The Council of Trent was a Catholic response to the Protestant Reformation of the early sixteenth century. It occurred under three popes: Paul III (r. 1534-1549), Julius III (r. 1550–1555), and Pius IV (r. 1559–1565).

In 1545, Pope Paul III called a general council that began a process of rejecting reconciliation with Protestants and cleaning up the abuses within the Catholic system and tradition. It also intended to reaffirm Catholic doctrine and ensure greater centralization for the Church.

There were numerous reforms and reaffirmations over the eighteen years the council lasted. There was reform in the appointment of bishops and priests. Bishops were not to be away from their diocese for more than three months a year. They were also to visit the parishes in the diocese at least once a year.

The Church wanted greater intellectual training for its clergy and established seminaries in each diocese. There was the revision of the Latin Bible which was deemed adequate for doctrinal proofs, the reshaping of Church administration, a reformulation of the Seven Sacraments, and changes in the use of indulgences.

The central administration of the Church was strengthened and many aspects that were particularly criticized by the Protestants, such as devotion to the Virgin Mary, the cult of saints, ecclesiastical hierarchy, the centrality of the priesthood, and pilgrimages, were reaffirmed. 

There was a review of the Sacraments of which seven were accepted while the Sacraments of Baptism and Confirmation were defined. There were changes in the use of indulgences and the reaffirmation of the doctrine of purgatory. The Catholic doctrine of transubstantiation had also come under heavy criticism from the Protestants and was defended. The Church maintained that Jesus Christ’s body and blood became present in the consecrated bread and wine during Holy Communion. The topics of excommunication and interdicts were discussed and instructed to be used sparingly and only when needed.

The Council of Trent also produced the Index of Forbidden Books through which the Church censored Catholic reading for several centuries until 1966. The Church’s rationale was for the benefit of its people. Just as an effective doctor strongly discourages his patient from consuming poison, so too the Catholic Church must protect the faithful by including books on the index that were considered immoral or theologically problematic. 

Beyond the Council of Trent there was a surge in Catholic missionary activity elsewhere. St. Ignatius Loyola (d. 1556) founded the Society of Jesus whose members became known as Jesuits. This body, with its strong emphasis on discipline and intellectual training, came to serve the papacy and contributed to the reconquest of certain parts of Europe for the Church, notably France, Germany, Poland, Austria, and other locations. In 1622, the Church set up the missionary body called the Congregation for the Propagation of the Faith.