When writing about putting on "the whole armor of God" in Ephesians 6, Paul begins to conclude the passage by repeating the concepts in Luke 21:36—praying always and watching (verse 18). He says in verse 16: ". . . above all, taking the shield of faith with which you will be able to quench all the fiery darts of the wicked one."
Albert Barnes' New Testament Commentary explains these fiery darts:
Paul here refers, probably, to the temptations of the great adversary, which are like fiery darts; or those furious suggestions of evil, and excitements to sin, which he may throw into the mind like fiery darts. They are blasphemous thoughts, unbelief, sudden temptation to do wrong, or thoughts that wound and torment the soul. In regard to them, we may observe:
(1) that they come suddenly, like arrows sped from a bow;
(2) they come from unexpected quarters, like arrows shot suddenly from an enemy in ambush;
(3) they pierce, and penetrate, and torment the soul, as arrows would that are on fire;
(4) they set the soul on fire, and enkindle the worst passions, as fiery darts do a ship or camp against which they are sent.
What happens when these fiery darts hit their target? The answer appears in James 1:13-15 (Contemporary English Version, CEV):
Don't blame God when you are tempted! God cannot be tempted by evil, and he doesn't use evil to tempt others. We are tempted by our own desires that drag us off and trap us. Our desires make us sin, and when sin is finished with us, it leaves us dead.
As Barnes says, these darts "enkindle the worst passions," or as James says, "our desires." Actually, these darts have been flying since the day we were born, doing their damage. Where is it better for us to deal with these darts: at the point of the shield or after they have hit their mark? Of course, at the shield!
II Samuel 22:31 tells us what our shield is: "As for God, His way is perfect; the word of the Lord is proven; He is a shield to all who trust in Him" (see also Genesis 15:1; Psalm 33:20; Proverbs 2:7). We are not the shield. Our faith is not the shield. God is the shield, using the same faith Jesus Christ had. If we let Him, God will protect us in our battles.
How do we erect this "shield of faith?" Notice these verses:
Matthew 17:19-21: Then the disciples came to Jesus privately and said, "Why could we not cast it out?" So Jesus said to them, "Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible for you. However, this kind does not go out except by prayer and fasting."
Psalm 18:30 (CEV): Your way is perfect, Lord, and your word is correct. You are a shield for those who run to you for help.
Along with fasting, Christ gives prayer as one of the antidotes to unbelief. David says that running to God for help, of which striving to pray always is the essence, will allow Him to be our shield, our source of power and strength (II Corinthians 3:5; 4:7).
Notice the first part of Matthew 26:41 from the New Life Bible: "Watch and pray so that you will not be tempted. . . ." Jesus repeats the instruction in Luke 21:36 but shows that the same process will build the shield of faith to protect us from the fiery darts of temptation.
Notice that the shield mentioned in Ephesians 6:16 can quench all the fiery darts—not some, not most, but all. Consider the great peace we would have if none of Satan's fiery darts ever reached their intended target! This sheds light on why Christ says in Matthew 11:30: "For My yoke is easy and My burden is light." We know He used every spiritual tool God makes available.