Final answer:
Martin Luther's complaint about Johann Tetzel was about the unethical sale of indulgences, which Luther believed falsely assured salvation for money and were not endorsed by the Bible. His wider criticisms of Church practices and opulence led to his excommunication and sparked the Protestant Reformation.
Step-by-step explanation:
Martin Luther's main complaint about Johann Tetzel was regarding the sale of indulgences to raise money for the rebuilding of St. Peter's Basilica in Rome. Indulgences were being marketed as a way to reduce time in purgatory and provided a false assurance of salvation in exchange for money. Luther, a Professor of Theology at Wittenberg, argued that such practices were not supported by the Bible and that forgiveness could only be granted by God - a doctrine known as justification by faith.
Luther's concerns extended beyond Tetzel's indulgences; he also criticized the Church's taxation of ordinary Germans and the delivery of Mass in a language they could not understand. Posting his Ninety-five Theses on the door of the Castle Church in Wittenberg was Luther's way of initiating a debate on these issues, not an intention to split from the Church. His refusal to recant his beliefs and his excommunication eventually led to the Protestant Reformation, a significant movement that permanently altered the course of Christian history.
Interestingly, Luther's critiques went beyond the theological; he also rebuked the Church for its lavish spending and opulent lifestyle, comparing the splendor of Rome to 'the who re of Babylon.' His advocacy for reform and transparency were instrumental in initiating changes within the Church itself.