John Calvin (Middle French: Jehan Cauvin; French: Jean Calvin [ʒɑ̃ kalvɛ̃]; 10 July 1509 – 27 May 1564) was a prominent French theologian, pastor, and reformer in Geneva during the Protestant Reformation. He was a principal figure in developing the system of Christian theology later known as Calvinism, which includes doctrines of predestination and God's absolute sovereignty in the salvation of human souls from eternal damnation. His doctrines were influenced by Augustinian and other Christian traditions.

Various Reformed Church movements, including Continental Reformed, Congregationalism, Presbyterianism, Waldensians, Baptist Reformed, Calvinist Methodism, and Reformed Anglican Churches, recognize him as a chief expositor of their beliefs.

Calvin was a tireless polemicist and apologetic writer, generating much controversy. He exchanged cordial and supportive letters with many reformers, including Philipp Melanchthon and Heinrich Bullinger. Besides his seminal work, Institutes of the Christian Religion, he wrote commentaries on most Bible books, confessional documents, and other theological treatises.

Originally trained as a humanist lawyer, Calvin broke from the Roman Catholic Church around 1530. After religious tensions led to deadly violence against Protestant Christians in France, Calvin fled to Basel, Switzerland, where he published the first edition of the Institutes in 1536. That year, he was recruited by William Farel to join the Reformation in Geneva, where he regularly preached sermons throughout the week. However, the city's governing council resisted their ideas, leading to his and Farel's expulsion.

Invited by Martin Bucer, Calvin moved to Strasbourg and became the minister of a church for French refugees. He continued supporting the reform movement in Geneva and was invited back in 1541 to lead the city's church. Upon his return, Calvin introduced new forms of church government and liturgy, despite opposition. During this time, Michael Servetus, regarded by both Roman Catholics and Protestants as heretical regarding the Trinity, was denounced by Calvin and executed in Geneva.

In his final years, Calvin promoted the Reformation in Geneva and Europe.