According to legend, Martin Luther nailed ninety-five theses to the door of Wittenberg Castle. This was a perfectly conventional way of initiating a theological debate at that time, and so there was nothing particularly controversial about Luther's actions. Nevertheless, the content of the theses proved to be dynamite; Luther wanted to debate the thorny issue of indulgences.
An indulgence was a remission of sin granted by the Church in return for money. The remission could be for yourself or for a family member, even for one who had died. For many Christian believers, indulgences seemed like a pretty good deal: peace of mind in return for a relatively small amount of money. However, Luther regarded the practice as an abomination, an example of the worldliness and corruption into which he believed the Church had sunk. As far as he was concerned, the sale of indulgences was little more than a tawdry scam that had no basis whatsoever in Scripture.
But the Church regarded his objections as sheer impudence; a mere Augustinian monk in a remote backwater of the Holy Roman Empire had no right to challenge the authority of Christ's Holy Body. But Luther and the Church were already at cross-purposes. Luther believed that the whole issue of indulgences boiled down to what was appropriate Christian teaching; the Church, however, regarded it as an issue relating to the exercise of its spiritual and temporal authority.