Let the high praises of God be in their mouth - Margin, as in Hebrew, in their throat. Literally, “Praises of God in their throat; and a sword of two edges in their hand.” That is, In the very work of executing the purposes of God on his enemies, there should be the feeling and the language of praise. Their hearts should be full of confidence in God; they should feel that they are engaged in his service; and while they defend themselves, or inflict punishment on the enemies of God, they should chant His praise. The idea is, that even in the work of war they might feel that they were engaged in the service of God, and that the passions usual in war should be subdued and kept under by the consciousness that they are mere instruments in the hand of God to accomplish His purposes. Perhaps the Hebrew word rendered “high praises” - רוממה rômemâh - may imply more than mere praise. It may embrace anything that is lofty and exalted, and may mean here that they would have the consciousness that they were engaged in high and lofty aims; that they were carrying out the great designs of God; that they were executing purposes more momentous than their own could be - even the eternal purposes of the Most High. This would give an importance, a dignity, an elevation to their conduct which could spring from no other source.
And a two-edged sword in their hand - literally, a sword of edges; that is, a sword with an edge on both sides of the blade. Roman swords were often made in this manner. They were made for piercing as well as for striking. See the notes at Hebrews 4:12.