In Ephesians 6:16, the Apostle Paul supplies the 4th way to stand firm by putting on the armor of God and that is to take up the shield of faith. At the outset, we must discuss the significance of the prepositional phrase at the beginning of v.16—in addition to all (NASB) or in all circumstances (ESV). In the original Greek text, it’s simply 2 words—in all or with all. It seems this phrase is here to emphasize the importance of faith—make sure you take the shield of faith along with your other pieces of armor. In light of the fiery arrows that are coming your way, you don’t just want the first three pieces of armor. Those are absolutely necessary, but insufficient to fully extinguish fiery arrows during the battle.
The same thing is true as it relates to the last piece of armor located at the end of v.17—the sword of the Spirit, which is the Word of God. Why isn’t that the only piece of necessary armor? Truth be told, you will not be able to stand firm against the devil if you do not believe the word of God. In other words, the word is ineffective if it is not accompanied by faith, meaning that if you do not entrust yourself to and rely fully upon God’s Word, then arrows will penetrate into your armor and severely injure you.
Think about it this way: What would you rather have to defend yourself against one or more fiery arrows?—a shield or just a sword. Once again, it’s critical to point out that the entire Christian life is a fight of faith and therefore any failure is due to a lack of faith or unbelief. If you are to stand firm, you are to entrust yourself to the word while following God’s will.