Luther's ninety-five theses of 1517 were an academic protest about a specific abuse. A campaign was selling indulgences in Germany, remitting the temporal penalty of sin, with the proceeds divided between the rebuilding of St Peter's and the repayment of a debt incurred by an archbishop to secure his own appointment. Luther's objections were initially pastoral: that the practice gave people false assurance and that a pope who could empty purgatory should do it out of love rather than for money. Whether he nailed the theses to the church door in Wittenberg is itself uncertain and disputed by historians.
The argument escalated because of what it exposed. Pressed in debate at Leipzig in 1519, Luther conceded that popes and councils could err, which moved the dispute from indulgences to authority. By 1520 he was publishing on the priesthood of all believers, on the sacraments, and against the papacy, and the answer to the question of what should decide a religious controversy had become scripture rather than the teaching office.
Justification by faith, worked out in these years, is the theological core: that a person is accepted by God as a gift received in faith rather than through any merit accumulated. Everything contested afterwards, the number of sacraments, the mass, the monastic life, the invocation of saints, follows from where that answer is placed.