Of the many disputes Martin Luther faced in his theological career, perhaps none was more unexpected and painful than his disagreement with Johannes Agricola.
Both men were born in the town of Eisleben, their families connected through the local mining industry. Agricola attended the University of Wittenberg and became an early adherent of Luther’s reforming theology, serving as his secretary at the 1519 Leipzig Debate. It is possible that Agricola even lit the fire outside the Elster Gate when the papal bull was burned in December 1520. 
Agricola testified that he was keenly aware of his moral failings from a young age: “[I] had a frightfully shocked heart and conscience, so that when I was young and went to school, I ran to the monasteries and Carthusian houses and wanted to receive comfort.”  In the teachings of Luther, Agricola found what he longed for: a message of complete forgiveness of sins by the grace of God alone. Early on, Agricola acted upon his reformational beliefs, marrying in 1520 despite being a cleric.
Agricola was probably given the assignment of preaching on the Old Testament at the town church, St. Mary’s, beginning in 1523.  He accompanied Luther on various journeys, including a crucial trip to their home region in 1524, in which Luther confronted the growing radicalism of the Peasants’ Revolt and likely made the decision to wed Katharina von Bora.  Agricola then became the schoolmaster in Eisleben, preparing a catechism in 1525 and devoting himself to Christian education. 
Agricola’s wife, Else, suffered a significant depression in 1527, and spent some time living with the Luthers, hoping it would improve her condition. The two families were close—their affection entirely mutual. But their relationships were pushed to the breaking point the following year when Agricola began attacking Philip Melanchthon’s teaching about the law of God.
The trouble started during the visitation of the Saxon churches in 1527, when Melanchthon and Luther encountered significant issues with Christian doctrine and practice. The new Lutheran teaching of justification by faith alone had led to misunderstandings in congregations where not only the parishioners, but also the clergy were ill-educated. Melanchthon began to stress the need for true contrition and repentance of one’s sins, and the continuing role of the law in the life of the believer.
Melanchthon was attempting to walk a fine line between Roman Catholic opponents, who taught that human works contribute to one’s justification before God, and a growing number of opponents in the radical Reformation who saw no need for good works in the Christian life at all. Melanchthon’s position was clearly outlined two years later in the Augsburg Confession.
“It is taught on our part that it is necessary to do good works, not that we should trust to merit grace by them, but because it is the will of God. It is only by faith that forgiveness of sins is apprehended, and that, for nothing. And because through faith the Holy Ghost is received, hearts are renewed and endowed with new affections, so as to be able to bring forth good works.” 
Agricola believed Melanchthon’s view of the law was contrary to Luther’s, perhaps because he was most familiar with Luther’s earlier work, which had a less developed law/gospel schema. The preaching of law and gospel had become a key concept in Reformation preaching and exegesis, spurred by Luther and Melanchthon’s new insights in the late 1510s and early 1520s. Therefore, a theological error in one’s distinction of law and gospel was considered a serious problem.
Contrary to Melanchthon, Agricola saw no role for the law following justification. Defending his teachings in 1537, Agricola wrote,
“In my whole work, I have followed this one thing very simply: that the preaching of the death of our Lord Jesus Christ should terrify and depress the minds and consciences of men—that is, it teaches repentance. Again, what I preach about Christ’s resurrection should raise the consciences and minds of those who have been frightened and depressed by the death of Christ—that is, it teaches the remission of sins.” 
This may seem to be good Lutheran theology. Indeed, Luther himself did not initially catch the key difference between Melanchthon and Agricola, characterizing it as a mere “war of words.”  But upon further inspection, Agricola’s error becomes clear.
Note that in Agricola’s description, the law is never mentioned. Instead, we see the death and resurrection of Jesus Christ. On which side of the law/gospel divide do Lutherans place the death and resurrection of Christ? On the gospel side, even as Agricola does. But with no positive role for the law in his theology, Agricola is forced to make the gospel do the work of the law. The death of Jesus Christ must “terrify” and “depress” consciences, leading them to repentance. Charles Arand, Robert Kolb, and James Nestigen have given the following description of Agricola’s thoughts, speaking from his perspective and not their own:
“A repentance born of the law is indistinguishable from any other fear-driven activity: it is mere self-protection. If there is going to be a real change of heart, the gospel must be preached ‘in all its sweetness.’ Hearing of Christ’s self-sacrifice, the hearer will be moved to self-examination and so, repenting of all that required Christ’s death, assay a new life.” 
But the death of Christ does not itself convict. Rather, it shows that the penalty of law breaking has been taken on by a Savior, and the work of redemption finished. Without a knowledge of the law, the judicial aspect of Christ’s sacrifice cannot be properly grasped. Agricola is forced into the odd position of insisting that the gospel itself drives a person to despair before leading them to consolation. In this, he deviated from Luther without realizing it. Attempting to escape the errors of medieval Catholic thinking, Agricola ended up making the same mistake of conflating law and gospel.
However, Luther did not immediately realize the severity of the problem. A conference was called at Torgau, and the disputants patched up their differences. Things then improved for a time. Agricola was part of the official delegations to the Diets of Speyer (1529) and Augsburg (1530), as was Philip Melanchthon. They seem to have tolerated one another’s presence. Luther lent support to Agricola when the latter insulted the duke of Württemberg and drew the ire of princes including Landgrave Philip I of Hesse. On account of this incident, Agricola was unable to attend formal negotiations in Schmalkald in 1537. Instead, Luther left Agricola in charge of his household, and Agricola preached at St. Mary’s Church in Luther’s place.  These are not the actions of men who do not trust each other.
As Agricola had long desired to leave his post in Eisleben and engage in more intense theological work, Luther informed him in 1536 that a position might be opening on the theological faculty in Wittenberg. Not waiting for confirmation that he could have the job, Agricola gathered up his wife and nine children and made for Wittenberg, taking up residence in the Luther home. Soon the old controversy reemerged, and this time Luther saw Agricola’s position as a betrayal of the gospel. Lyndal Roper has summarized the disagreement thus.
“Agricola was putting the Crucifixion in place of the Law, that is, God’s law, through which we come to recognize sin…Agricola put the subjective feelings of the believer at the heart of salvation—something that Luther refused to do—and his theology, with its concern for troubled consciences, moved too quickly to focus on the forgiveness of sin and to relieve the individual’s misery.” 
Agricola shared many of Luther’s pastoral concerns, but ultimately diverged from him doctrinally. The result, after numerous denunciations and attempts at reconciliation, was that Agricola never held Luther’s confidence again. In 1540, Agricola left for Berlin. However, Agricola’s wife and daughters were allowed to stay with the Luthers again in 1545, long after relations had broken down. What we should conclude from that is unclear, but it is certain that Agricola never played the same role in the Wittenberg movement again. As Arand, Kolb, and Nestigen concluded,
“Luther recognized that law and gospel cannot be defined simply by their superficial content—God’s demands in the form of injunctions and God’s love in sending Christ to the cross, for instance—but must also be defined by the impact which the Holy Spirit accomplished through that content. If God’s love in Christ places a crushing burden of guilt upon a sinner, its message is functioning theologically as law.” 
We must be careful not to make the same mistake.
 Roper, Lyndal. Martin Luther: Renegade and Prophet (New York: Random House, 2016), 360.
 Förstemann, Carl Eduard. Neues Urkundenbuch zur Geschichte der evangelischen Kirchen-Reformation, Erster Band (Hamburg: Friedrich Werthers, 1842), 298.
 Brecht, Martin. Martin Luther: Shaping and Defining the Reformation, 1521-32, trans. James L. Schaaf (Minneapolis: Fortress Press, 1994), 123.
 I conclude that Luther made the decision during this trip because he was evidently encouraged in this regard by his father during the visit to Mansfield, then wrote in a letter at the tail end of the trip of his intention to wed Katharina.
 Brecht, 273.
 Article XX, Of Good Works. https://bookofconcord.org/augsburg-confession/
 Arand, Charles P., Robert Kolb, and James A. Nestingen. The Lutheran Confessions: History and Theology of the Book of Concord (Minneapolis: Fortress Press, 2012), 71.
 Arand, 70-1.
 Roper, 360-1.
 Roper, 362.
 Arand, 166-7.