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

Its Meaning

Deliverance from the condemning sentence of the divine law is the fundamental blessing in divine salvation: so long as we continue under the curse, we can neither be holy nor happy. But as to the precise nature of that deliverance, as to exactly what it consists of, as to the ground on which it is obtained, and as to the means whereby it is secured, much confusion now obtains. Most of the errors which have been prevalent on this subject arose from the lack of a clear view of the thing itself, and until we really understand what justification is, we are in no position to either affirm or deny anything concerning it. We therefore deem it requisite to devote a whole chapter unto a careful defining and explaining this word "justification," endeavoring to show both what it signifies, and what it does not connote. Between Protestants and Romanists there is a wide difference of opinion as to the meaning of the term "justify": they affirming that to justify is to make inherently righteous and holy; we insisting that to justify signifies only to formally pronounce just or legally declare righteous. Popery includes under justification the renovation of man's moral nature or deliverance from depravity, thereby confounding justification with regeneration and sanctification. On the other hand, all representative Protestants have shown that justification refers not to a change of moral character, but to a change of legal status; though allowing, yes, insisting, that a radical change of character invariably accompanies it. It is a legal change from a state of guilt and condemnation to a state of forgiveness and acceptance; and this change is owing solely to a gratuitous act of God, founded upon the righteousness of Christ (they having none of their own) being imputed to His people. "We simply explain justification to be an acceptance by which God receives us into His favor and esteems us as righteous persons; and we say that it consists in the remission of sins and the imputation of the righteousness of Christ. . . justification, therefore, is no other than an acquittal from guilt of him who was accused, as though his innocence has been proved.

Since God, therefore, justifies us through the mediation of Christ, He acquits us, not by an admission of our personal innocence, but by an imputation of righteousness; so that we, who are unrighteous in ourselves, are considered as righteous in Christ" (John Calvin, 1559). "What is justification? Answer: Justification is an act of God's free grace unto sinners, in which He pardons all their sins, accepts and accounts their persons righteous in His sight; not for anything wrought in them, or done by them, but only for the perfect obedience and full satisfaction of Christ, by God imputed to them, and received by faith alone" (Westminster Catechism, 1643). "We thus define the gospel justification of a sinner: It is a judicial, but gracious act of God, whereby the elect and believing sinner is absolved from the guilt of his sins, and has a right to eternal life adjudged to him, on account of the obedience of Christ, received by faith" (H. Witsius, 1693). "A person is said to be justified when he is approved of God as free from the guilt of sin and its deserved punishment; and as having that righteousness belonging to him that entitles to the reward of life" (Jonathan Edwards, 1750). Justification, then, refers not to any subjective change wrought in a person's disposition, but is solely an objective change in his standing in relation to the law. That to justify cannot possibly signify to make a person inherently righteous or good is most clearly to be seen from the usage of the term itself in Scripture. For example, in Proverbs 17:15 we read, "He who justifies the wicked, and he who condemns the just, even they both are abomination to the LORD": now obviously he who shall make a "wicked" person just is far from being an "abomination to the LORD," but he who knowingly pronounces a wicked person to be righteous is obnoxious to Him. Again; in Luke 7:29 we read, "And all the people that heard Him, and the publicans, justified God": how impossible it is to make the words "justified God" signify any moral transformation in His character; but understand those words to mean that they declared Him to be righteous, and all ambiguity is removed.

Once more, in 1 Timothy 3:16 we are told that the incarnate Son was "justified in (or "by") the Spirit": that is to say, He was publicly vindicated at His resurrection, exonerated from the blasphemous charges which the Jews had laid against Him.

1. 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 favor. 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 nothing but an eternal banishment from His presence, is received into His favor and given a right to all the blessings which Christ has, by His perfect satisfaction, purchased for His people. In substantiation of the above definition, the meaning of the term "justify" may be determined, first, by its usage in Scripture. "And Judah said, What shall we say unto my lord? what shall we speak? or how shall we clear (this Hebrew word "tsadaq" always signifies "justify") ourselves?" (Genesis 44:16). Here we have an affair which was entirely a judicial one.

Judah and his brethren were arraigned before the governor of Egypt, and they were concerned as to how they might procure a sentence in their favor. "If there be a controversy between men, and they come unto judgment, that the judges may judge them; then they shall justify the righteous, and condemn the wicked" (Deuteronomy 25:1). Here again we see plainly that the term is a forensic one, used in connection with the proceedings of law-courts, implying a process of investigation and judgment. God here laid down a rule to govern the judges in Israel: they must not "justify" or pass a sentence in favor of the wicked: compare 1 Kings 8:31, 32. "If I justify myself, mine own mouth shall condemn me: if I say, I am perfect, it shall also prove me perverse" (Job 9:20): the first member of this sentence is explained in the second—"justify" there cannot signify to make holy, but to pronounce a sentence in my own favor. "Then was kindled the wrath of Elihu . . . against Job . . . because he justified himself rather than God" (Job 32:2), which obviously means, because he vindicated himself rather than God. "That you might be justified when you speak, and be clear when You judge" (Psalm 51:4), which signifies that God, acting in His judicial office, might be pronounced righteous in passing sentence. "But Wisdom is justified of her children" (Matthew 11:19), which means that they who are truly regenerated by God have accounted the wisdom of God (which the scribes and Pharisees reckoned foolishness) to be, as it really is, consummate wisdom: they cleared it of the calumny of folly.

2. The precise force of the term "to justify" may be ascertained by noting that it is the antithesis of "to condemn." Now to condemn is not a process by which a good man is made bad, but is the sentence of a judge upon one because he is a transgressor of the law. "He who justifies the wicked, and he who condemns the just, even they both are abomination to the LORD" (Proverbs 17:15 and cf. Deuteronomy 25:1). "For by your words you shall be justified, and by your words you shall be condemned" (Matthew 12:37). "It is God that justifies. Who is he who condemns?" (Romans 8:33, 34). Now it is undeniable that "condemnation" is the passing of a sentence against a person by which the punishment prescribed by the law is awarded to him and ordered to be inflicted upon him; therefore justification is the passing of a sentence in favor of a person, by which the reward prescribed by the law is ordered to be given to him.

3. That justification is not an experimental change from sin to holiness, but a judicial change from guilt to no condemnation may be evidenced by the equivalent terms used for it. For example, in Romans 4:6 we read, "Even as David also describes the blessedness of the man, unto whom God imputes righteousness without works": so that legal "righteousness" is not a habit infused into the heart, but a gift transferred to our account. In Romans 5:9, 10 to be "justified by Christ's blood" is the same as being "reconciled by His death," and reconciliation is not a transformation of character, but the effecting of peace by the removal of all that causes offense.

4. From the fact that the judicial side of our salvation is propounded in Scripture under the figures of a forensic trial and sentence.

"(1) A judgment is supposed in it, concerning which the Psalmist prays that it may not proceed on the terms of the law: Psalm 143:2.

(2) The Judge is God Himself: Isaiah 50:7, 8.

(3) The tribunal whereon God sits in judgment is the Throne of Grace: Hebrews 4:16.

(4) A guilty person. This is the sinner, who is so guilty of sin as to be obnoxious to the judgment of God: Romans 3:18.

(5) Accusers are ready to propose and promote the charge against the guilty person; these are the law (John 5:45), conscience (Romans 2:15), and Satan: Zechariah 3:2, Revelation 12:10.

(6) The charge is admitted and drawn up in a 'handwriting' in form of law, and is laid before the tribunal of the Judge, in bar to the deliverance of the offender: Colossians 2:14.

(7) A plea is prepared in the gospel for the guilty person: this is grace, through the blood of Christ, the ransom paid, the eternal righteousness brought in by the Surety of the covenant: Romans 3:23, 25, Daniel 9:24.

(8) Hereunto alone the sinner betakes himself, renouncing all other apologies of defensatives whatever: Psalm 130:2, 3; Luke 18:13.

(9) To make this plea effectual we have an Advocate with the Father, and He pleads His own atoning sacrifice for us: 1 John 2:1, 2.

(10) The sentence hereon is absolution, on account of the sacrifice and righteousness of Christ; with acceptance into favor, as persons approved of God: Romans 8:33, 34; 2 Corinthians 5:21" (John Owen). From what has been before us, we may perceive what justification is not. First, it differs from regeneration. "Whom He called, them He also justified" (Romans 8:30).

Though inseparably connected, effectual calling or the new birth and justification are quite distinct. The one is never apart from the other, yet they must not be confounded. In the order of nature regeneration precedes justification, though it is in no sense the cause or ground of it: none is justified until he believes, and none believe until quickened. Regeneration is the act of the Father (James 1:18), justification is the sentence of the Judge. The one gives me a place in God's family, the other secures me a standing before His throne. The one is internal, being the impartation of divine life to my soul: the other is external, being the imputation of Christ's obedience to my account. By the one I am drawn to return in penitence to the Father's house, by the other I am given the "best robe" which fits me for His presence. Second, it differs from sanctification. Sanctification is moral or experimental, justification is legal or judicial. Sanctification results from the operation of the Spirit in me, justification is based upon what Christ has done for me. The one is gradual and progressive, the other is instantaneous and immutable. The one admits of degrees, and is never perfect in this life; the other is complete and admits of no addition. The one concerns my state, the other has to do with my standing before God. Sanctification produces a moral transformation of character, justification is a change of legal status: it is a change from guilt and condemnation to forgiveness and acceptance, and this solely by a gratuitous act of God, founded upon the imputation of Christ's righteousness, through the instrument of faith alone. Though justification is quite separate from sanctification, yet sanctification ever accompanies it. Third, it differs from forgiveness. In some things they agree. It is only God who can forgive sins (Mark 2:7) and He alone can justify (Romans 3:30). His free grace is the sole moving cause in the one (Ephesians 1:7) and of the other (Romans 3:24). The blood of Christ is the procuring cause of each alike (Matthew 26:28, Romans 5:9).

The objects are the same: the persons that are pardoned are justified, and the same that are justified are pardoned; to whom God imputes the righteousness of Christ for their justification to them He gives the remission of sins; and to whom He does not impute sin, but forgives it, to them He imputes righteousness without works: (Romans 4:6-8). Both are received by faith (Acts 26:18; Romans 5:1). But though they agree in these things, in others they differ. God is said to be "justified" (Romans 3:4), but it would be blasphemy to speak of Him being "pardoned"—this at once shows the two things are diverse. A criminal may be pardoned, but only a righteous person can truly be justified. Forgiveness deals only with a man's acts, justification with the man himself. Forgiveness respects the claims of mercy, justification those of justice. Pardon only remits the curse due unto sin; in addition justification confers a title to Heaven. Justification applies to the believer with respect to the claims of the law, pardon with respect to the Author of the law. The law does not pardon, for it knows no relaxation; but God pardons the transgressions of the law in His people by providing a satisfaction to the law adequate to their transgressions. The blood of Christ was sufficient to procure pardon (Ephesians 1:7), but His righteousness is needed for justification (Romans 5:19).

Pardon takes away the filthy garments, but justification provides a change of clothing (Zechariah 3:4). Pardon frees from death (2 Samuel 12:13), but righteousness imputed is called "justification of life" (Romans 5:18). The one views the believer as completely sinful, the other as completely righteous. Pardon is the remission of punishment, justification is the declaration that no ground for the infliction of punishment exists. Forgiveness may be repeated unto seventy times seven, justification is once for all. From what has been said in the last paragraph we may see what a serious mistake it is to limit justification to the mere forgiveness of sins. Just as "condemnation" is not the execution of punishment, but rather the formal declaration that the accused is guilty and worthy of punishment; so "justification" is not merely the remission of punishment but the judicial announcement that punishment cannot be justly inflicted—the accused being fully conformed to all the positive requirements of the law in consequence of Christ's perfect obedience being legally reckoned to his account. The justification of a believer is no other than his being admitted to participate in the reward merited by his Surety. Justification is nothing more or less than the righteousness of Christ being imputed to us: the negative blessing issuing therefrom is the remission of sins; the positive, a title to the Heavenly inheritance. Beautifully has it been pointed out that "We cannot separate from Immanuel His own essential excellency. We may see Him bruised and given like beaten incense to the fire, but was incense ever burned without fragrance, and only fragrance being the result?

The name of Christ not only cancels sin, it supplies in the place of that which it has canceled, its own everlasting excellency. We cannot have its nullifying power only; the other is the sure concomitant. So was it with every typical sacrifice of the law. It was stricken: but as being spotless it was burned on the altar for a sweet-smelling savor. The savor ascended as a memorial before God: it was accepted for, and its value was attributed or imputed to him who had brought the vicarious victim. If therefore, we reject the imputation of righteousness, we reject sacrifice as revealed in Scripture; for Scripture knows of no sacrifice whose efficacy is so exhausted in the removal of guilt as to leave nothing to be presented in acceptableness before God" (B.W. Newton). "What is placing our righteousness in the obedience of Christ, but asserting that we are accounted righteous only because His obedience is accepted for us as if it were our own? Wherefore Ambrose appears to me to have very beautifully exemplified this righteousness in the blessing of Jacob: that as he, who had on his own account no claim to the privileges of primogeniture, being concealed in his brother's habit, and invested with his garment, which diffused a most excellent odor, insinuated himself into the favor of his father, that he might receive the blessing to his own advantage, under the character of another; so we shelter ourselves under the precious purity of Christ" (John Calvin).