5 (ASV)

They shall rejoice. In mentioning here joy, jubilee, and the high praises of God, he shows even more clearly from the effects it would produce that he is not speaking of a common benefit from God. For if the deliverance of the people had not been remarkable, there would have been no occasion for such joy, and even triumph.

And by these expressions, he suggests that the people would not be brought back from exile only to be immediately dispersed again, but to flourish in the enjoyment of every kind of blessing. For this reason, he mentions couches, teaching them to expect daily rest under the divine protection.

He declares that they would be equipped with arms and power, not only to ward off enemies but also to put them to flight on every side, thereby reducing to subjection kings and nations that formerly ruled over them. By swords of a double-mouth, or two-edged, are meant those that cut on both sides, because at that time swords only had one edge.