John Calvin (French: Jean Calvin, born Jehan Cauvin: 10 July 1509 – 27 May 1564) was an influential French theologian and pastor during the Protestant Reformation. He was a principal figure in the development of the system of Christian theology later called Calvinism. Originally trained as a humanist lawyer, he broke from the Roman Catholic Church around 1530. After religious tensions provoked a violent uprising against Protestants in France, Calvin fled to Basel, Switzerland, where he published the first edition of his seminal work The Institutes of the Christian Religion in 1536.

In that year, Calvin was recruited by William Farel to help reform the church in Geneva. The city council resisted the implementation of Calvin and Farel's 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 was eventually invited back to lead its church.

Following his return, Calvin introduced new forms of church government and liturgy, despite the opposition of several powerful families in the city who tried to curb his authority. During this time, the trial of Michael Servetus was extended by libertines in an attempt to harass Calvin. However, since Servetus was also condemned and wanted by the Inquisition, outside pressure from all over Europe forced the trial to continue. 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.

Calvin was a tireless polemic 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 the Institutes, he wrote commentaries on most books of the Bible, as well as theological treatises and confessional documents. He regularly preached sermons throughout the week in Geneva. Calvin was influenced by the Augustinian tradition, which led him to expound the doctrine of predestination and the absolute sovereignty of God in salvation of the human soul from death and eternal damnation.

Calvin's writing and preachings provided the seeds for the branch of theology that bears his name. The Reformed and Presbyterian churches, which look to Calvin as a chief expositor of their beliefs, have spread throughout the world.