Fidei defensor (Latin for Defender of the Faith) was a title granted on October 17, 1521 by Pope Leo X to King Henry VIII of England due to Henry's book Assertio Septem Sacramentorum (written with the uncredited assistance of Thomas More) which defended the sacramental nature of marriage and the supremacy of the Pope.
The Latin version of the title, abbreviated to FD, is seen on all current British coins. It was first placed on coins in 1714 in the reign of George I of England.
Most Commonwealth Realms where the British Monarch is head of state omit the title "defender of the faith" from their country's official title given to the Queen.
Recently, Charles, Prince of Wales has said that he would like to recast the title as Defender of Faith to avoid the appearance of favouring one religion above another.
Fidei defensor or Defender of the Faith has been one of the subsidiary titles of the English (and later British) Monarchs since it was granted on October 17, 1521 by Pope Leo X to Tudor King Henry VIII of England (some other major Catholic Kingdoms have obtained similar pious titles, such as Apostolic King).
When the Tudor king broke with Rome and established himself as head of the Church of England, from the papal point of view the worst attack on the faith (or rather, and more to the point, on the Catholic Church) since Luther, the title was revoked by Pope Paul III.
Most Commonwealth Realms where the British Sovereign is head of state omit the title "defender of the faith" from their country's full official title given to the Monarch, while maintaining the initial By the Grace of God, e.g.