Final answer:
The Society of Jesus, or Jesuits, was the Catholic monastic order founded to combat the spread of Protestantism during the Counter-Reformation.
Step-by-step explanation:
The Catholic monastic order founded to stop the spread of the Reformation and reverse the gains of Protestants was the Society of Jesus, commonly known as the Jesuits. Established by Ignatius of Loyola, a Spanish noble, in the 1540s, the Jesuits took on several roles to rejuvenate the Catholic Church during the period of the Catholic Reformation, also known as the Counter-Reformation. They focused on the education of young Catholic men, the conversion of non-Christians, and acted as advisers to Catholic rulers. Additionally, they set out on missions to parts of the world that had been colonized as predominantly Catholic and attempted to reconvert areas that had become Protestantized during the Reformation.
Among their various strategies, the Jesuits also influenced the establishment of seminaries for the proper training of priests, returning religious orders to their spiritual foundations, and fostering new spiritual movements that emphasized a personal relationship with Christ. They actively participated in the political realm as well, including the Roman Inquisition and efforts to stem Protestantism in Europe.