The Acts of Supremacy are two acts passed by the Parliament of England in the 16th century that established the English monarchs as the head of the Church of England

 

                                                     Queen Katherine_of_Aragon

The Acts of Supremacy are two acts passed by the Parliament of England in the 16th century that established the English monarchs as the head of the Church of England; two similar laws were passed by the Parliament of Ireland establishing the English monarchs as the head of the Church of Ireland. The 1534 Act declared King Henry VIII and his successors as the Supreme Head of the Church, replacing the Pope. This first Act was repealed during the reign of the Catholic Queen Mary I. The 1558 Act declared Queen Elizabeth I and her successors the Supreme Governor of the Church, a title that the British monarch still holds.

Royal supremacy is specifically used to describe the legal sovereignty of the King (i.e., civil law) over the law of the Church in England.

https://en.wikipedia.org/wiki/Acts_of_Supremacy#First_Act_of_Supremacy_1534

Henry VIII's Act of Supremacy was repealed in 1554 during the reign of his staunchly Roman Catholic daughter, Queen Mary I. Upon her death in November 

1558, her Protestant half-sister Elizabeth I succeeded to the throne. The first Elizabethan Parliament passed the Act of Supremacy 1558,[nb 1] which declared Elizabeth the Supreme Governor of the Church of England, instituted an Oath of Supremacy, requiring anyone taking public or church clergymen to swear allegiance to the monarch as head of the Church and state. Anyone refusing to take the oath could be charged with treason.[12]

The use of the term Supreme Governor as opposed to Supreme Head pacified some Catholics and those Protestants concerned about a female leader of the Church of England. Elizabeth, who was a politique,[citation needed][13] did not prosecute nonconformist laymen, or those who did not follow the established rules of the Church of England unless their actions directly undermined the authority of the English monarch, as was the case in the vestments controversy. 

Thus, it was through the Second Act of Supremacy that Elizabeth I officially established the now reformed Church of England. This was a part of the Elizabethan Religious Settlement

https://en.wikipedia.org/wiki/Elizabethan_Religious_Settlement