Martin Luther's Ninety-Five Theses were originally intended to spark reform within the Catholic Church by identifying what Luther saw as departures from Biblical precepts. Luther was a monk, who had lived an austere an ascetic life, who was eventually ordained and then sent to study and eventually teach theology at the University of Wittenberg. In his meditations as a monk and studies of the Bible, he began to formulate the doctrine of salvation by faith rather than works. He was particularly concerned that works, and particularly the rituals of the church, turned our emphasis away from grace and from goodness as a response to grace.
The particular event that sparked the theses was the sale of indulgences by Johann Tetzel. Theologically, the notion of indulgences is built upon a concept of a treasury of merit stored up by Jesus Christ and the saints, a sort of storehouse overflowing with virtues. These merits, like the sacrifice of Jesus, could help save people who themselves had insufficient merits to go to Heaven or reduce the time sinners would need to spend in Purgatory. The Roman Catholic Church, in need of money for rebuilding St. Peter's, hit upon the idea of selling indulgences for cash, treating the donation of money to the church as a sort of "good work" and asserting that the Church had control over the treasury of merits and could dispense the stored up merits of the saints in return for cash.
Luther saw the sale of indulgences as bad for two reasons. First, and most obviously, he saw it as corrupt and a sign of the church succumbing to luxury and materialism. Second, and more importantly, he saw it as part of a theology which focused on the institutional power of the Church and the clergy as a road to salvation, and made salvation something humans could work for, rather than seeing salvation as flowing from divine grace. He emphasized that humans could only be saved by God, not by humans or human institutions.