In asking the question, what is the Reformed definition of justification; one will find various answers because of the multifaceted nature of the Reformed tradition. Maybe it would be better to begin by asking what is it to be Reformed? Undoubtedly, that answer could only come in the form of a multi-volume work or a month long summit of Reformed scholastics and even then the answer would be nuanced unto complete murkiness. So with that in mind, my strategy for answering this question is to lay out various definitions of the broad Reformed tradition and attempt to characterize the soul that bleeds through all of them.
Before beginning this quest for the soul of the Reformed definition, I must confess that this section will be completely unoriginal. I plan on quoting from various confessions and catechisms because it is best if the Reformed tradition speaks for itself. I hope only to contribute a summary of a conversation in which there is a crowd of people, all vocalizing their thoughts at the same time.
Strangely enough, I start with the Lutheran consensus found in The Formula of Concord of 1576. Is the Lutheran definition even a Reformed definition? No, but I consider the Lutheran tradition, especially in regards to justification, a kind of Siamese twin attached to the heart and mind of Reformed tradition. Are they different? Yes. Are similarities and interdependencies? Most certainly. By observing the Lutheran view we will gain a perspective on how unique and similar the Reformed tradition is in comparison to its Protestant sister.
The Formula of Concord of 1576 art iii states these views on justification:
By unanimous consent… it is taught in our churches… that sinners are justified before God… alone by faith in Christ, so that Christ alone is our righteousness.
We teach… the whole Christ, in his sole, most absolute obedience which he rendered to the Father even unto death, as God and man… thereby merited for us the remission of all our sins and eternal life.
For he bestows and imputes to us the righteousness of the obedience of Christ; for the sake of that righteousness we are received by God into favour and accounted righteous.
We… recognize Christ… and confide in him:…. For his obedience’s sake we have by grace the remission of sins, are accounted righteous… and attain eternal salvation.
In this historical confession we see that Justification is forensic in nature. Meaning that Christ’s death leads to the remission of sins and His obedience is accounted to the believer. The righteousness of Jesus is objective in that the one receives the legal declaration of innocence and receives the benefits of eternal life because Jesus earned it on behalf of all those who have faith in Him.
The Heidelberg Catechism is Reformed but was also influenced by Lutheranism. In my opinion, this is evidence that the Reformed understanding of justification is always in the cultural and theological context of Lutheranism. It was finished in 1563 and it has two relevant teachings on the topic of justification:
Question 60: How are you righteous before God? Answer: Only by true faith in Jesus Christ. In spite of the fact that my conscience accuses me that I have grievously sinned against all the commandments of God, and have not kept any one of them, and that I am still ever prone to all that is evil, never the less, God, without any merit of my own, out of pure grace, grants me the benefits of the perfect expiation of Christ, imputing to me his righteousness and holiness as if I had never committed a single sin or had ever been sinful, having fulfilled myself all the obedience which Christ has carried out for me, if only I accept such favour with a trusting heart.’
Question 61: Why do you say that you are righteous by faith alone? Answer: Not because I please God by virtue of the worthiness of my faith, but because the satisfaction, righteousness and holiness of Christ alone are my righteousness before God, and because I can accept it a make it mine in no other way than by faith alone.
Again, we might notice a continued theme. The Heidelberg catechetical answer to how a Christian is righteous before God is to know that we have broken the commandments of God and posses no inherent righteousness apart from faith in Christ. We also see the word “imputation” included in this catechism which implies that a Christian’s righteousness (via Christ) is externally applied through faith. Justification in this sense is completely forensic.
The Westminster Confession of Faith is probably the most widely used and accepted reformed confession. It was formulated in the 1640s by the Westminster divines who were assembled to revise the Thirty-Nine Articles of the Church of England. This confession contains a distinguishing Reformed definition of Justification. Concerning justification, the Westminster Confession of Faith reads:
Those of whom God effectually calleth, he also freely justifieth: not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for anything wrought in them, or done by them, but for Christ’s sake alone; not by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on him and his righteousness by faith; which faith they have not of themselves, it is the gift of God.
This Reformed confession is similar to previous one in that it states justification is the imputation of Christ’s “obedience and satisfaction.” This confession develops this idea further by stating the logical order where justification comes from God’s effectual call. In essence, one can be justified only after one has received God’s prevenient grace that leads to a person’s justification through his faith in Christ.
One significant difference in this confession compared to the previous documents is that it tends to make negative assertions about justification. Not that they are negative about justification but they fully develop the idea of justification when they state what is not. Previous documents do this implicitly, where as the Westminster confession is very explicit when it states that one is not justified by infused righteousness, not by anything done in them, not by an infusion of faith, the act of believing, or any kind of evangelical righteousness. This whittles down the systematic idea of justification to forensic imputation and leaves no room for any kind of digression. Compared to the previous statements, the Westminster confession takes the strongest stance on justification by defining it this way.
In light of examining these confessional documents we can start to get a feel for what the Reformed definition of justification is. James Orr (1844-1913) was a Scottish Presbyterian minister who was also a professor in Church History. He has one of the deftest Reformed summaries of justification when he writes:
The chief points in the doctrine of justification at the Reformation [were] (1) that justification is of God’s free grace and not of works; (2) that it is through faith alone; (3) that it includes the forgiveness of sins and the pronouncing of the sinner righteous before God; (4) that it is to be distinguished from the internal change we designate regeneration and sanctification, and does not proceed on the ground of this change; (5) that it is nevertheless mere amnesty, but has its ground in the perfect righteousness of Christ, and the atonement made by Him for sin; and (6) that it is instantaneous and complete, and act of God never to be repeated, – these cardinal points on which all the Reformers were at one, were fixed, I believe, beyond the power of future to recall.
If one could capture the soul of the Reformed concept of justification then James Orr has come the closest to it. This definition ties all the previous confessions into one convenient paragraph. He covers extensively what justification is and what it is not. It is entirely external and should not be confused with God’s internal working of regeneration and sanctification. It is a one time act that Christ has accomplished for all those who have faith in Him. This is what James Orr sees as the Reformation understanding of justification and I agree with Him completely. But if we wanted to sum it up even more “concisely”, J.I. Packer, a Puritan scholar, neatly states that justification is “the fact that God forgives believing sinners, reckons them righteous and treats them as righteous.”
 Art. iii.i
 Art iii.ii
 Art. iii.iii
 Part of the reason why this Catechism was formed was to relieve tensions between Calvinists and Lutherans in the Palitinate province of Germany. The Spirit of the Reformation Study Bible (Grand Rapids: Zondervan, 2003), p. 2143.
 Chapter XI, art. i
 James Orr, The Progress of Dogma, first published 1901 (Grand Rapids, MI: Eerdmans Pub, 1952), pp. 268, 269.
 J. I. Packer, Concise Theology (Wheaton, IL: Tyndale House Publishers, Inc, 1993), Pgs. 164-166.