Martin Luther wrote that “The reason why some people do not understand why faith alone justifies is that they do not know what faith is.” Yet more from McGrath ‘s book:
Three points relating to Luther’s idea of faith may be singled out as having special importance to his doctrine of justification….these points are:
Faith has a personal, rather than a purely historical, reference.
Faith concerns trust in the promises of God.
Faith unites the believer to Christ.
I have often spoken about two different kinds of faith. The first goes like this: you believe it is true that Christ is the person who is described and proclaimed in the gospels, but you do not believe that he is such a person for you. You doubt if you can receive that from him, and you think: ‘Yes, I’m sure he is that person for someone else (like Peter and Paul, and for religious and holy people). But is he that person for me? Can I confidently expect to receive everything from him that the saints expect?’ You see, this faith is nothing. It receives nothing of Christ, and tastes nothing of him either. It cannot feel joy, not love of him or for him. This is a faith related to Christ, but not a faith in Christ…The only faith which deserves to be called Christian is this: you believe unreservedly that it is not only for Peter and the saints that Christ is such a person, but also for you yourself–in fact, for you more than anyone else (emphasis added).
It is necessary that anyone who is about to confess his sins put his trust only and completely in the most gracious promise of God. That is, he must be certain that the one who has promised forgiveness to whoever confesses his sins will most faithfully fulfill this promise…[W]e are not to glory on account of the worthiness or adequacy of our confession (because there is no such worthiness or adequacy) but on account of the truth and certainty of God’s promises.
…Even if your faith is weak, I still have exactly the same treasure and the same Christ as others. There is no difference…It is like two people, each of whom owns a hundred gulden. One may carry them around in a paper sack, the other in an iron chest. But despite these differences, they both own the same treasure. Thus the Christ who you and I own is one and the same, irrespective of the strength or weakness of your faith or mine.
I like this restatement by McGrath:
The content of faith thus matters far more than its intensity. It is pointless to trust passionately in someone who is not worthy of trust; even a modicum of faith in someone who is totally reliable is vastly to be preferred. Trust is not, however, an occasional attitude. For Luther, it is an undeviating trusting outlook upon life, a constant stance of conviction of the trustworthiness of the promises of God.
How faith unites the believer with Christ is more mysterious. Luther again:
Faith unites the soul with Christ as a bride is united with her bridegroom. As Paul teaches us, Christ and the soul become one flesh by this mystery (Ephesians 5:31-2). And if they are one flesh and the marriage is real– in fact, it is the most perfect of all marriages, …then it follows that everything that they have is held in common, whether good or evil. So the believer can boast of the glory in whatever Christ possesses, as though it were his or her own; and whatever the believer has, Christ claims as his own. Let us see how this works and how it benefits us. Christ is full of grace, life and salvation. The human soul is full of sin, death and damnation. Now let faith come between them. Sin, death and damnation will then be Christ’s; and grace, life and salvation will be the believer’s.