Over all, take up the shield of the faith, by which you will be able to quench all the fiery darts from the malignantly evil one (Ephesians 6:16).
The shield of faith protects us from all the fiery darts of the malignantly evil one (Ephesians 6:16). These darts include doubt and other persuasive methods designed to incite us to act independently from God. This type of shield is large and intended for comprehensive, offensive protection—like a door, as illustrated in Revelation 4:1.
Faith is taking God at His Word (Hebrews 11:1). Therefore, our faith must be based upon a promise. The promises we have from God serve as a powerful defense against Satan’s attacks, especially in the area of doubt.
Doubt is not merely uncertainty; rather, it is the act of questioning whether God permits a certain action—perceiving that it contradicts His standard—and then seeking to justify doing it anyway. For example, even eating can be done in doubt. Anything done while doubting is sin for a Christian (Romans 14:23). No situation justifies violating the commandments we are under as Christians. We are to love other Christians as Christ loves us and to abide in Him.
We do not overcome doubt through law, for “the law is not of faith” (Galatians 3:12). This is a teaching of men that causes us to be unsettled in all our ways (Ephesians 4:14). The righteous live out from faith (Romans 1:17). Therefore, when doubt arises—when we seek to justify an action we know contradicts God's standard for us—the shield of faith counters that attitude.