When we (Protestants) say that people are 'justified by faith', what do we mean? Are we declared righteous on the basis of a general credulity? Is it faith, per se, that justifies, or is there a particular character to justifying faith?
John Owen, a man not much given to brevity, defines the object of justifying faith as "the Lord Jesus Christ himself, as the ordinance of God, in his work of mediation for the recovery and salvation of lost sinners, and as unto that end proposed in the promise of the gospel". And he goes on to helpfully unpack that in four dimensions, which we might summarise thus:
1. "The Lord Jesus Christ himself" - that is to say, the faith which justifies is personal faith, trust in Christ himself. It is not mere assent to facts. We all know what it means to trust a person; that is what faith is, and the trusted person is the Lord Jesus Christ.
2. "as the ordinance of God" - in other words, faith in Christ views him not as a general person, but as the person given by God the Father to bring about our salvation. So justifying faith is not only grounded in the person of Christ as the Son; it also looks to the Father as the one who sends him. Knowing that Christ is the one sent by God for the purpose of recovering and saving lost sinners, those who are justified put their trust in him.
3. "for the recovery and salvation of lost sinners" - the effect of Christ's work is salvation, and so justifying faith has an eye on that as its end goal. (Owen fudges a little here, to my mind, arguing that although justifying faith ought to lead one to believe that one's own sins are forgiven, that does not belong to its essential nature - I struggle with that; how can one trust Christ for salvation and not trust that one is actually saved?) Still, justifying faith is not directly faith that one is justified - this could well be mere presumption. It is faith in Christ as the one sent to effect justification.
4. "as unto that end proposed in the promise" - faith which justifies is faith which looks at God's promises extended to us in the gospel and leans on Christ in the promises. All the promises of the gospel - every good thing which God offers to believers - is found in Christ and based in his work as mediator. Therefore, faith which regards the promises is faith which can be traced to Christ himself.
Justifying faith is trust in Jesus as the one sent by the Father for our good; trust in Jesus as the one who came for our salvation; trust in Jesus as the one in whom all God's promises are yes and amen.