The Reformed Orthodox were very clear that faith is justifying only because it is receptive of Christ. Heinrich Heppe collects evidence of this in his, Reformed Dogmatics, pp.553 and 554:
The condition to which the attribution of Christ’s righteousness is attached is not the performance of a work (for by works fallen man can merit nothing nor satisfy God in any way), but only faith in Christ and his work of redemption. But this faith effects justification not as a meritorious work or as the root of good works, but purely as a causa instrumentalis, not for one moment as a condition fulfillable by man,�for a condition of justification can only be laid down by the law, but not by free grace, and the single real condition of justification is perfect obedience to the law.
Mastricht (VI, vi, 14): “It is worth while inquiring how faith inflows into justification.�(1) It does not do so as the meritorious cause of it.�Nor (2) because of faith;�but we are justified through faith”. Witsius (III, viii, 52): “Nor does it seem to me an accurate statement, that faith is the condition which the gospel demands of us, that we may be held righteous and innocent with God. Strictly speaking the condition of justification is nothing but perfect obedience. � This the law enacted. Nor did the gospel substitute another; it teaches that the law has been satisfied by our sponsor Christ. It is at once the duty of faith to accept and by accepting to make its own satisfaction offered for it.” Crocius 1223: “So not only are those works excluded from the act of justification, which are emitted by a man before faith and conversion, but also those which proceed from faith.” Burmann (VI, v, 25): “Indeed faith is so opposed to works in this matter that it even excludes itself, if it is considered as a work. Although regarded by itself it is a work, in justification it is not regarded after this manner but purely as an instrumental work.” Bucan (XXXI, 34): “In what sense are we said to be justified by faith? It is not regarded in its own intrinsic dignity or merit, nor as a work or a new quality in us, nor in its force and efficacy minus love; nor because it has love added to it or works through love; nor because faith imparts the Spirit of Christ, by whom the believer is rendered just because we are bidden seek righteousness not in ourselves but in Christ; but because it seeks and embraces the righteousness offered in the Gospel Rom. 1:16,17. As regards justification faith is a purely passive thing, bringing nothing of ours to conciliate God, but receiving from Christ what we lack.”
(Formatting altered for greater ease in reading)
Faith is not a meritorious cause of justification; faith does not meet the demands of the law; faith does not justify because it produces good works, nor do the good works that proceed from faith play any role in justification. The only reason we are justified by faith is that faith apprehends Christ our righteousness.