The phrase “sword of the Spirit” is found only once in Scripture, in Ephesians 6:17. The sword is one of the pieces of spiritual armor Paul tells the Ephesian Christians to put on as part of the “full armor of God” that will enable us to stand our ground against evil (Ephesians 6:13).
The sword is both an offensive and defensive weapon used by soldiers or warriors. In this case it is a weapon belonging to the Holy Spirit. Swords were used to protect oneself from harm or to attack the enemy to overcome or kill him. In both cases it was necessary for a solider to get rigid training on the proper use of the sword to get maximum protection. All Christian soldiers need the same rigid training to know how to properly handle the Sword of the Spirit, “which is the word of God.” The sword that Paul refers to here is the Holy Scriptures. We know from 2 Timothy 3:16–17 that the word of God is from the Holy Spirit and written by men. Since every Christian is on the spiritual battle with the satanic and evil forces of this world, we need to know how to handle the Word properly. Only then will it be an effective defense against evil, but it will also be an offensive weapon we use to “demolish strongholds” of error and falsehood (2 Corinthians 10:4–5).
God refers to His Word as a sword in Hebrews 4:12. Here the Word is described as living and active and sharper than a double-edged sword. The Roman sword was commonly made in this manner. The fact that it had two edges made it easier to penetrate, as well as to cut in every way. The idea is that of piercing, or penetrating; the Word of God reaches the “heart,” the very center of action, and lays open the motives and feelings of those it touches.
The purpose of the sword of the Spirit—the Bible—is to make us strong and able to withstand the evil onslaughts of Satan, our enemy (Psalm 119:11, 33–40, 99–105). The Holy Spirit uses the power of the Word to save souls and then to give them spiritual strength to be mature soldiers for the Lord in fighting this corrupt and evil world we live in. The more we know and understand the Word of God, the more useful we will be in doing the will of God and the more effective we will be in standing against the enemy of our souls.