By John Piper:
In New Testament times swords were not for digging, shaving, or whittling. They were for killing. The only reason Peter cut off Malchus’s ear was that he missed (John 18:10).
But Herod didn’t miss: “He killed James the brother of John with the sword” (Acts 12:2).
Many saints have felt the full force of the sword: “They were stoned, they were sawn in two, they were killed with the sword” (Hebrews 11:37). So it was and will be: “If anyone is to be slain with the sword, with the sword must he be slain” (Revelation 13:10).
That’s what swords are for. So when Paul calls the word of God the “sword of the Spirit” in Ephesians 6:17, he is serious—something must be put to death. And it is not people. Christians don’t kill people to spread our faith; we die to spread our faith.
The link in Paul’s mind is given in Romans 8:13.
If by the Spirit you put to death the deeds of the body, you will live.
The word of God is the sword of the Spirit. The Sword is for putting to death. And by the Spirit we put to death our sinful deeds. So I conclude that the way we kill our sins is with the Spirit’s sword, the word of God.
All temptations to sin have power by lying. The are “deceitful desires” (Ephesians 4:22). They tell us that the pleasure of the sin is worth it. The killing blow against these lies is the power of God’s truth. Hence the sword of the Spirit, God’s word, is the weapon to use.
As John Owen said, “Be killing sin, or it will be killing you.” That is what swords are for, especially the Bible.