Mary Tudor burned Protestants at the stake because she was a devout Catholic and believed that heresy was a crime punishable by death. She saw the Protestant Reformation as a threat to her authority and the Catholic faith, and she used the power of the state to suppress it. Mary's persecution of Protestants was particularly severe during the years 1555-1558, when she was known as "Bloody Mary." During this time, over 300 Protestants were burned at the stake. Mary's persecution of Protestants was eventually unsuccessful, as Protestantism continued to grow in England and eventually became the dominant religion.