The Church started selling indulgences to earn money. These were documents prepared by the church and bought by individuals either for themselves or on behalf of the dead that would release them from punishment from their sins, and decreased time in purgatory.
Poor people spent their money and didn't have enough to survive.
Martin Luther was angry with Pope Leo X's indulgences so he wrote 95 Theses against the church for selling indulgences. These were nailed on the University of Wittenberg's chapel door.
With the new invention of the printing press the word spread a lot quicker and caused a lot of debate.
Pope Leo X excommunicated Martin Luther.
Martin Luther was hidden. One of his most great achievements was translating the bible into German. With the new printing press it was spread to everyone and everyone could read the bible and find out what truly happened in the bible.