How did Martin Luther challenge papal authority in the Ninety-five Theses?

In his Ninety-five Theses, Martin Luther challenged papal authority by attacking the theological and financial abuses associated with the sale of indulgences. His critique focused on three primary areas:

• Financial Abuses: Luther argued that the papacy was exploiting the poor to fund the construction of St. Peter’s Basilica. He asserted that if the pope truly understood the poverty of the German people, he would prefer the church to remain unbuilt rather than be funded by the suffering of his followers.1
• Doctrinal Abuses: Luther questioned the pope’s jurisdiction over purgatory. He famously argued that if the pope possessed the power to release souls from purgatory, he should do so out of love and necessity rather than for money.21
• Religious Authority: By implying that the true treasury of the church was the gospel rather than the Treasury of Merit, Luther undermined the basis of papal control over salvation.13

When the papacy responded by declaring his position heretical, Luther further challenged papal authority by denying the infallibility of both popes and church councils, declaring Scripture to be the only valid basis of authority.1