The Ninety-five Theses or Disputation on the Power and Efficacy of Indulgences is a list of propositions for an academic disputation written in 1517 by Martin Luther, professor of moral theology at the University of Wittenberg, Germany, at the time controlled by the Electorate of Saxony. Retrospectively considered to signal the start of the Protestant Reformation and the birth of Protestantism, the document advances Luther's positions against what he saw as the abuse of the practice of clergy selling plenary indulgences, which were certificates believed to reduce the temporal punishment in purgatory for sins committed by the purchasers or their loved ones. In the Theses, Luther claimed that the repentance required by Christ in order for sins to be forgiven involves inner spiritual repentance rather than merely external sacramental confession. He argued that indulgences led Christians to avoid true repentance and sorrow for sin, believing that they could forgo it by obtaining an indulgence. These indulgences, according to Luther, discouraged Christians from giving to the poor and performing other acts of mercy, which he attributed to a belief that indulgence certificates were more spiritually valuable. Though Luther claimed that his positions on indulgences accorded with those of the Pope, the Theses challenge a 14th-century papal bull stating that the pope could use the treasury of merit and the good deeds of past saints to forgive temporal punishment for sins. The Theses are framed as propositions to be argued in debate rather than necessarily representing Luther's opinions, but Luther later clarified his views in the Explanations of the Disputation Concerning the Value of Indulgences.

Luther sent the Theses enclosed with a letter to Albert of Brandenburg, Archbishop of Mainz, on 31 October 1517, a date now considered the start of the Reformation and commemorated annually as Reformation Day. Luther may have also posted the Ninety-five Theses on the door of All Saints' Church and other churches in Wittenbergin accordance with University customon 31 October or in mid-November. The Theses were quickly reprinted and translated, and distributed throughout Germany and Europe. They initiated a pamphlet war with the indulgence preacher Johann Tetzel, which spread Luther's fame even further. Luther's ecclesiastical superiors had him tried for heresy, which culminated in his excommunication in 1521. Though the Theses were the start of the Reformation, Luther did not consider indulgences to be as important as other theological matters which would divide the church, such as justification by faith alone and the bondage of the will. His breakthrough on these issues would come later, and he did not see the writing of the Theses as the point at which his beliefs diverged from those of the Roman Catholic Church.

The Reformation (alternatively named the Protestant Reformation or the European Reformation) was a major movement within Western Christianity in 16th-century Europe that posed a religious and political challenge to the Catholic Church and in particular to papal authority, arising from what were perceived to be errors, abuses, and discrepancies by the Catholic Church. The Reformation was the start of Protestantism and the split of the Western Church into Protestantism and what is now the Roman Catholic Church. It is also considered to be one of the events that signify the end of the Middle Ages and the beginning of the early modern period in Europe.Prior to Martin Luther, there were many earlier reform movements. Although the Reformation is usually considered to have started with the publication of the Ninety-five Theses by Martin Luther in 1517, he was not excommunicated until January 1521 by Pope Leo X. The Edict of Worms of May 1521 condemned Luther and officially banned citizens of the Holy Roman Empire from defending or propagating his ideas. The spread of Gutenberg's printing press provided the means for the rapid dissemination of religious materials in the vernacular. Luther survived after being declared an outlaw due to the protection of Elector Frederick the Wise. The initial movement in Germany diversified, and other reformers such as Huldrych Zwingli and John Calvin arose. In general, the Reformers argued that salvation in Christianity was a completed status based on faith in Jesus alone and not a process that requires good works, as in the Catholic view. Key events of the period include: Diet of Worms (1521), formation of the Lutheran Duchy of Prussia (1525), English Reformation (1529 onwards), the Council of Trent (1545–63), the Peace of Augsburg (1555), the excommunication of Elizabeth I (1570), Edict of Nantes (1598) and Peace of Westphalia (1648). The Counter-Reformation, also called the Catholic Reformation or the Catholic Revival, was the period of Catholic reforms initiated in response to the Protestant Reformation. The end of the Reformation era is disputed.