This dates back to the reign of King Henry VIII, who was granted the title ‘Defender of the Faith’ in 1521 by Pope Leo X. When Henry VIII renounced the spiritual authority of the Papacy in 1534 he was proclaimed ‘supreme head on earth’ of the Church of England. This was repealed by Queen Mary I but reinstated during the reign of Queen Elizabeth I, who was proclaimed ‘Supreme Governor’ of the Church of England.
At Her Late Majesty The Queen’s Coronation in 1953, she was anointed by the Archbishop of Canterbury and took an oath to “maintain and preserve inviolably the settlement of the Church of England, and the doctrine worship, discipline, and government thereof, as by law established in England”.
The Queen formally appoints Archbishops, Bishops and Deans of the Church of England. Church of England deacons and parish priests also swear an oath of allegiance to the Sovereign.
In 1970 The Queen became the first Sovereign to inaugurate and address the General Synod in person. After this, Her Late Majesty inaugurated and addressed the opening session of the General Synod every five years.
To read more from the national Church of England website: The Queen: her faith and the Church of England | The Church of England