The following is from The Doctrine of Justification by Arthur Pink:

Its Nature

Justification, strictly speaking, consists in God's imputing to His elect the righteousness of Christ, that alone being the meritorious cause or formal ground on which He pronounces them righteous: the righteousness of Christ is that to which God has respect when He pardons and accepts the sinner. By the nature of justification we have reference to the constituent elements of the same, which are enjoyed by the believer. These are, the non-imputation of guilt or the remission of sins, and second, of the investing of the believer with a legal title to Heaven. The alone ground on which God forgives any man's sins, and admits him into His judicial favor, is the vicarious work of his Surety—that perfect satisfaction which Christ offered to the law on his behalf. It is of great importance to be clear on the fact that Christ was "made under the law" not only that He might redeem His people "from the curse of the law" (Galatians 3:13), but also that they might "receive the adoption of sons" (Galatians 4:4, 5), that is, be invested with the privileges of sons. This grand doctrine of justification was proclaimed in its purity and clarity by the Reformers—Luther, Calvin, Zanchius, Peter Martyr, etc.; but it began to be corrupted in the seventeenth century by men who had only a very superficial knowledge of it, who taught that justification consisted merely in the removal of guilt or forgiveness of sins, excluding the positive admittance of man into God's judicial favor: in other words, they restricted justification unto deliverance from Hell, failing to declare that it also conveys a title unto Heaven.

This error was perpetuated by John Wesley, and then by the Plymouth Brethren, who, denying that the righteousness of Christ is imputed to the believer, seek to find their title to eternal life in a union with Christ in His resurrection. Few today are clear upon the twofold content of justification, because few today understand the nature of that righteousness which is imputed to all who believe. To show that we have not misrepresented the standard teachings of the Plymouth Brethren on this subject, we quote from Mr. W. Kelly's "Notes on Romans." In his "Introduction" he states, "There is nothing to hinder our understanding 'the righteousness of God' in its usual sense of an attribute or quality of God" (p. 35). But how could an "attribute" or "quality" of God be "upon all them that believe" (Romans 3:22)? Mr. Kelly will not at all allow that the "righteousness of God" and "the righteousness of Christ" are one and the same, and hence, when he comes to Romans 4 (where so much is said about "righteousness" being imputed to the believer) he evacuates the whole of its blessed teaching by trying to make out that this is nothing more than our own faith, saying of Abraham, "his faith in God's word as that which he exercised, and which was accounted as righteousness" (p. 47). The "righteousness of Christ" which is imputed to the believer consists of that perfect obedience which He rendered unto the precepts of God's law and that death which He died under the penalty of the law. It has been rightly said that, "There is the very same need of Christ's obeying the law in our stead, in order to the reward, as of His suffering the penalty of the law in our stead in order to our escaping the penalty; and the same reason why one should be accepted on our account as the other... To suppose that all Christ does in order to make atonement for us by suffering is to make Him our Savior but in part. It is to rob Him of half His glory as a Savior. For if so, all that He does is to deliver us from Hell; He does not purchase Heaven for us" (Jonathan Edwards).

Should anyone object to the idea of Christ "purchasing" Heaven for His people, he may at once be referred to Ephesians 1:14, where Heaven is expressly designated "the purchased possession." The imputation to the believer's account of that perfect obedience which his Surety rendered unto the law for him is plainly taught in Romans 5:18, 19, "Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life. For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous." Here the "offence" or "disobedience" of the first Adam is set over against the "righteousness" or "obedience" of the last Adam, and inasmuch as the disobedience of the former was an actual transgression of the law, therefore the obedience of the latter must be His active obedience unto the law; otherwise the force of the apostle's antithesis would fail entirely. As this vital point (the chief glory of the gospel) is now so little understood, and in some quarters disputed, we must enter into some detail. The one who was justified upon his believing sustained a twofold relation unto God: first, he was a responsible creature, born under the law; second, he was a criminal, having transgressed that law—though his criminality has not canceled his obligation to obey the law any more than a man who recklessly squanders his money is no longer due to pay his debts.

Consequently, justification consists of two parts, namely, an acquittal from guilt, or the condemnation of the law (deliverance from Hell), and the receiving him into God's favor, on the sentence of the law's approval (a legal title to Heaven). And therefore, the ground upon which God pronounces him just is also a double one, as the one complete satisfaction of Christ is viewed in its two distinct parts: namely, His vicarious obedience unto the precepts of the law, and His substitutionary death under the penalty of the law, the merits of both being equally imputed or reckoned to the account of him who believes. Against this it has been objected, "The law requires no man to obey and die too." To which we reply in the language of J. Hervey (1750), "But did it not require a transgressor to obey and die? If not, then transgression robs the law of its right, and vacates all obligation to obedience. Did it not require the Surety for sinful men to obey and die? If the Surety dies only, He only delivers from penalty. But this affords no claim to life, no title to a reward—unless you can produce some such edict from the Court of Heaven— "Suffer this, and you shall live." I find it written 'In keeping Your commandments there is great reward' (Psalm 19:11), but nowhere do I read, "In undergoing Your curse, there is the same reward." Whereas, when we join the active and passive obedience of our Lord—the peace-speaking Blood with the Life-giving righteousness—both made infinitely meritorious and infinitely efficacious by the divine glory of His person, how full does our justification appear! How firm does it stand!" It is not sufficient that the believer stand before God with no sins upon him—that is merely negative.

The holiness of God requires a positive righteousness to our account—that His law be perfectly kept. But we are unable to keep it, therefore our Sponsor fulfilled it for us. By the blood-shedding of our blessed Substitute the gates of Hell have been forever shut against all those for whom He died. By the perfect obedience of our blessed Surety the gates of Heaven are opened wide unto all who believe. My title for standing before God, not only without fear, but in the conscious sunshine of His full favor, is because Christ has been made "righteousness" unto me (1 Corinthians 1:30). Christ not only paid all my debts, but fully discharged all my responsibilities. The law-giver is my law-Fulfiller. Every holy aspiration of Christ, every godly thought, every gracious word, every righteous act of the Lord Jesus, from Bethlehem to Calvary, unite in forming that "best robe" in which the seed royal stand arrayed before God. Yet sad to say, even so widely-read and generally-respected a writer as the late Sir Rob. Anderson, said in his book, "The Gospel and Its Ministry" (Chapter on Justification by Blood), "Vicarious obedience is an idea wholly beyond reason; how could a God of righteousness and truth reckon a man who has broken law to have kept law, because someone else has kept it? The thief is not declared to be honest because his neighbor or his kinsman is a good citizen." What a pitiable dragging down to the bar of sin-polluted human reason, and a measuring by worldly relations, of that divine transaction wherein the "manifold wisdom of God" was exercised! What is impossible with men is possible with God.

Did Sir Robert never read that Old Testament prediction wherein the Most High God declared, "Therefore, behold, I will proceed to do a marvelous work among this people, even a marvelous work and a wonder: for the wisdom of their wise men shall perish, and the understanding of their prudent men shall be hid" (Isaiah 29:14)? It is pointed out that, "In the human realm, both innocence and righteousness are transferable in their effects, but that in themselves they are untransferable." From this it is argued that neither sin nor righteousness are in themselves capable of being transferred, and that though God treated Christ as if He were the sinner, and deals with the believer as though he were righteous, nevertheless, we must not suppose that either is actually the case; still less ought we to affirm that Christ deserved to suffer the curse, or that His people are entitled to be taken to Heaven. Such is a fair sample of the theological ignorance of these degenerate times, such is a representative example of how divine things are being measured by human standards; by such sophistries is the fundamental truth of imputation now being repudiated.

Rightly did W. Rushton, in his "Particular Redemption," affirm, "In the great affair of our salvation, our God stands single and alone. In this most glorious work, there is such a display of justice, mercy, wisdom and power, as never entered into the heart of man to conceive, and consequently, can have no parallel in the actions of mortals. 'Who has declared this from ancient time? who has told it from that time? have not I the LORD? and there is no God else beside Me; a just God and a Savior; there is none beside Me': Isaiah 45:21." No, in the very nature of the case no analogy whatever is to be found in any human transactions with God's transferring our sins to Christ or Christ's obedience to us, for the simple but sufficient reason that no such union exists between worldlings as obtains between Christ and His people. But let us further amplify this counter-imputation. The afflictions which the Lord Jesus experienced were not only sufferings at the hands of men, but also enduring punishment at the hand of God: "it pleased the LORD to bruise Him" (Isaiah 53:10); "Awake, O sword, against My Shepherd, and against the man that is My Fellow, says the LORD of hosts: smite the shepherd" (Zechariah 13:7) was His edict.

But lawful "punishment" presupposes criminality; a righteous God had never inflicted the curse of the law upon Christ unless He had deserved it. That is strong language we are well aware, yet not stronger than what Holy Writ fully warrants, and things need to be stated forcibly and plainly today if an apathetic people is to be aroused. It was because God had transferred to their Substitute all the sins of His people that, officially, Christ deserved to be paid sin's wages. The translation of our sins to Christ was clearly typed out under the law: "And Aaron shall lay both his hands upon the head of the live goat, (expressing identification with the substitute), and confess over him all the iniquities of the children of Israel, and all their transgressions in all their sins, putting them upon the head of the goat (denoting transference), and shall send him away by the hand of a fit man into the wilderness: And the goat shall bear upon him all their iniquities unto a land not inhabited" (Leviticus 16:21, 22). So too it was expressly announced by the Prophets: "The LORD has laid on Him the iniquity of us all... He shall bear their iniquities" (Isaiah 53:6, 11).

In that great Messianic Psalm, the 69th, we hear the Surety saying, "O God, You know My foolishness; and My sins are not hid from You" (verse 5)—how could the spotless Redeemer speak thus, unless the sins of His people had been laid upon Him? When God imputed sin to Christ as the sinner's Surety, He charged Him with the same, and dealt with Him accordingly. Christ could not have suffered in the stead of the guilty unless their guilt had been first transferred to Him. The sufferings of Christ were penal. God by act of transcendent grace (to us) laid the iniquities of all that are saved upon Christ, and in consequence, divine justice finding sin upon Him, punished Him. He who will by no means clear the guilty must strike through sin and smite its bearer, no matter whether it be the sinner himself or One who vicariously takes his place. But as G.S. Bishop well said, "When justice once strikes the Son of God, justice exhausts itself."

The atonement of Christ was contrary to our processes of law because it rose above their finite limitations! Now as the sins of him who believes were, by God, transferred and imputed to Christ so that God regarded and treated Him accordingly—visiting upon Him the curse of the law, which is death; even so the obedience or righteousness of Christ is, by God, transferred and imputed to the believer so that God now regards and deals with him accordingly—bestowing upon him the blessing of the law, which is life. And any denial of that fact, no matter by whoever made, is a repudiation of the cardinal principle of the gospel. "The moment the believing sinner accepts Christ as his Substitute, he finds himself not only freed from his sins, but rewarded: he gets all Heaven because of the glory and merits of Christ (Romans 5:17). The atonement, then, which we preach is one of absolute exchange (1 Peter 3:18). It is that Christ took our place literally, in order that we might take His place literally—that God regarded and treated Christ as the Sinner, and that He regards and treats the believing sinner as Christ. "It is not enough for a man to be pardoned. He, of course, is then innocent—washed from his sin—put back again, like Adam in Eden, just where he was. But that is not enough. It was required of Adam in Eden that he should actually keep the command. It was not enough that he did not break it, or that he is regarded, through the Blood, as though he did not break it. He must keep it: he must continue in all things that are written in the book of the law to do them. How is this necessity supplied? Man must have a righteousness, or God cannot accept him. Man must have a perfect obedience, or else God cannot reward him" (G.S. Bishop). That necessary and perfect obedience is to be found alone in that perfect life, lived by Christ in obedience to the law, before He went to the cross, which is reckoned to the believer's account.

It is not that God treats as righteous one who is not actually so (that would be a fiction), but that He actually constitutes the believer so, not by infusing a holy nature in his heart, but by reckoning the obedience of Christ to his account. Christ's obedience is legally transferred to him so that he is now rightly and justly regarded as righteous by the divine law. It is very far more than a naked pronouncement of righteousness upon one who is without any sufficient foundation for the judgment of God to declare him righteous. No, it is a positive and judicial act of God "whereby, on the consideration of the mediation of Christ, He makes an effectual grant and donation of a true, real, perfect righteousness, even that of Christ Himself unto all that do believe, and accounting it as theirs, on His own gracious act, both absolves them from sin, and grants them right and title unto eternal life" (John Owen). It now remains for us to point out the ground on which God acts in this counter-imputation of sin to Christ and righteousness to His people. That ground was the Everlasting Covenant. The objection that it is unjust the innocent should suffer in order that the guilty may escape loses all its force once the Covenant-Headship and responsibility of Christ is seen, and the covenant-oneness with Him of those whose sins He bore. There could have been no such thing as a vicarious sacrifice unless there had been some union between Christ and those for whom He died, and that relation of union must have subsisted before He died, yes, before our sins were imputed to Him. Christ undertook to make full satisfaction to the law for His people because He sustained to them the relation of a Surety. But what justified His acting as their Surety?

He stood as their Surety because He was their Substitute: He acted on their behalf, because He stood in their room. But what justified the substitution? No satisfactory answer can be given to the last question until the grand doctrine of everlasting covenant oneness comes into view: that is the great underlying relation. The federal oneness between the Redeemer and the redeemed, the choosing of them in Christ before the foundation of the world (Ephesians 1:4), by which a legal union was established between Him and them, is that which alone accounts for and justifies all else. "For both He who sanctifies and they who are sanctified are all of one: for which cause He is not ashamed to call them brethren" (Hebrews 2:11). As the Covenant-Head of His people, Christ was so related to them that their responsibilities necessarily became His, and we are so related to Him that His merits necessarily become ours. Thus, as we said in an earlier chapter, three words give us the key to and sum up the whole transaction: substitution, identification, imputation—all of which rest upon covenant-oneness. Christ was substituted for us, because He is one with us—identified with us, and we with Him. Thus God dealt with us as occupying Christ's place of worthiness and acceptance. May the Holy Spirit grant both writer and reader such an heart-apprehension of this wondrous and blessed truth, that overflowing gratitude may move us unto fuller devotedness unto Him who loved us and gave Himself for us.