Which of the following was a key belief of Martin Luther?

Martin Luther's key belief that all people are equal in the eyes of God fundamentally challenged the prevailing views of authority and hierarchy in the Church during his time. He emphasized the importance of individual faith and the belief that salvation is accessible to all, not restricted to an elite class or mediated solely by the clergy. This idea was central to his call for reform within the Catholic Church, advocating for the priesthood of all believers, which posited that everyone has the right to read and interpret the Scriptures without needing an intermediary. This belief was a driving force behind the Protestant Reformation, encouraging people to question established religious practices and doctrines, thus promoting a more personal relationship with God.