in the history of Christianity, the Roman Catholic efforts directed in the 16th and early 17th centuries both against the Protestant Reformation and toward internal renewal; the Counter-Reformation took place during roughly the same period as the Protestant Reformation, actually (according to some sources) beginning shortly before Martin Luther's act of nailing the Ninety-Five Theses to the church door (1517).

Early calls for reform grew out of criticism of the worldly attitudes and policies of the Renaissance popes and many of the clergy. New religious orders and other groups were founded to effect a religious renewal—e.g., the Theatines, the Capuchins, the Ursulines, and especially the Jesuits. Later in the century, John of the Cross (John of the Cross, Saint) and Teresa of Ávila (Teresa of Ávila, Saint) promoted the reform of the Carmelite order and influenced the development of the mystical tradition. Francis of Sales (Francis of Sales, Saint) had a similar influence on the devotional life of the laity.

There was little significant papal reaction to the Protestants or to demands for reform from within the Roman Catholic Church before mid-century. Pope Paul III (reigned 1534–49) is considered to be the first pope of the Counter-Reformation. It was he who in 1545 convened the Council of Trent (Trent, Council of). The council, which met intermittently until 1563, responded emphatically to the issues at hand. Its doctrinal teaching was a reaction against the Lutheran emphasis on the role of faith and God's grace and against Protestant teaching on the number and nature of the sacraments. Disciplinary reforms attacked the corruption of the clergy. There was an attempt to regulate the training of candidates for the priesthood; measures were taken against luxurious living on the part of the clergy, the appointment of relatives to church office, and the absence of bishops from their dioceses. Prescriptions were given about pastoral care and the administration of the sacraments.

The Roman Inquisition, an agency established in 1542 to combat heresy, was more successful in controlling doctrine and practice than similar bodies in those countries where Protestant princes had more power than the Roman Catholic Church. Political and military involvement directed against Protestant growth is most clearly reflected in the policies of Emperor Charles V and in those of his son Philip II, who was associated with the Spanish Inquisition.

Various theologians—especially the Jesuit Robert Bellarmine (Bellarmine, Saint Robert)—attacked the doctrinal positions of the Reformers, but there was no one to rival the theological and moral engagement evident in the writings of Luther or the eloquence and passion characteristic of the works of John Calvin. Roman Catholics tended to emphasize the beliefs and devotional subjects that were under direct attack by the Protestants—e.g., the Real Presence of Christ in the Eucharist, the Virgin Mary, and St. Peter.

A major emphasis of the Counter-Reformation was an ongoing missionary endeavour in parts of the world that had been colonized by predominantly Roman Catholic countries. The work of such men as Francis Xavier and others in Asia and of missionaries in the New World was rewarded with millions of baptisms, if not true conversions. There were also attempts to reconvert areas of the world that had once been Roman Catholic—e.g., England and Sweden.