When in the late 1520s the Catholic authorities of England tried to buy up and burn all copies of William Tyndale's English translation of the Bible, they were attempting to stop the spread of what they viewed as a dangerous plague of heresies spreading out from Luther's Germany. The plague was the Protestant Reformation, a movement opposed to crucial aspects of both the belief system and the institutional structure of Roman Catholicism.

Many of the key tenets of the Reformation were not new: they had been anticipated in England by the teachings of the theologian and reformer John Wycliffe in the fourteenth century. But Wycliffe and his followers, known as Lollards, had been suppressed, and, officially at least, England in the early sixteenth century had a single religion, Catholicism, whose acknowledged head was the Pope in Rome. In 1517, drawing upon long-standing currents of dissent, Martin Luther, an Augustinian monk and professor of theology at the University of Wittenberg in Germany, challenged the authority of the Pope and attacked several key doctrines of the Catholic Church. According to Luther, the Church, with its elaborate hierarchical structure centered in Rome, its rich monasteries and convents, and its enormous political influence, had become hopelessly corrupt, a conspiracy of venal priests who manipulated popular superstitions to enrich themselves and amass worldly power. Luther began by vehemently attacking the sale of indulgences — certificates promising the remission of punishments to be suffered in the afterlife by souls sent to Purgatory to expiate their sins. These indulgences, along with other spiritual and temporal powers claimed by the Pope, had no foundation in the Bible, which in Luther's view was the only legitimate source of religious truth. Christians would be saved not by scrupulously following the ritual practices fostered by the Catholic Church — observing fast days, reciting the ancient Latin prayers, endowing chantries to say prayers for the dead, and so on — but by faith and faith alone.

This challenge spread and gathered force, especially in Northern Europe, where major leaders like the Swiss pastor Ulrich Zwingli and the French theologian John Calvin established institutional structures and elaborated various and sometimes conflicting doctrinal principles. Calvin, whose thought came to be particularly influential in England, emphasized the obligation of governments to implement God's will in the world. He advanced too the doctrine of predestination, by which, as he put it, "God adopts some to hope of life and sentences others to eternal death." God's "secret election" of the saved made Calvin uncomfortable, but his study of the Scriptures had led him to conclude that "only a small number, out of an incalculable multitude, should obtain salvation." It might seem that such a conclusion would lead to passivity or even despair, but for Calvin predestination was a mystery bound up with faith, confidence, and an active engagement in the fashioning of a Christian community.

The Reformation had a direct and powerful impact on those realms where it gained control. Monasteries were sacked, their possessions seized by princes or sold off to the highest bidder; the monks and nuns, expelled from their cloisters, were encouraged to break their vows of chastity and find spouses, as Luther and his wife, a former nun, had done. In the great cathedrals and in hundreds of smaller churches and chapels, the elaborate altar-pieces, bejeweled crucifixes, crystal reliquaries holding the bones of saints, and venerated statues and paintings were attacked as "idols" and often defaced or destroyed. Protestant congregations continued, for the most part, to celebrate the most sacred Christian ritual, the Eucharist or Lord's Supper, but they did so in a profoundly different spirit from the Catholic Church, more as commemoration than as miracle, and they now prayed not in the old liturgical Latin but in the vernacular.

The Reformation was at first vigorously resisted in England. Indeed, with the support of his ardently Catholic chancellor, Thomas More, Henry VIII personally wrote (or at least lent his name to) a vehement, often scatological attack on Luther's character and views, an attack for which the Pope granted him the honorific title "Defender of the Faith." Protestant writings, including translations of the Scriptures into English, were seized by officials of the church and state and burned. Protestants who made their views known were persecuted, driven to flee the country or arrested, put on trial, and burned at the stake. But the situation changed decisively when Henry decided to seek a divorce from his first wife, Catherine of Aragon, in order to marry Anne Boleyn.

Catherine had given birth to six children, but since only a daughter, Mary, survived infancy, Henry did not have the son he craved. Then as now, the Catholic Church did not ordinarily grant divorce, but Henry's lawyers argued on technical grounds that the marriage was invalid (and therefore, by extension, that Mary was illegitimate and hence unable to inherit the throne). Matters of this kind were far less doctrinal than diplomatic: Catherine, the daughter of Ferdinand of Aragon and Isabella of Castile, had powerful allies in Rome, and the Pope ruled against Henry's petition for a divorce. A series of momentous events followed, as England lurched away from the Church of Rome. In 1531 Henry charged the entire clergy of England with having usurped royal authority in the administration of canon law (the ecclesiastical law governing faith, morals, and disciplines, including such matters as divorce). Under extreme pressure, including the threat of confiscations and imprisonment, the Convocation of the Clergy begged for pardon, made a donation to the royal coffers of over £100,000, and admitted that the king was "supreme head of the English Church and clergy" (modified by the rider "as far as the law of Christ allows"). The next year the Convocation submitted to the demand that the king be the final arbiter of canon law: one day later Thomas More resigned his post.

In 1533 Henry's marriage to Catherine was officially declared null and void, and on June 1 Anne Boleyn was crowned as queen. The king was promptly excommunicated by Pope Clement VII. In the following year, the parliamentary Act of Succession confirmed the effects of the divorce and required an oath from all adult male subjects confirming the new dynastic settlement. Thomas More and John Fisher, the Bishop of Rochester, were among the small number who refused. The Act of Supremacy, passed later in the year, formally declared the king to be "Supreme Head of the Church in England" and again required an oath to this effect. In 1535 and 1536 further acts made it treasonous to refuse the oath of royal supremacy or, as More had tried to do, to remain silent. The first victims were three Carthusian monks who rejected the oath — "How could the king, a layman," said one of them, "be Head of the Church of England?" — and in May 1535, they were hanged, drawn, and quartered. A few weeks later, Fisher and More were convicted and beheaded. Between 1536 and 1539 the monasteries were suppressed and their vast wealth seized by the crown.

Royal defiance of the authority of Rome was a key element in the Reformation but did not by itself constitute the establishment of Protestantism in England. On the contrary, in the same year that Fisher and More were martyred for their adherence to Roman Catholicism, twenty-five Protestants, members of a sect known as Anabaptists, were burned for heresy on a single day. Through most of his reign, Henry remained an equal-opportunity persecutor, ruthless to Catholics loyal to Rome and hostile to many of those who espoused Reformation ideas, though many of these ideas gradually established themselves on English soil.

Even when Henry was eager to do so, it proved impossible to eradicate Protestantism, as it would later prove impossible for his successors to eradicate Catholicism. In large part this tenacity arose from the passionate, often suicidal heroism of men and women who felt that their souls' salvation depended upon the precise character of their Christianity and who consequentially embraced martyrdom. It arose too from a mid-fifteenth-century technological innovation that made it almost impossible to suppress unwelcome ideas: the printing press. Early Protestants quickly grasped that with a few clandestine presses they could defy the Catholic authorities and flood the country with their texts. "How many printing presses there be in the world," wrote the Protestant polemicist John Foxe, "so many blockhouses there be against the high castle" of the Pope in Rome, "so that either the pope must abolish knowledge and printing or printing at length will root him out." By the end of the sixteenth century, it was the Catholics who were using the clandestine press to propagate their beliefs in the face of Protestant persecution.

The greatest insurrection of the Tudor age was not over food, taxation, or land but over religion. Most people conformed, more or less willingly, to the structural and doctrinal changes commanded by the king and his ministers, but there were pockets of resistance, particularly in the north of England, from those who were loyal to the traditional religious order of Roman Catholicism and who resented the attempt to subordinate the church to the authority of the state. On Sunday, October 1, 1536, stirred up by their vicar, the parishioners of Louth in Lincolnshire, in the north of England, rose up in defiance of the ecclesiastical visitation sent to enforce royal supremacy. The rapidly spreading rebellion, which became known as the Pilgrimage of Grace, was led by the lawyer Robert Aske. The city of Lincoln fell to the rebels on October 6, and though it was soon retaken by royal forces, the rebels seized cities and fortifications throughout Yorkshire, Durham, Northumberland, Cumberland, Westmoreland, and northern Lancashire. Carlisle, Newcastle, and a few castles were all that were left to the king in the north. The Pilgrims soon numbered forty thousand, led by some of the region's leading noblemen. The Duke of Norfolk, representing the crown, was forced to negotiate a truce, with a promise to support the rebels' demands that the king restore the monasteries, shore up the regional economy, suppress heresy, and dismiss his evil advisers.

The Pilgrims kept the peace for the rest of 1536, on the naive assumption that their demands would be met. Then, early in 1537, Henry moved suddenly to impose order and capture the ringleaders. One hundred and thirty people, including lords, knights, heads of religious houses, and, of course, Robert Aske, were executed.