Aegidius Hunnius didn't only write on the subject of Justification in the Theses Opposed to Huberianism (linked here). Among other things, he wrote a book (about 250 pages long) called The Article of the Gracious Justification of Sinful Man before God, Explained by Means of Questions and Answers. It'll take me a long time to finish translating that work, especially since I'm currently working on others, but here is a brief series of excerpts.
The Article of the Gracious Justification of Sinful Man before God: Questions and Answers(Hunnius, p.17 ff)
What does the word “justify” mean in the present discussion?
In a human judgment, they are said “to be justified” who are pronounced free from the guilt of the crimes of which they were accused. (The Scripture speaks in this sense in Deut. 25: If a case arises and they go to judgment, the righteous man should be justified and the ungodly man condemned, as this word “justifying” is understood in both Proverbs 17 and Is. 5). In the same way, understanding the word in the same forensic usage, they are said to be justified before God who, fleeing to the Throne of Grace, are absolved from the guilt of sin and from damnation, and are reckoned as righteous by the imputation of the righteousness of Christ, which consists in His obedience.
In order that each part may be examined in order, give me such a definition of justification that embraces the sum of the whole treatment that will follow.
Justification is the act of God by which He deigns to consider the man who is frightened by the awareness of sins and who flees to the Throne of Grace with pure mercy, through and for the sake of the merit of Christ, apprehended by faith; and, having forgiven him his sins, He reckons him as righteous, free from damnation, and also an heir of eternal life, without any human merit and without any view of God toward the virtues or the works of man.
What kind of definition is this?
It is a causal definition, seeing that the true causes are being enumerated and the false causes removed.
How many causes of justification are there?
Three. First is grace, that is, the gracious favor of God. Second: The obedience of Christ. Third: Faith.
Why do you number the causes in this order?
I put the grace of God in first place because this was given to us, as the Apostle testifies, before times of eternity, and it is also the source and beginning of the remaining causes, since it occurred by the mercy and grace of God that the Son was sent into the world to satisfy God in our place. Faith, on the other hand, since it is considered relatively to the obedience of Christ as the instrument that apprehends the thing that is apprehended, necessarily presupposes that which is apprehended, namely, the merit of Christ. In the order of causes, the merit of Christ comes before our faith, although in the case of the fathers, who lived before the Messiah was born and suffered, their faith (temporally speaking) existed prior to the suffering of the Lord, as they were naturally looking forward toward that which was to come. Still, if you consider the order of causes, the suffering of the Son comes first before God, who justifies (who views the merit of His Son outside the realm of time). Similarly, if you weigh the order of causes and effects, the suffering of Christ, is naturally prior to the salvation of the patriarchs (for this depends on the suffering of Christ as the effect brought about by the cause), although if you view the priority of time, the fathers gained that salvation before the Lord suffered—indeed, before He was born into the world.
(Hunnius, p.66 ff)
Explain more clearly what this means.
Scripture locates our justification in a twofold imputation, one positive, the other negative. Positive, in that the righteousness of Christ is imputed to us, which otherwise would not dwell in us. Negative, in that that which does dwell in us, namely, sins, are not imputed. The Apostle writes about both in Romans 4, with these words: As David also explains the blessedness of the man to whom God imputes righteousness apart from works: Blessed are they whose iniquities are forgiven and whose sins are covered. Blessed is the man to whom the Lord will not impute sin. Here you hear of the imputation of righteousness (which, for us, is Christ in His obedience); you hear likewise of the non imputation of sin. Indeed, our justification is located in both, even as one reads again of this latter non-imputation in 2 Cor. 5, God was in Christ, reconciling the world to Himself, not imputing sins to them.
As I understand your thinking, you believe that a man who is righteous by nature is justified?
When God justifies a man, He does not justify one who is righteous by nature. For He finds none, in that all have sinned, all have become useless and unrighteous, and fall short of the glory of God. Indeed, Christ did not come for the sake of righteous people, but to save sinners, to seek and to save what was lost. Paul does not hesitate to affirm in this regard that God justifies the ungodly (Rom. 4)—not the ungodly as he continues in his ungodliness, but as he acknowledges it and turns to the Throne of Grace, Jesus Christ.
But Proverbs 17 says that it is an abomination before God if someone justifies an ungodly man. Knowing this, how, then, shall we attribute this to God Himself?
There is a great difference between the two. To be sure, in a human judgment (where the imputation of a foreign righteousness has no place), it is called “justifying the ungodly” when he who is neither righteous by his own nor by a foreign righteousness is reckoned and pronounced righteous, without respect to or intervention of any righteousness—that is, by a false and unjust sentence. But in God’s judgment, where a foreign righteousness is valid, the ungodly is justified, not without any righteousness at all, but, since he lacks his own righteousness (and is for this very reason “ungodly” by nature), nonetheless, he is clothed by faith with a foreign righteousness which most certainly can be and is imputed to believers by God, as was just demonstrated.
What is the means of this imputation?
Faith. As it is written, Abraham believed God, and it was imputed to him as righteousness. And again, To him who does not work, but believes in Him who justifies the ungodly, his faith is imputed as righteousness.
You said that the righteousness of Christ is imputed to us. Now it appears from the testimonies that have been produced that faith is imputed for righteousness?
These things are not at odds with one another. For each is imputed to us in its own way. The righteousness of the obedience of Christ is the very thing that is imputed, that is, because of which we are reckoned as righteous and by which we are called “righteous.” But faith is the means of that imputation as it lays hold of the righteousness imputed by God. In this sense, faith itself, insofar as it embraces the righteousness of the obedience of the Savior, is said to be imputed as righteousness.
So then, you are making faith a third cause of our justification?
Very much so. And this together with the Prophets and Apostles, who have set forth that justification of faith illustrated by the example of Abraham (Gen. 15, Rom. 4). Indeed, there are also other testimonies that confirm that man is justified by faith: John 3, God so loved the world that He gave His only-begotten Son, that everyone who believes in Him should have eternal life. Acts 13, Let it be known to you that through this Man the remission of sins is announced to you, and through Him everyone who believes is justified from all the things from which you could not be justified through the law of Moses. And this phrase is often used by the Apostles from the Prophets: The righteous will live by faith. Paul makes this the proposition of his entire disputation in the introduction of the Epistle to the Romans, writing: The righteousness of God is revealed through it (the Gospel), from faith to faith, as it is written: But the righteous will live by his faith. And in Romans 3: But the righteousness of God through faith in Jesus Christ to all and upon all those who believe. And again: We judge that a man is justified by faith apart from the works of the law. And: The inheritance is given by faith, so that [it may be] according to grace, so that the promise may be firm. Many such sayings occur here and there (Acts 10, Gal. 2, Phil. 3, Heb. 11, and elsewhere).
But what do you here understand by “faith”?
Justifying faith is not only the knowledge in the intellect by which we hold the chief articles of the doctrine that has been divinely passed down, which includes that Christ has been crucified, dead, buried, resurrected, etc., but is above all the confidence of the heart with which a man trustingly rests in the satisfaction obtained through the death of Christ.