Final answer:
The phrase from Romans 1:17 that influenced Martin Luther's concept of salvation was "The just shall live by faith." This understanding sparked the Protestant Reformation, as Luther challenged the Catholic Church's practice of selling indulgences and asserted that salvation is a free gift of grace accessible through faith alone.
Step-by-step explanation:
The phrase found in Romans 1:17 that led Martin Luther to the understanding of salvation by faith alone was "The just shall live by faith." This phrase was pivotal in Luther's theological breakthrough, where he recognized that salvation cannot be earned through good works but is a free gift of God's grace through faith. It was a cornerstone for the Protestant Reformation, fundamentally challenging the Catholic Church's teachings of that era, which encompassed practices such as the sale of indulgences.
Luther's comprehension of this phrase brought about his formulation of the Ninety-five Theses, which argued against the sale of indulgences and proposed the idea of justification by faith. This act not only propelled him to eventually be excommunicated from the Catholic Church but also ignited a movement that led to the establishment of Protestantism, with different Christian denominations embracing the idea that faith is paramount for salvation.
Moreover, Luther's theological reformation spread across Europe, influencing other reformers like John Calvin, who further developed the concept of predestination. Luther's reliance on scripture alone as the source of Christian belief also led to a departure from several traditional Catholic practices, thereby shaping the course of Christianity to this day.