Protestantism is one of the major groupings within Christianity. It has been defined as "any of several church denominations denying the universal authority of the Pope and affirming the Reformation principles of justification by faith alone, the priesthood of all believers, and the primacy of the Bible as the only source of revealed truth" and, more broadly, to mean Christianity outside "of a Catholic or Eastern church". It is a movement that began in Germany in the early 16th century as a reaction against medieval Roman Catholic doctrines and practices, especially in regards to salvation, justification, and ecclesiology. The doctrines of the over 33,000 Protestant denominations vary, but most include the Ten Commandments, justification by grace through faith alone, known as Sola Gratia and Sola Fide respectively, the priesthood of all believers, and the Bible as the supreme authority in matters of faith and morals, known as Sola Scriptura, Latin for "by scripture alone".

In the 16th century, the followers of Martin Luther established the evangelical (Lutheran) churches of Germany and Scandinavia. Reformed churches in Hungary, Scotland, Switzerland and France were established by John Calvin and other reformers such as Huldrych Zwingli. In addition, John Knox established a Calvinist communion in the Church of Scotland and also the Reformed Church in Hungary. The Church of England became independent of papal authority and influenced by some Reformation principles. There were also reformation movements throughout continental Europe known as the Radical Reformation which gave rise to the Anabaptist, Moravian, and other pietistic movements.