Final answer:
Indulgences are the acts taught by the Catholic Church as necessary for salvation, which Protestant reformers like Martin Luther countered with the doctrine of salvation through faith alone.
Step-by-step explanation:
The acts that the Catholic Church teaches are necessary to obtain salvation, which were countered by Luther and other Protestant reformers who believed in salvation through faith alone, are called indulgences. These indulgences were ways to reduce the time spent in purgatory to atone for sins before reaching heaven. Martin Luther's primary concern was that these indulgences allowed people, especially the wealthy, to 'purchase' their way to forgiveness, which he believed was contrary to the doctrine of justification by faith as it is not based on human actions but entirely upon God's grace.Luther was also critical of the fact that indulgences seemed to suggest that salvation could be earned or bought, rather than being a free gift from God received through faith. His Ninety-five Theses challenged this practice and sparked the Protestant Reformation. The Catholic Church, in response to the reformers, addressed the question of indulgences at the Council of Trent by forbidding their sale and emphasizing a combination of faith and good works for salvation.