Perhaps another way to clearly note the differences between the Reformation view and the Roman Catholic view of justification is to pay attention to Aristotelian categories of causation.
The rediscovery of Aristotle was important to the development of the Western Christian tradition. In terms of justification, Catholic theology differentiates between at least four causes of justification. These four Aristotelian causes are the (a) material cause, (b) efficient cause, (c) formal cause, and (d) final cause. For Aristotle, these causes helped offer an explanation of the changes we observe in the natural world. The Medieval Church employed levels of causation in various ways. One use of Aristotelian causation was to explain the doctrine of justification. At the Council of Trent (1545–1563), Catholic theologians took up the doctrine of justification in the sixth session (1546). Eventually, a decree on justification was published in 1547. Responding in part to the Reformers, the decree clearly defined justification in transformative (rather than declarative) terms and used Aristotelian categories to explain five causes of justification. Alister McGrath provides a helpful chart that summarizes the levels of causation as codified by Trent.
Final Cause The glory of God and eternal life Efficient Cause The mercy of God Meritorious Cause The passion of Christ Instrumental Cause The sacrament of baptism Formal Cause The righteousness of God
|Final Cause||The glory of God and eternal life|
|Efficient Cause||The mercy of God|
|Meritorious Cause||The passion of Christ|
|Instrumental Cause||The sacrament of baptism|
|Formal Cause||The righteousness of God|
Though there is some agreement between Protestants and Catholics on certain aspects of causation (e.g. final cause and efficient cause), the basic difference seems to lie in how one understands the formal cause. In the chart provided by McGrath, “the righteousness of God” (or “justice of God”) is an ambiguous phrase, with Rome and the Reformers understanding it differently. But it is an important phrase and the difference between Rome and the Reformers is more than semantic. Peter Toon summarizes the issue and helps clarify the divide between Protestants and Catholics on justification:
On the formal cause of justification, that by which God actually pronounces and accepts a sinner as righteous, there had never been agreement. The traditional Roman Catholic position was that at baptism God infuses into the soul his divine grace and that this grace purifies the soul. On seeing this infused righteousness in a human being God accepts him or justifies him. This new grace of the soul is thus the formal cause of justification and is at the same time the means of sanctification. With this view Protestant scholars had no sympathy. They argued that once God’s grace enters the soul it becomes human righteousness and no human righteousness is sufficient in quality to be the basis for justification and full acceptance with the eternal God. So they pointed to the external righteousness of Christ the Mediator and argued that his righteousness was imputed or reckoned to the Christian as the formal cause of acceptance of justification. Within both of these camps, the Roman and the Protestant, there was a limited variety of teaching within the fixed limits of either the infused, inherent righteousness or the external righteousness of Christ, as the formal cause.
The distinction, then, is between an inherent and alien righteousness. For Rome, God justifies the sinner because, in some sense, the sinner was now a saint. They were truly and inherently righteous and thus qualified to stand before God’s tribunal because of their internal form: they are a righteous person. As we shall see below, this is not how the Reformation tradition will explain the formal cause of justification. Generally speaking, for the Reformers, the “righteousness of God” was a reference to God graciously imputing the righteousness of Christ to the sinner. This imputed righteousness served as the formal cause of justification.
Given the differences between Catholicism and the Reformers in terms of how a sinner was justified in the sight of God, the energy Reformed theologians gave to articulating a robust doctrine of justification makes sense. It is not surprising to find Luther, Melanchthon, Calvin, and their theological children spilling large amounts of ink in order to clarify, defend, and promote justification sola fide. The very essence of the gospel, how sinners were made right with God, stood or fell on this point. If the conscience of a sinner would be soothed and confidence to stand before God’s grand tribunal acquired, the doctrine of justification needed to be understood and embraced. So, though there are complex reasons for the Protestant Reformation overall, doctrine was a central concern. At the heart of the doctrinal controversy was the gospel, again, cast in terms of justification by faith alone. The point in this here is simply that the biblical doctrine of justification is something theologians in every era of church history have wrestled with to one degree or another.
Today, justification continues to enjoy widespread discussion. Thus, justification is by no means a settled doctrine and my last several posts are small attempts to contribute to the conversation. Admittedly, however, I write from a Protestant perspective that believes John Calvin, John Owen, and contemporaries like Thomas Schreiner, John Piper, and Michael Horton are basically correct in how they articulate justification. May God cause this reformed perspective to flourish for the good of all people and the glory of Christ.
 Though Aristotle was not warmly embraced by all, Aquinas found Aristotle useful and used Aristotelian ideas at will.
 The four causes of justification parallel what historians note concerning the causes of the Reformation. For instance, the material principle of the Reformation was justification by grace alone through faith alone. The formal principle was Scripture (sola Scriptura). For why we employ such language, see Justin Taylor, “Why Do We Call Them the Formal and Material Principles of the Reformation,” (https://www.thegospelcoalition.org/blogs/justin-taylor/why-do-we-call-them-the-formal-and-material-principles-of-the-reformation/).
 Alister McGrath, iustitia Dei, 325. In addition, J. V. Fesko provides a similar chart in his chapter on the Reformed doctrine of justification (Fesko, “The Ground of Religion,” in Barret, The Doctrine on Which the Church Stands or Falls, 706).
 There is evidence of some diversity of opinion in terms of Aristotelian categories. John Owen, for instance, devotes a chapter in his work on justification to the “formal cause.” Francis Turretin, however, believes the main difference lies in the meritorious cause. Yet, as we shall see below, though they disagree over which Aristotelian category to use, the substance of their arguments are the same.
 Peter Toon, Evangelical Theology, 1833-1856: A Response to Tractarianism, New Foundations Theological Library (Atlanta: John Knox Press, 1979), 145–146.
 Matthew Barrett writes, “most fundamentally, the Reformation was a theological movement, caused by doctrinal concerns” (Matthew Barrett, “Introduction,” Reformation Theology, 44).
 Timothy George writes, “Protestantism was born out of the struggle for the doctrine of justification by faith alone. For Luther this was not simply one doctrine among others but “the summary of all Christian doctrine” “the article by which the church stands or falls” (Timothy George, Theology of the Reformers, 25th anniversary, rev. ed. (Nashville, TN: B & H Academic, 2013), 63).
 The doctrine of justification is being discussed across denominational aisles. Protestant-Catholic dialogues are taking fresh looks at justification. A. N. S. Lane, Justification by Faith in Catholic-Protestant Dialogue: An Evangelical Assessment, Scholars’ Editions in Theology (London; New York: T&T Clark, 2006). The Reformanda Initiative is another ministry engaging Evangelicals and Catholics in dialogue. According to their website, “Reformanda exists to identify, unite, equip, and resource evangelical leaders to understand Roman Catholic theology and practice, to educate the evangelical Church and to communicate the Gospel” (https://www.reformandainitiative.org/about-us-1). The New Perspective on Paul (NPP) has sought to rework justification in light of the work of E. P. Sanders, J. D. G. Dunn, and N. T. Wright SOURCES. Lutheran discussions over the doctrine continue to take form The “Finnish” School. Furthermore, how we understand the nature of justifying faith is something evangelicals are discussing. Within the realm of New Testament (NT) studies, some scholars are wrestling with pistis (faith) and how we have understood the word itself (Matthew W. Bates, Salvation by Allegiance Alone: Rethinking Faith, Works, and the Gospel of Jesus the King (Grand Rapids, MI: Baker Academic, 2017); Matthew W. Bates, Gospel Allegiance: What Faith in Jesus Misses for Salvation in Christ (Grand Rapids, MI: Brazos Press, 2019); Nijay K. Gupta, Paul and the Language of Faith (Grand Rapids, MI: William B. Eerdmans Publishing Company, 2020).