Final answer:
Martin Luther, a German monk and Professor of Theology, is best known for his criticism of indulgences and Catholic Church practices, leading to the Protestant Reformation.
Step-by-step explanation:
The most famous critic of the practices of "Indulgences" and the theology of the Catholic Church, who effectively used the printing press to publish his own theology, was Martin Luther. Luther, a German monk and Professor of Theology at the University of Wittenberg, sparked the Protestant Reformation. His opposition to the sale of indulgences, the Catholic Church's taxation of ordinary Germans, and the delivery of Mass in Latin led to his drafting the Ninety-five Theses. This document challenged the practice of selling indulgences as a means to reduce time in purgatory and questioned the authority of the pope, notably on the claim of him being 'God on earth'. His actions and use of the printing press to disseminate his theses greatly facilitated the spread of his message and laid the foundation for the Protestant Reformation.