Greg Bahnsen: Christ's perfect righteousness make justification possible in terms of God's perfect nature

Greg Bahnsen explains that God's justice and grace are manifested in the full justification of sinners through Christ's righteousness in obeying the precepts and satisfying the penalties:

God could only forgive sins in a manner which is consistent with His holiness; in salvation righteousness and peace must kiss each other (Ps. 85:9f). Consequently, salvation with[out] justification is impossible, and justification without righteousness is inconceivable. There must be perfect righteousness in the reign of God’s grace for our salvation. Therefore, Scripture centers on the obedience of Christ – both active and passive – because it is the necessary requirement for the full justification of sinners. (Emphasis Bahnsen; Theonomy in Christian Ethics, p. 152; source: John Otis)

Archibald Alexander: Justification is grounded in the imputation of Christ's active and passive obedience

Archibald Alexander explains that Christ's preceptive and penal obedience alone are the basis for our pardon and acceptance with God:

When God imputes the righteousness of Christ to a sinner, he actually bestows it upon him for all the purposes of his complete justification. The sinner owes a righteousness to the law, which he cannot pay; but God in mercy reckons to him the perfect righteousness of another. For the sake then of Christ's satisfaction to the precept and penalty of the law he is pardoned and accepted as having a perfect righteousness in his Surety. (Archibald Alexander, A Treatise on Justification by Faith)

A.W. Pink: Justification is a legal declaration of satisfaction and acceptance

A.W. Pink on forensic justification and Christ's penal and preceptive obedience:

Justification has to do solely with the legal side of salvation. It is a judicial term, a word of the law courts. It is the sentence of a judge upon a person who has been brought before him for judgment. It is that gracious act of God as Judge, in the high court of Heaven, by which He pronounces an elect and believing sinner to be freed from the penalty of the law, and fully restored unto the Divine favour. It is the declaration of God that the party arraigned is fully conformed to the law; justice exonerates him because justice has been satisfied. Thus, justification is that change of status whereby one, who being guilty before God, and therefore under the condemning sentence of His Law, and deserving of nought but an eternal banishment from His presence, is received into His favour and given a right unto all the blessings which Christ has, by His perfect satisfaction, purchased for His people. (A.W. Pink, Justification, ch. 2: Its Meaning)

John Calvin: Christ's obedience alone is reckoned to us as righteousness

John Calvin on Christ's righteousness imputed to us:

To declare that by him alone we are accounted righteous, what else is this but to lodge our righteousness in Christ's obedience, because the obedience of Christ is reckoned to us as if it were our own? (Calvin, Institutes, III:11:23)

James Buchanan: Imputation of Sin and Righteousness

James Buchanan on the relation of the three imputations within Christian theology:

Take the three cases of Imputation which have been specified and compare them with one another. We find that in two out of the three a change of moral character is the invariable concomitant or consequent of imputation; for the imputation of Adam's guilt to his posterity was connected with their loss of original righteousness and the corruption of their whole nature. And the imputation of Christ's righteousness to His people is connected, in like manner, with their renewal and sanctification. But we also find that, in the third case--which is as real and as complete an instance of imputation as either of the other two--the imputation of our sins to Christ was not connected with any change in His holy character, or with the infusion of any, even the slightest, taint of moral evil. Whence we infer that imputation, so far from consisting in, is not even invariably connected with the infusion of moral qualities. We find again, that in two out of the three cases, representative and personal agency are so clearly distinguished as to make it manifest, that the party to whom anything is imputed is not supposed to have had any active participation in the doing of it: for our sins were really, and in the full sense of the term, imputed to Christ as our Substitute, yet He had no share in the commission of them. And His righteousness is, in like manner, imputed to us for our Justification, yet we had no share with Him in “finishing the work which the Father had given Him to do.” Whence we infer that, in the third case--the imputation of Adam's guilt to his posterity--it is so far from being necessary to suppose our personal participation in his act, that such a supposition would go far to destroy the doctrine of Imputation altogether, by setting aside the fundamental distinction between the agency of the representative and that of those who were represented by him. We find again that in all the three cases, imputation, whether of sin or of righteousness, is founded on a federal relation subsisting between one and many--for Adam was constituted the head and representative of his race, and Christ the Substitute and Surety of His people. This relation may be fitly described as amounting to a union between them, in virtue of which they are regarded and treated as being, in some respects, one. But this union is not such as to destroy the distinction between their respective personalities or to confound their several acts: for it is still true, that the representative was personally different from those whom he represented, and that his obedience or disobedience was his own act and not theirs, although it is imputed to them. (Dr. James Buchanan, The Immediate and Only Ground of Justification: The Imputed Righteousness of Christ)

Lorraine Boettner: Three Essential Imputations in Christian Theology

Lorraine Boettner on the biblical doctrine of imputation:

In Christian theology there are three separate and distinct acts of imputation. In the first place Adam's sin is imputed to all of us, his children, that is, judicially set to our account so that we are held responsible for it and suffer the consequence of it. This is commonly known as the doctrine of Original Sin. In the second place, and in precisely the same manner, our sin is imputed to Christ so that He suffers the consequences of it. And in the third place Christ's righteousness is imputed to us and secures for us entrance into heaven. We are, of course, no more personally guilty of Adam's sin than Christ is personally guilty of ours, or than we are personally meritorious because of His righteousness. In each case it is a judicial transaction. We receive salvation from Christ in precisely the same way that we receive condemnation and ruin from Adam. In each case the result follows because of the close and official union which exists between the persons involved. To reject any one of these three steps is to reject an essential part of the Christian system. (Lorraine Boettner, The Atonement; quoted in Rushdoony's Systematic Theology, p. 593)

John MacArthur: Justification by faith is always attacked first by the enemy

John MacArthur on the Reformation doctrine of justification by faith:

The Reformation doctrine of justification by faith is, and has always been, the number one target of the enemy’s attack. It provides the foundation of the bridge that reconciles God and man—without that key doctrine, Christianity falls. But the doctrine that the Reformers so painstakingly clarified, even spilled blood over, has become so muddled today that many Protestants barely recognize it. Sadly, there are some who react against a clear presentation of justification, calling it nothing more than useless hair-splitting.

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What’s so important about the doctrine of justification by faith alone? It is the doctrine upon which the confessing church stands or falls. Without it there is no salvation, no sanctification, no glorification—nothing. You wouldn’t know it to look at the state of Christianity today, but it really is that important. (John MacArthur, Justification by Faith)