My favorite illustration of what faith does in justification is tongs. Let’s say you are canning green beans. You’ve got your jars in the boiling pot, and it’s time to take out the jars. With what do you remove them? Your bare hands? Not likely, unless you are a leper and cannot feel it. You need something that will lay hold of the jar and lift it out without harming you.
In a similar way, Christ’s righteousness is holy and untouchable by the bare soul. It is a consuming fire. And yet, we need such a righteousness if our guilt towards the law can be expiated. How can we lay hold of such a righteousness? It is by faith alone. Works will not work to do this, since works is our bare soul (sinful and stained) trying to touch the infinite holiness of God. We need faith, which is God-given (see Ephesians 2). Faith is not really a thing, in and of itself. Faith is defined by faith’s object, which is the person and work of Jesus Christ.
In the time of the Reformation, the relationship of faith to justification and what constituted faith was an extremely important question. Very precise distinctions came about as a result of the debates. For instance, there are different causes of justification, which must be distinguished. The ultimate cause (the reason for it) of our justification is the glory of God. The material cause of our justification is the person and work of Jesus Christ (the material cause is the “what” of what caused our justification). The immediate cause (in space and time, the cause that actually effects it in our lives; the material cause of Christ effected our salvation in principle, whereas the Holy Spirit applies it to our lives) is the Holy Spirit. And finally the instrumental cause of justification (the tongs by which we lay hold of it) is faith alone, completely and utterly apart from works. It is vitally important here that in justification, faith is alone. Faith is not alone when considering the Christian life as a whole. But in justification, faith is most certainly alone.
Lastly, when we define faith in justification, we must be very careful not to sneak in works through the back door. It is not “faith formed by love” which justifies. Rather “faith formed by love” sanctifies. Faith has three elements: knowledge, assent, and trust. Knowledge is the easy part: you need to know God through Jesus Christ. Assent is also easy: you need to agree to what you know. Trust is the hard part. Trust means that you entrust your soul to God. It is the difference between knowing that a chair will hold you up, agreeing that it will, but also actually sitting in it, trusting that the chair will hold your weight. In this sense, we are not talking about the continuous trust in Christ which springs from this initial trust. Justification is a one-time act that occurs in an instant at the time-point of faith. Rather, we are talking about a one-time entrustment of the soul to God (and even this is God-given, lest our perpetually glory-hungry souls seek to steal something from the glory of God).
