|El Greco, c1600-10|
A Dominican, as Cardinal Ghislieri he prosecuted eight French Bishops for heresy. He also stood firm against nepotism, rebuking his predecessor Pope Pius IV to his face when he wanted to make a 13-year old member of his family a cardinal and subsidise a nephew from the Papal treasury.
As Pope he acted quickly to restore discipline and morality, and to implement effectively the decrees of the Council of Trent.
He is most famous for promulgating the Tridentine Missal in 1570 which reflected the ancient practices of the Church of Rome, but necessarily of many other places, and thus in effect, if not in law, suppressing many legitimate rites such as the Sarum.
He also took strong measures with rather mixed results, against Protestants. In France he dismissed a Cardinal and several bishops who had been pursuing a policy of tolerance towards the Huguenots. And he excommunicated Elizabeth I of England in the bull Regnans in Excelsis, and urged her subjects to rebel against her, a measure that resulted in a much tougher policy of repression and many martyrdoms.
He also formed the Holy League, which enabled the defeat of the Turks at the Battle of Lepanto.