Like Chemnitz and Hunnius, Polycarp Leyser also wrote of various "causes" that are involved in the justification of sinners. Like the other orthodox Lutheran theologians, Leyser speaks of faith as the "instrumental cause" of justification. In other words, this is the means by which God justifies sinners. Faith is not the one who justifies sinners (that is, the efficient cause). Nor is it what motivates God to justify sinners (the interior motivating cause), nor is it that which earns the justification of the sinner (the meritorious cause). Instead, the Word of God and the faith to which the Word gives birth are the "how" of justification. All of these "causes" are involved, so to speak of justification as already having "taken place" or "happened" apart from any of these "causes," including the "how" of faith, is, Scripturally speaking, nonsense. As the Apology says in IV:67,
Or as the Formula of Concord enumerates the causes in SD:III:25 (without using the word "cause"):
The interior motivating cause is not any preceding merit of ours, nor any subsequent satisfaction, but only the free and infallible mercy of God, who sees the miseries of the human race, procures their redemption, and freely justifies us without any condition of the law having been fulfilled by us.
The exterior motivating or meritorious cause is the obedience, suffering, death and resurrection of Christ, the Son of God and of Man, by which He has perfectly made satisfaction to the Law for us, has made atonement for sins, has defeated death, has conquered Satan, and with the gates of Hades having been broken, has freed us for the freedom of the sons of God.
The formal cause is no inhering quality in us, nor is it the essential righteousness of God dwelling in us. But it is the remission of our sins and the righteousness of Christ alone which the heavenly Father imputes to us as our own, and by this He pronounces us to be righteous.
The instrumental cause with regard to God is the ministry of Word and Sacrament, in which God opens His heavenly treasures, hidden in the Son, and offers them to all men without discrimination or condition.
The instrumental cause with regard to us is faith, which acknowledges the fullness of the divine promise about Christ, offered in the Word and sealed in the Sacraments; embraces it with firm assent; and rests in it with great confidence that has no doubt concerning its salvation.
And since no works of the Law, neither preceding nor present nor subsequent, constitute any cause (either of merit or of application), therefore we rightly and piously declare that man is justified by faith alone in Christ.
The final cause of this free justification is not only that the dignity of God’s righteousness may be acknowledged, but also that our consciences, afflicted by sin in general, may have peace before God.
The effects are: adoption as sons of God, regeneration, the indwelling of God, vivification, eternal life, and innumerable other things.