Final answer:
Martin Luther believed justification of the soul comes through faith alone, marking a pivotal shift from the Catholic Church's emphasis on good works and indulgences for salvation.
Step-by-step explanation:
According to Martin Luther, the soul is justified by faith alone. Luther's spiritual crisis led him to the conclusion that it was impossible to earn salvation through good works as mandated by the Catholic Church. He found solace in the words of St. Paul, "The just shall live by faith" (Romans 1:17), which he interpreted to mean that salvation is a gift of God's grace, received through faith rather than by works.
This realization marked a significant theological shift, emphasizing personal faith over the church's doctrine, and it eventually led to the Protestant Reformation. Consequently, Luther protested against the sale of indulgences, which was a common practice in the Catholic Church at that time, where it was taught that buying indulgences could reduce the time in purgatory.