"our confession makes clear that faith (biblically understood) is an appropriating instrument, not something God sees in us which he then rewards.

Since there is no merit in faith, but only in the object of faith—Jesus Christ, faith must seen as an instrument, a means of reception, which links to the saving merits which are found in Jesus Christ alone. As our confession notes, “for faith is only the instrument by which we embrace Christ our righteousness; He imputes to us all His merits and as many holy works as He has done for us and in our place. Therefore Jesus Christ is our righteousness, and faith is the instrument that keeps us with Him in the communion of all His benefits. When those benefits have become ours, they are more than sufficient to acquit us of our sins.”

As Warfield correctly points out,
The place of faith in the process of salvation, as biblically conceived, could scarcely, therefore, be better described than by the use of the scholastic term `instrumental cause.’

Faith is not the ground of our salvation in general, nor of our justification in particular. Faith is the means, or the instrument, through which we receive the righteousness of another, namely that of Jesus Christ, who alone fulfilled all righteousness through his own perfect and personal obedience to the commandments of God. J. I. Packer is absolutely correct, when he points out that faith is “an appropriating instrument, an empty hand outstretched to receive the free gift of God’s righteousness in Christ.”11 Faith doesn’t do anything which causes or motives God to act. Faith is not something which God sees in us and which he rewards. Faith only receives with empty hands that which God freely offers to us—the saving merit of his dear Son earned through all our Lord’s holy works, when he obeyed the law of Moses, and did what the first Adam failed to do, fulfill all of the demands of the covenant of works.

Perhaps it would help to think of it this way. Having considered that which God demands of us in the law (perfect obedience), and then as a result realizing both our sin and our unworthiness, faith is that instrument (those outstretched hands) which humbly receives that which God freely offers to us—the merits of another, merit which will meet the demands of the law. We are, of course, speaking of the perfect obedience of Jesus Christ and his sacrificial death which satisfies God’s justice, the benefits of which can only be received with the empty hands of faith. Faith doesn’t do anything. Faith only receives what has already been done. Faith, then, is the instrument through which we receive a righteousness which is more than sufficient to justify the worst of sinners.

What, then, should we take with us from our discussion of faith which will help us live lives of gratitude before God?

Since we are all sinners, we have not the slightest chance of entering heaven clothed in our own righteousness, which is filthy rags. There is no ceremony we can perform, no amount of good works we can do, which can remove or turn aside God’s righteous anger toward our sins. What we need is for someone to take away our guilt and provide us with a righteousness which can pass God’s holy gaze. All of this is to be found in the life and death of Jesus Christ. Jesus came as the mediator of the covenant of grace, as the great high priest, who not only perfectly obeyed the law of God, but who offered himself up as a sacrifice for sin, a sacrifice sufficient to satisfy God’s just and righteous anger toward our sin.

But the work of Christ does us no good unless and until we confess our sins, renounce our own righteousness, and then, through faith, receive all the saving merits of Jesus Christ—merit which is more than sufficient to save from our sins and to render us as righteous and holy. Through faith, all that Jesus Christ is becomes ours. For he is our righteousness and our sanctifier. And being found in him, we are not only regarded as righteous as he is, but that same Lord who has justified us, now begins the life-long process of killing off the remnants of Adam and conforming us to the image of Jesus himself. And all of this comes to us only through the outstretched hands of faith. For having received the merit of our Lord’s holy works–which are more than sufficient to render us righteous before the Holy God, so that we are justified by faith apart from works—how can we go on living in sin? We cannot! For the faith which receives the merits of Christ fills our hearts with gratitude for all that Christ has done for us. The faith which justifies, is also the faith which transforms us. Amen!"

~ Kim Riddlebarger (Sermons on the Belgic Confession, Art. 22)