Luther gained support from people and criticism from the Church. He originally was a monk and a priest in the Catholic church and also a key figure.
Selling of Indulgences (Catholic concept of salvation). Luther didn't like the fact that people had to buy Indulgences or reduced the punishment after death. In other words, buying an Indulgence could not get you to heaven.
Luther realized that only faith (in the ultimate goodness of Jesus), not good deads, could save a person. No good works, rituals etc. would save a person if they did not believe.
Martin Luther made the church a better place now that people can read the bible, strict rules, eliminated existing problems, different beliefs, peace love and, understanding. Luther made the Catholic Church reflect on the long doings.