Calvinism (also called the Reformed tradition, Reformed Christianity, Reformed Protestantism, or the Reformed faith) is a major branch of Protestantism that follows the theological tradition and forms of Christian practice set down by John Calvin and other Reformation-era theologians. Paul Helm, a well-known Reformed theologian, appropriately uses the term Augustinian Calvinism due to John Calvin's self-admitted adherence to Augustine's theology. "Augustine is so wholly within me, that if I wished to write a confession of my faith, I could do so with all fullness and satisfaction to myself out of his writings."
Calvinists broke from the Roman Catholic Church in the 16th century. Calvinists differ from Lutherans on the real presence of Christ in the Eucharist, theories of worship, and the use of God's law for believers, among other things. The term Calvinism can be misleading, because the religious tradition which it denotes has always been diverse, with a wide range of influences rather than a single founder. In the context of the Reformation, Huldrych Zwingli began the Reformed tradition in 1519 in the city of Zürich. His followers were instantly labeled Zwinglians, consistent with the Catholic practice of naming heresy after its founder. Very soon, Zwingli was joined by Martin Bucer, Wolfgang Capito, William Farel, Johannes Oecolampadius and other early Reformed thinkers.
The namesake of the movement, French reformer John Calvin, converted to the Reformed tradition from Roman Catholicism only in the late 1520s or early 1530s as it was already being developed. The movement was first called Calvinism, referring to John Calvin, by Lutherans who opposed it. Many within the tradition find it either an indescriptive or an inappropriate term and would prefer the word Reformed to be used instead. The most important Reformed theologians include John Calvin, Huldrych Zwingli, Martin Bucer, William Farel, Heinrich Bullinger, Peter Martyr Vermigli, Theodore Beza, and John Knox. In the twentieth century, Abraham Kuyper, Herman Bavinck, B. B. Warfield, J. Gresham Machen, Karl Barth, Martyn Lloyd-Jones, Cornelius Van Til, Gordon Clark, and R. C. Sproul were influential. Contemporary Reformed theologians include J. I. Packer, John MacArthur, Timothy J. Keller, David Wells, and Michael Horton.
Reformed churches may exercise several forms of ecclesiastical polity; most are presbyterian or congregationalist, though some are episcopalian. Calvinism is largely represented by Continental Reformed, Presbyterian, and Congregationalist traditions. The biggest Reformed association is the World Communion of Reformed Churches with more than 100 million members in 211 member denominations around the world. There are more conservative Reformed federations such as the World Reformed Fellowship and the International Conference of Reformed Churches, as well as independent churches.