Faith is a Shield - Ephesians 6:16

...taking the shield of faith wherewith ye shall be able to quench all the fiery darts of the wicked.

Faith is a shield against temptation. To the seeker of righteousness, this is a great gift. A shield is the most effective armament against arrows. Even plate armor can be pierced. But with a shield, you have to be paying attention to the adversary. If they flank you, you are still vulnerable. That is to say, no falling asleep on the job. We have to stay active and alert. Such is faith, it is a principle of action and is rendered ineffective by inactivity.