Every day at Lauds in the Benedictine tradition, you pray Psalms 148-150; these Psalms, in fact, give this office its name of Laudes. These Psalms begin ‘Alleluia!’ and are filled with exhortations to praise the Lord — Laudate dominum in Latin.
In the midst of the praise, at Psalm 149:6, we meet this:
Let the praises of God be in their mouth, / and a two-edged sword in their hands;
For some reason, this image always sticks in me. Maybe it’s the rhythm of Coverdale’s verse. Arresting as it is, it’s not exactly the sort of thing Christians today are comfortable with, especially when we read that the sword is for vengeance. I wouldn’t be surprised if some of the mainline liberal Psalters have quietly expunged it along with the end of Psalm 137 (like Canada’s BCP).
Now, I haven’t checked any of the Fathers or medieval exegetes on this, but — what do we think that two-edged sword is?
The patristic and, therefore, medieval principles of interpreting the Bible are that the Bible is always right. The Bible interprets itself. The Bible is always about Christ and/or His mystical body, the Church. The literal sense is never to be ignored, but we are called to dig deeper through allegory, typology, etc. And, which should be common to all Christian reading of Scripture: Jesus trumps all.
Attempting, then, to think like the Fathers, we should admit that executing vengeance is something many of them would be uncomfortable with. Are there clues in the verse as to what it means for Christians today? Unlike a modern(ist) reading, this verse cannot be left as a historical relic. It speaks today, to our situation.
Well, where else do we see a two-edged sword in Scripture? Hebrews 4:12:
For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
This immediately draws us to Ephesians 6:17, in the discussion of the full armour of God, where the Sword of Spirit is the Word of God. What we don’t always think on is Ephesians 6:18, which is what we are to do whilst wearing this armour:
Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and supplication for all saints
We’ll come back to this.
Revelation has a few relevant sword references:
And he had in his right hand seven stars: and out of his mouth went a sharp twoedged sword: and his countenance was as the sun shineth in his strength. (Rev. 1:16)
Repent; or else I will come unto thee quickly, and will fight against them with the sword of my mouth. (Rev. 2:16)
And out of his mouth goeth a sharp sword, that with it he should smite the nations (Rev. 19:15)
And the remnant were slain with the sword of him that sat upon the horse, which sword proceeded out of his mouth: and all the fowls were filled with their flesh. (Rev. 19:21)
The Revelation verses are all about the two-edged sword coming out of the mouth of the visionary Christ, the Rider on the White Horse. It is not insignificant that the sword comes from his mouth — Christ is God the Word, after all. And our Ephesians and Hebrews verses refer to the word of God — in this case presumably Scripture — as being a sword.
What, then, is the two-edged sword of Psalm 149:6? It is the Word of God, being wielded by God’s people in battle against the Enemy — not men, but the world, the flesh, and the devil.
And when do we take up this two-edged sword? According to Ephesians 6:18, in prayer. The battle for man’s soul (the Psychomachia) occurs on our knees. Do not forget Saint Antony (fourth-century) when he was confronted with all the denizens of Hell. He proclaimed Psalm 27:3:
Though an host should encamp against me, my heart shall not fear
John Cassian (d. c. 435) recommends saying over and over again Psalm 70:1:
O God, make speed to save me; O Lord, make haste to help me.
Let us, then, take up this two-edged sword in our hands, and get on our knees and fight.
Classically Christian 
That passage isn’t saying we as saints be on our knees fighting however. Prayer is most certainly a powerful weapon. But that passage in particular is speaking in military terms. There are other passages that allude to the saints co-ruling with Christ with a “rod of iron”.
The nations that are disarming according to the passage in Isaiah are possibly doing so because the rule of Christ and His saints will be impossible to overcome. The passage simply says “many” will disarm but it doesn’t say “all” would disarm.
Moreover the first mention of a weapon in the Bible is the sword from a Cherubim. How were angels allowed to carry swords and where did the Cherubim get it from? The Word of God must have allowed it. This sword appears to have been a literal weapon but with unknown technology…what kind of sword is flaming and turns in all directions?
So yes it appears the saints will be armed against the elites of this world upon Christ’s return but in the meantime, saints are to be subject to authority. These weapons will not be of this world….it will be impossible to go against Christ and His saints…but nonetheless the world will try to go against it anyway.