Final answer:
Martin Luther believed that every Christian was a priest because he argued that all Christians were part of the spiritual estate and had the same rights and access to God's grace.
Step-by-step explanation:
Martin Luther believed that every Christian was a priest because he argued that all Christians were part of the spiritual estate and had the same rights and access to God's grace. He believed that the Catholic Church had distorted the teachings of Christianity by claiming that only bishops and priests had the authority to perform sacraments and absolve sins. Luther's belief in the priesthood of all believers became one of the foundational principles of the Protestant Reformation.