Final answer:
The Catholic Church, through the Council of Trent, embarked on a vigorous reinforcement of Catholic doctrine and internal reforms during the sixteenth century to prevent Catholics from deviating from Church teachings in the face of Protestant challenges.
Step-by-step explanation:
To ensure that no Catholic deviated from doctrine, the Catholic Church initiated several measures during the sixteenth century, especially in response to the Protestant Reformation. The most significant measure was the Council of Trent, which took place between 1545 and 1563. This council was crucial in countering the challenges posed by Protestant theologians like Martin Luther and John Calvin. It affirmed important Catholic doctrines such as the necessity of both faith and good works for salvation, the acceptance of both scripture and tradition as sources of divine authority, and the doctrine of transubstantiation. Moreover, the Catholic Reformation brought about internal reforms, including the improvement of clergy education, the prohibition of indulgence sales, and the establishment of new religious orders such as the Jesuits, which focused on education and missionary work. These efforts were aimed at reigniting Catholic spirituality and codifying Catholic doctrines to prevent defection among the faithful.