Final answer:
Martin Luther was a central figure in the Protestant Reformation who was labeled a heretic by the Catholic Church due to his critique of church practices and doctrines, including the sale of indulgences and the authority of the Pope.
Step-by-step explanation:
Martin Luther played a fundamental role in the Protestant Reformation, challenging the practices and doctrines of the Roman Catholic Church. A heretic, in the context of the Church's response to Luther's actions, is someone who holds and promotes beliefs that are contrary to the established doctrines of the church.
Luther's public criticism, especially his stance against the sale of indulgences, led to his excommunication and branding as a heretic. His reforms proposed that faith alone was needed for salvation and that the scriptures were the only source of Christian authority, differing significantly from the Catholic teachings of that time.