Ephesians 6:16 (AMP)
16 Lift up over all the [covering] shield of saving faith, upon which you can quench all the flaming missiles of the wicked [one].
Doubt is used by Satan to steal our faith. We are to wear our armor of Christ to protect us from the enemy. The shield protects us from the enemy. When the enemy attacks us is when we need to lift up (use) our faith. Doubt however, is used to steal our faith and the boldness we have inside us. The enemy would like for us to be cowardly.
Doubt can just nag us all of the time. We don’t even realize how much we doubt throughout the day. When doubt comes to us we cannot wish it away, we have to lift up our shield of faith and believe that everything will come to pass.