The Radical Reformation is a term usually referring to those 16th century groups who rejected both the Roman Catholic tradition and the ongoing Protestant alternatives to it, in the name of what they considered truer forms of Christianity, most notably those who became known as Anabaptists. As a result, they were persecuted by Catholics and Protestants alike and their ideas and lives were bitterly attacked, often without a genuine knowledge of what they stood for.
Early forms of the Radical Reformation were often millenarian, focusing on the imminent end of the world. This was particularly notable in the rule of John of Leiden over the city of Münster in 1535, which was ultimately crushed by the forces of the Catholic Bishop of Münster and the Lutheran Landgrave of Hesse. After the fall of Münster, several small groups continued to adhere to revolutionary Anabaptist beliefs. The largest and most important of these groups, the Batenburgers, persisted in various forms into the 1570s.
In contrast to Roman Catholicism, mainstream Evangelical (Lutheran) and Reformed (Zwinglian, Calvinist) Protestant movements, the Radical Reformation generally abandoned the idea of the "Church Visible" as distinct from the "Church Invisible." Thus, the Church only consisted of the tiny community of believers, who accepted Jesus Christ by adult baptism, called "believer's baptism".
In addition to the Anabaptists, other strains of the Radical Reformation have been identified. Notably, George Hunston Williams, the great categorizer of the Radical Reformation, considered early forms of Unitarianism (such as that of the Socinians, and exemplified by Michael Servetus), and other trends that disregarded the Nicene christology still accepted by both Catholics, Orthodox, and the two larger Protestant groups, as part of the Radical Reformation.