Final answer:
Martin Luther discredited the medieval church, criticized the clergy's political involvement, emphasized spiritual equality, and believed that Scripture alone should be the basis of Christian belief, initiating the Protestant Reformation.
Step-by-step explanation:
The historical figure who discredited the medieval church, declared clergy's involvement in politics as sinful, preached spiritual equality, and viewed Scripture as the basis of all true belief was Martin Luther. Luther, a German monk and theology professor, initiated the Protestant Reformation in 1517 by posting his 95 Theses. These theses challenged the Roman Catholic Church's practices, such as selling indulgences, and emphasized that salvation is achieved through faith alone, not by purchasing indulgences or performing good works. Luther also advocated for the translation of Scripture into vernacular languages so that everyone could read and understand the Bible, leading to greater spiritual equality.