The Puritans were a group of English Reformed Protestants in the 16th and 17th centuries who sought to "purify" the Church of England from its "Catholic" practices, maintaining that the Church of England was only partially reformed.
Puritanism in this sense was founded as an activist movement within the Church of England. The founders, clergy exiled under Mary I, returned to England shortly after the accession of Elizabeth I of England in 1558.
Puritanism played a significant role in English history during the first half of the 17th century. One of the most effective stokers of anti-Catholic feeling was John Pym, whose movement succeeded in taking control of the government of London the time of the Grand Remonstrance of 1641.
Puritans were blocked from changing the established church from within and were severely restricted in England by laws controlling the practice of religion. Their beliefs, however, were transported by the emigration of congregations to the Netherlands, and later to New England in North America, and by evangelical clergy to Ireland (and later to Wales), and were spread into lay society and parts of the educational system, particularly certain colleges of the University of Cambridge. They took on distinctive beliefs about clerical dress and in opposition to the episcopal system, particularly after the 1619 conclusions of the Synod of Dort were resisted by the English bishops. They largely adopted Sabbatarianism in the 17th century, and were influenced by millennialism.