To take the shield of faith is to appropriate the promises of God on our behalf, confident that what He says will come to pass, and that He will protect us in the midst of the fiery darts of Satan. Likewise, Peter admonishes us to ‘be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour. Resist him, steadfast in the faith …’ (1 Peter 5:8-9)
What kind of faith is being commended here? Certainly it is not mere mental assent to the facts about God. James 2:19 says that even the demons have this kind of faith. Neither can temporary faith be meant – the kind that in a wave of emotions says, “I believe,” but only endures for a while (Mt 13:21) and then disappears. Faith to do the miraculous also cannot be meant, for even Judas Iscariot, as one of the apostles, could cast demons out of others. There is but one kind of faith that remains – a faith that in the words of John overcomes the world (1 Jn 5:4) – it is a justifying faith. Justifying faith more than gives mental assent to the facts of the Gospel, it embraces them for oneself.
