Defender of the Faith is a title that was given to the King or Queen of England by the Pope in 1521. It means that the ruler is responsible for protecting the Catholic faith and defending it against any enemies or threats.
At that time, the English monarch was Henry VIII, and he was a devout Catholic. He was even given the title by the Pope because he had written a book against a man named Martin Luther who was speaking out against the Catholic Church.
However, as years went by, Henry VIII became more and more upset with the Pope and the Catholic Church. He wanted to divorce his wife and marry another woman, but the Pope would not allow it. Henry VIII eventually decided to break away from the Catholic Church and start his own religion called the Church of England.
Even though he had broken away from the Catholic Church, Henry VIII still held onto the title of Defender of the Faith. Today, the title is still used by the British monarch, but it now refers to the defense of all religions and faiths, not just the Catholic Church.