One of the greatest of Popes: hammer of heretics, champion of Lepanto, defender of Christendom, preserver of the faith, and author of the foundational Quo Primum. His feast day is today.
From The Liturgical Year, by Dom Prosper Gueranger:
“The whole life of Pius V was a combat. His pontificate fell during those troubled times when Protestantism was leading whole countries into apostasy. Italy was not a prey that could be taken by violence: artifice was therefore used, in order to undermine the Apostolic See and thus develop the whole Christian world in the darkness of heresy. Pius defended the Peninsula with untiring devotedness from the danger that threatened her. Even before he was raised to the Papal Throne he frequently exposed his life by his zeal in opposing the preaching of false doctrines. Like Peter the Martyr, he braved every danger and was the dread of the emissaries of heresy. When seated on the Chair of Peter, he kept the innovators in check by fear, roused the sovereigns of Italy to energy and by measures of moderate severity drove back beyond the Alps the torrent that would have swept Christianity from Europe had not the Southern States thus opposed it. From that time forward, Protestantism has never made any further progress: it has been wearing itself out by doctrinal anarchy. We repeat it: this heresy would have laid all Europe waste, had it not been for the vigilance of the pastor who animated the defenders of truth to resist it where it already existed, and who set himself as a wall of brass against its invasion in the country where he himself was the master.
Another enemy, taking advantage of the confusion caused in the West by Protestantism, organized an expedition against Europe. Italy was to be its first prey. The Ottoman fleet started from the Bosphorus. This again would have meant the ruin of Christendom but for the energy of the Roman Pontiff, our Saint. He gave the alarm, and called the Christian Princes to arms. Germany and France, torn by domestic factions that had been caused by heresy, turned a deaf ear to the call. Spain alone, together with Venice and the little Papal fleet, answered the summons of the Pontiff. The Cross and the Crescent were soon face-to-face in the Gulf of Lepanto. The prayers of Pius V decided the victory in favour of the Christians, whose forces were much inferior to those of the Turks… [W]e cannot omit to mention today the prediction uttered by the holy Pope, on the evening of the great day of October 7, 1571. The battle between the Christian and Turkish fleets lasted from six o’clock in the morning till late in the afternoon. Towards evening, the Pontiff suddenly looked up towards heaven, and gazed upon it in silence for a few seconds. Then turning to his attendants, he exclaimed: ‘Let us give thanks to God! The Christians have gained the victory!’ The news soon arrived at Rome; and thus, Europe once more owed her salvation to a Pope! The defeat at Lepanto was a blow from which the Ottoman Empire has never recovered: its fall dates from that glorious day.
The zeal of this holy Pope for the reformation of Christian morals, his establishment of the observance of the laws of discipline prescribed by the Council of Trent and his publication of the new Breviary and Missal have made his six years’ pontificate to be one of the richest periods of the Church’s history…”
O God, Who for the overthrow of the enemies of Thy Church and for the restoration of divine worship didst vouchsafe to choose blessed Pius as Supreme Pontiff: grant that we may be defended by his patronage and so cleave to Thy service, that overcoming all the wiles of our enemies, we may rejoice in perpetual peace. Through our Lord Jesus Christ Thy Son, Who liveth and reigneth with Thee in the unity of the Holy Ghost, God, world without end. Amen.
–From the Collect of the Mass of St. Pius V