Pope Pius V, OP, born Antonio Ghislieri, was head of the Catholic Church and ruler of the Papal States from 7 January 1566 to his death, in May 1572. He is venerated as a saint of the Catholic Church. He is chiefly notable for his role in the Council of Trent, the Counter-Reformation, and the standardization of the Roman Rite within the Latin Church, known as Tridentine mass. Pius V declared Thomas Aquinas a Doctor of the Church.

The Council of Trent, held between 1545 and 1563 in Trent, now in northern Italy, was the 19th ecumenical council of the Catholic Church. Prompted by the Protestant Reformation at the time, it has been described as the embodiment of the Counter-Reformation.