The Oath of Supremacy, originally imposed by King Henry VIII of England through the Act of Supremacy 1534, but repealed by his daughter, Queen Mary I of England and reinstated under Mary's sister, Queen Elizabeth I of England under the Act of Supremacy 1559, provided for any person taking public or church office in England to swear allegiance to the monarch as Supreme Governor of the Church of England. Failure to do so was to be treated as treasonable. The Oath was later extended to include Members of Parliament and people studying at universities.
Text of the Oath as published in 1559:
I, A. B., do utterly testify and declare in my conscience that the Queen's Highness is the only supreme governor of this realm, and of all other her Highness's dominions and countries, as well in all spiritual or ecclesiastical things or causes, as temporal, and that no foreign prince, person, prelate, state or potentate hath or ought to have any jurisdiction, power, superiority, pre-eminence or authority ecclesiastical or spiritual within this realm; and therefore I do utterly renounce and forsake all foreign jurisdictions, powers, superiorities and authorities, and do promise that from henceforth I shall bear faith and true allegiance to the Queen's Highness, her heirs and lawful successors, and to my power shall assist and defend all jurisdictions, pre-eminences, privileges and authorities granted or belonging to the Queen's Highness, her heirs or successors, or united or annexed to the imperial crown of this realm. So help me God, and by the contents of this Book.
Roman Catholics who refused to take the Oath of Supremacy, for example Sir Thomas More, were indicted for treason on charges of praemunire. Such persons were generally executed.
Some of the language in the Oath would later inspire the United States Oath of Citizenship.