Final answer:
Martin Luther became an Augustinian monk following a life-altering experience during a thunderstorm at St. Anne's Chapel in 1505, and this eventually led him to initiate the Protestant Reformation.
Step-by-step explanation:
The frightening experience that compelled Martin Luther to give up a career as a lawyer and become an Augustinian monk was a thunderstorm at St. Anne's Chapel. This event occurred in 1505 when Luther vowed to St. Anne, the patroness of miners, that he would become a monk if she saved him from the storm. This transformative moment led Luther on a path that eventually sparked the Protestant Reformation. His nailing of the Ninety-five Theses to the door of the Castle Church in Wittenberg was a key event in challenging the practices and doctrines of the Roman Catholic Church, such as the sale of indulgences. Luther's actions and subsequent excommunication led to the development of Protestantism as a separate branch of Christianity.