During the reign of Henry VIII, the Act of Supremacy established the Church of England.
The true term is actually "Acts of Supremacy" because there were two that were passed. The first Act of Supremacy was passed in November 1534. This was the one that Henry VIII passed. The primary Act of Supremacy was actually repealed by Queen Mary I in 1554. It was then reinstated by her sister (Queen Elizabeth I) when she took the throne; which made her the Supreme Head of the Church of England.
Before the Act of Supremacy, the Pope was the Supreme Head of the Church of England.
