John Calvin (; Middle French: Jehan Cauvin; French: Jean Calvin [ʒɑ̃ kalvɛ̃]; 10 July 1509 – 27 May 1564) was a French theologian, pastor and reformer in Geneva during the Protestant Reformation. He was a principal figure in the development of the system of Christian theology later called Calvinism, including its doctrines of predestination and of God's absolute sovereignty in the salvation of the human soul from death and eternal damnation. Calvinist doctrines were influenced by and elaborated upon the Augustinian and other Christian traditions. Various Congregational, Reformed and Presbyterian churches, which look to Calvin as the chief expositor of their beliefs, have spread throughout the world.

Calvin was a tireless polemicist and apologetic writer who generated much controversy. He also exchanged cordial and supportive letters with many reformers, including Philipp Melanchthon and Heinrich Bullinger. In addition to his seminal Institutes of the Christian Religion, Calvin wrote commentaries on most books of the Bible, confessional documents, and various other theological treatises.

Calvin was originally trained as a humanist lawyer. He broke from the Roman Catholic Church around 1530. After religious tensions erupted in widespread deadly violence against Protestant Christians in France, Calvin fled to Basel, Switzerland, where in 1536 he published the first edition of the Institutes. In that same year, Calvin was recruited by Frenchman William Farel to join the Reformation in Geneva, where he regularly preached sermons throughout the week. However, the governing council of the city resisted the implementation of their ideas, and both men were expelled. At the invitation of Martin Bucer, Calvin proceeded to Strasbourg, where he became the minister of a church of French refugees. He continued to support the reform movement in Geneva, and in 1541 he was invited back to lead the church of the city.

Following his return, Calvin introduced new forms of church government and liturgy, despite opposition from several powerful families in the city who tried to curb his authority. During this period, Michael Servetus, a Spaniard regarded by both Roman Catholics and Protestants as having a heretical view of the Trinity, arrived in Geneva. He was denounced by Calvin and burned at the stake for heresy by the city council. Following an influx of supportive refugees and new elections to the city council, Calvin's opponents were forced out. Calvin spent his final years promoting the Reformation both in Geneva and throughout Europe.