The Protestant Reformation started when Martin Luther made a mild comment about the Roman Catholic Church in the early 16th century. This quickly became time for changes in the church.
The Reformation was 'sparked' when Martin Luther created the 95 Theses and nailed them to the door of the parish church in Wittenberg on October 17, 1517. After that, the sales of indulgences dropped greatly. Luther was then declared a heretic and excommunicated.