Martin Luther was declared a heretic and outlaw in 1521. In the years that followed, dozens of city councils and princely rulers nevertheless undertook changes in their domains to reform church and polity in the Lutheran manner. In practice, introducing reformation in a particular place involved violations of local customary law, canon law, civil law, or imperial law. Thus, reformation spawned litigation. Attempts throughout the first half of the sixteenth century to resolve disputes that had arisen out of local reformations through arbitration, imperial-level negotiations, religious colloquies, and a church council did little to stop “old-faith” (altgläubig) clergy and church authorities from suing proto-Protestant princes and cities. The most consequential litigation of this period took place in the Imperial Chamber Court, a Roman civil law court that had been established in 1495. With a stable location (that did not travel with the Emperor), and judges appointed by both Emperor and Estates, the Imperial Chamber Court was the first Empire-wide judicial forum that aspired to be independent of the personal justice of the Emperor. In this relatively new, and initially unstable, Roman law court, old-faith litigants sued princes and cities for violating the Land-Peace, confiscating church property, seizing jurisdiction, and other illegal acts. Because its first-instance litigants were primarily princes and cities directly subject to the Emperor, the litigation also had the character of public law.This dissertation is a socio-legal history of Reformation-related litigation that appeared in the Imperial Chamber Court prior to the 1555 Augsburg Religion-Peace. It offers a new kind of legal history of the Reformation. The legal history of the early German Reformation has primarily been the territory of intellectual historians, theologians, and scholars of public law. Their focus on political negotiations, watershed treaties, theological writings, and top-down legislative output, while invaluable, tell us only part of the story. My work, by contrast, elevates the importance of civil litigation as a distinctly important forum in which the Reformation unfolded. I show that legal praxis—the work of classification, of performative speech acts, and of experimentation in high-stakes contexts—are just as important, if not more important, than politics and doctrine for understanding the legal significance of the Reformation. The Reformation holds a particular potency in the historiography of the Christian West as an originary moment for forms of political arrangement and social life that would recognize and acknowledge internal Christian difference, and, eventually, an ever-expanding circle of worldviews. In particular, historians have tended to look to the Augsburg Religion-Peace of 1555 as the headwaters variously of modern secularism, religious freedom, the rule of law, and sovereignty. I argue that we need to look to the early decades of the Reformation to understand what precisely the Peace was aiming to settle and contain. A careful examination of civil litigation can help us account for the most consequential legal transformation of the early Reformation period: while in 1521 Lutheranism was outlawed as a heresy (in the Edict of Worms), in 1555 it was recognized as a legal confession (in the Augsburg Peace). Put another way, we cannot understand how the Holy Roman Empire got from Worms to Augsburg without understanding the Reformation cases.This dissertation also provides a new genealogy for “religion” as a secular legal category. While historians have tended to look to the Enlightenment for a point of origins of “religion” as an academic category, or to the Augsburg Peace as its origins as a political-legal category, in fact, the question of what counted as a “matter of religion” was a key issue as early as the 1520s in the context of Reformation litigation. My research shows how the deep ambiguity of the term “religion” in these cases was decisive in shaping the Augsburg Peace and imperial public law.Finally, my dissertation offers a new way of understanding the impact of Protestantism on modern law. Rather than examining how Protestant rulers reformed law in their domains, or the writings of theologians like Luther and Melanchthon on law, my dissertation analyzes the ways in which Protestant litigants experimented with imperial law in the context of high-stakes litigation. I identify certain patterns of usage that can help explain some of the features of the post-1555 imperial system—including its recognition of multiple “religious parties,” its increased investment in the consolidation of state institutions to manage agonistic difference, and the formation of two distinct legal interpretive universes along confessional lines which eventually destabilized the Augsburg system in the seventeenth century.For readers interested in law and legal history, they will find an exposition of fundamental questions of lawmaking in the Holy Roman Empire, and the way this legal culture shaped how the Reformation unfolded in the German lands. For readers interested in the Reformation, they will find a new approach to considering the role of law in this period, and its consequences on modern law. For readers interested in secularism, they will find a fresh scene in which the religion category gets re-invented, and a detailed examination of the role of the Reformation in producing the particular intractability of “religion” as we have inherited it in late modernity.