The Protestant Reformation was, from the empire's constitutional perspective, the single most important event in its eight-hundred-year history. The religious schism produced by Martin Luther's theological campaign of 1517 onward did not merely divide the empire's Christians into two (later three) confessional communities; it fractured the constitutional doctrine that the empire was a single confessional polity under a single ecclesiastical hierarchy headed by a Catholic emperor crowned by a Catholic pope. By the time the religious wars of the sixteenth century were nominally settled in the Peace of Augsburg of 1555 — and definitively settled, after another ninety years of religious-political conflict, in the Peace of Westphalia of 1648 — the empire had become a constitutional federation in which the religious affiliation of each territorial unit was a matter for its sovereign prince rather than for the emperor or any common ecclesiastical authority. The political-theological idea of an imperial Christendom was dead.
Luther in 1517
Luther's Ninety-Five Theses, posted (or, according to recent scholarship, simply circulated in letters) at Wittenberg on the 31st of October 1517, were a relatively narrow attack on the indulgence-selling preaching of Johann Tetzel in the archdiocese of Mainz. Tetzel was raising funds for the rebuilding of St Peter's Basilica in Rome through the sale of plenary indulgences whose theological claims went substantially beyond what Catholic doctrine endorsed. Luther's intent in writing the Theses was probably academic — he was a doctor of theology at Wittenberg and the city's parish priest — but the rapid circulation of his Theses in print, in both Latin (for the scholarly audience) and German (for the popular audience), produced a public controversy that escalated beyond his immediate control.
Between 1518 and 1521 Luther's theological position radicalised. The Heidelberg Disputation of 1518 articulated his theology of the cross. The Leipzig Debate with Johann Eck in 1519 produced his rejection of the supremacy of the pope and of the infallibility of general councils. His three foundational tracts of 1520 — To the Christian Nobility of the German Nation, On the Babylonian Captivity of the Church, and On the Freedom of a Christian — articulated his doctrines of universal priesthood, of justification by faith alone, and of scriptural authority alone (the three classical "alones" of the Reformation). He was excommunicated by Pope Leo X in June 1520 and burned the bull Exsurge Domine at Wittenberg in December.
The Diet of Worms of 1521 summoned Luther before the new emperor Charles V (elected in 1519). Charles, by family Burgundian-Spanish and by personal disposition a conventional Catholic, expected Luther to recant. Luther, in the famous appearance of the 18th of April, refused: "Here I stand, I can do no other." (The exact phrase may be apocryphal; the substance is not.) The Edict of Worms of the 25th of May 1521 declared Luther an outlaw of the empire and his writings illegal. The Edict was not enforced. The Saxon elector Frederick the Wise sheltered Luther at the Wartburg castle for the next eleven months while the Lutheran movement spread across Saxony, Hesse, and the imperial cities of central and southern Germany.
The Schmalkaldic League
By the mid-1520s the religious division was institutionalised in the imperial diet through opposing princely blocs. The Lutheran princes — Saxony, Hesse, Brandenburg-Ansbach, several northern German bishops who had converted with their territories, and a substantial number of the imperial free cities — formed the Schmalkaldic League in 1531, named for the Thuringian town of Schmalkalden where it was inaugurated. The League was a defensive military and political alliance of the Protestant estates against the emperor and the Catholic estates. The Catholic counter-bloc included the Habsburg hereditary lands, the Bavarian Wittelsbach territories, the prince-bishoprics and abbeys, and the southern Catholic free cities. The constitutional principle that the empire's religious affairs were determined at the imperial diet by majority vote had become inoperable: the two confessions could not, in practice, be outvoted into agreement.
The Augsburg Confession of 1530 — drafted principally by Philip Melanchthon, presented to Charles V at the Diet of Augsburg — was the formal Lutheran statement of doctrine, intended as a basis for theological reconciliation but functioning, in practice, as the founding document of the Lutheran confessional community. It would remain, in the post-1555 settlement, the legal definition of who counted as a "Protestant of the Augsburg Confession" for the purposes of the imperial religious peace. The Reformed (Calvinist) churches, by contrast, would have no legal standing in the empire until the Peace of Westphalia of 1648.
Charles V and the Schmalkaldic War
Charles V's reign (1519–1556) was the high point of Habsburg dynastic empire — by inheritance, election and marriage he ruled Spain, the Netherlands, Naples, Sicily, the Indies, Austria, Burgundy, and the imperial title — but it was also the period in which the religious settlement collapsed. Charles was a Spanish-trained Catholic who deeply opposed the Reformation but who was perpetually distracted by his three other major conflicts: the Italian Wars with France, the Ottoman threat in the Mediterranean and Hungary (Süleyman the Magnificent reached Vienna in 1529), and the Spanish-colonial administration. He attempted, repeatedly through the 1520s and 1530s, to summon a general council that would resolve the religious dispute; the papacy resisted, and when the Council of Trent finally opened in 1545 it was clear that the council would not be a reconciling instrument but a counter-reforming one.
The Schmalkaldic War of 1546–1547 was Charles's attempt to settle the religious question by military means. He concluded a temporary peace with the Ottomans, recruited papal subsidies, and defeated the Schmalkaldic forces at the Battle of Mühlberg on the 24th of April 1547. The Saxon elector Johann Friedrich was captured and forced to surrender the electoral dignity to his cousin Maurice (a Lutheran but a Habsburg ally). Charles imposed the Augsburg Interim of 1548 — a temporary religious settlement that allowed Protestants clerical marriage and lay communion in both kinds but otherwise restored Catholic doctrine. The Interim was unenforceable. Maurice of Saxony, having achieved his electoral goals, switched sides in 1551 and led a Protestant counter-invasion that nearly captured Charles at Innsbruck in May 1552. Charles fled across the Brenner pass on horseback, abandoning his imperial baggage train.
The Peace of Augsburg
The Peace of Augsburg of the 25th of September 1555 — negotiated by Charles's brother Ferdinand, since Charles himself was preparing his own abdication — established the principle of cuius regio, eius religio: each prince of the empire was entitled to determine the religious confession of his territory between the two recognised confessions (Catholic and Lutheran of the Augsburg Confession). Subjects who could not in conscience conform were granted the right of emigration with their property. Ecclesiastical territories (the prince-bishoprics and abbeys) were declared subject to the "ecclesiastical reservation": a bishop or abbot who converted to Lutheranism would lose his territorial jurisdiction. The Calvinist Reformed churches were not recognised. Charles V abdicated in stages between 1555 and 1556, retiring to the monastery of Yuste in Spain; the empire passed to his brother Ferdinand I, the Spanish crown to his son Philip II.
The Peace of Augsburg lasted, with periodic breaches, for sixty-three years. The structural tensions it had not resolved — the ecclesiastical-reservation problem, the unrecognised Calvinist confession, the question of which territories had converted before and after the cut-off date of 1552 — would accumulate across the late sixteenth century and finally rupture in the catastrophe that opened the Thirty Years' War. That war is the next chapter.
End of Chapter IV