Chloe, Mary I was a devout Catholic. He father, Henry VIII broke with the church of Rome and appointed himself Supreme Head of the Church of England when the Pope refused to grant him a divorce from her mother, Catherine of Aragon so that he could marry Anne Boleyn. Catherine always argued that she was Henry's true wife and the Mary was a legitimate princess (Henry declared her a bastard after the divorce). He then left her mother to die in a cold, damp castle whilst Protestantism spread throughout the country.
When Henry died, knowing that Mary would turn the country catholic again her enemies placed Lady Jane Grey on the throne instead. She was executed. She went on a rampage against 'heretics' to turn her country back to the 'true faith' and burnt non Catholics at the stake :|