It is not enough that they have believed; they must continue believing. The truth and its evidence must be habitually before their minds. Everything depends on that. They are safe ‘if they keep in memory what has been preached to them;’ not otherwise. The truth works effectually towards the resistance of the wicked one, but only in him who believes it, and only in the degree in which he believes it. It is faith that makes the Christian strong for combat. Let him lose sight of the truth and its evidence, and, like Samson shorn of his locks, he is weak as another man. Whenever he staggers through unbelief, he becomes powerless in resisting the great adversary. It is he only who puts on the whole armour of God that can stand in the evil day; but it is the believer alone who can put on and wear and wield that armour. It is the girdle of truth believed that can alone gird up the loins of the mind. The breastplate is the righteousness which is of God by faith. The well-roughed shoes, of the preparation of the gospel of peace, which are necessary to enable the spiritual soldier to stand firm in the slippery field of temptation, can be worn only by them who believe that gospel. The shield, which enables him to quench all the fiery darts of the wicked one, is the shield of faith. The hope, which is the helmet of salvation, can grace no brow but the brow of the believer, for hope rests on faith; the sword of the Spirit, which is the word of God, can be wielded only by the arm of the believer; and the prayer which is necessary to secure the right and the effectual use of all those pieces of spiritual armour, is the prayer of faith.
John Brown, The Christian’s Great Enemy