What does Ephesians 6:17 mean?

Helmets are essential in battle. A helmet can protect against stones, hand weapons, projectiles, fists, impacts with the ground or other attacks aimed at the head. Soldiers knew one hit to the head could mean disaster in battle. For this reason, the helmet does more to put a soldier at ease than almost any other piece of armor. Paul associates the helmet in the armor of God with salvation. Salvation is ultimately the best protection against Satan, since nothing, even Satan, can separate us from the love of God in Christ (Romans 8:37–39).

In addition, Paul mentions "the sword of the Spirit, which is the word of God." This is the first offensive weapon mentioned. The sword was used to kill and defeat enemies during attack. The typical Roman sword was not a long, cumbersome weapon. Rather, they were short-bladed, easy to draw, and quick in combat. Paul uses the term machairan here, which specifically refers to a short-bladed sword of this type. In the same way, God's Word helps to defeat our enemies during spiritual attacks. During the temptations of Jesus by Satan, Jesus used Scripture on all three occasions to overcome temptation (Matthew 4:1–11). Those who study and know Scripture can best "strike back" against temptation and prevent the Devil from knocking them off of their post.

It should also be noted that swords, as powerful weapons, are subject to misuse. A properly-wielded sword, used as the soldier has been taught, is a capable tool. But, a soldier who swings a sword wildly or carelessly is liable to harm themselves or others. The same is true of the Bible, the "sword of the Spirit." Improper, careless, or rebellious use of the Bible causes pain, harm, and spiritual damage.