What can we learn from this event today? Why does Martin Luther matter?
1). Luther reminds us the "church" can err. The Roman Catholic Church was selling indulgences - they had drifted away from the Scriptures. If it happened back then, it can happen today.
2). On January 3, 1521 Luther was excommunicated from the Catholic Church. He was warned of his writings and views, and he refused to recant them. The word "Protestant" comes from the word, "Protest." All Protestant churches are in essence a "Protest" church of the Roman Catholic and Orthodox churches. When he nailed his theses to the door four years earlier, he probably didn't intend to begin a new branch of Christianity and be removed from the Catholic Church.
The two main doctrines Protestants reject from the Reformation to today are papal supremacy and the sacraments. The doctrine of salvation reemerged as faith alone - by rejecting the sacraments of salvation the Catholic Church taught, Luther began the greatest revival in Christian history. Luther's rejection led to salvation.
3). Luther translated the Bible into German, the language of the people. The common people in Germany during the 16th century, did not know Latin. Luther translated the Bible into German and used the newly invented printing press to print mass copies. Regular Joe and Jane all of a sudden were able to read the Scriptures for themselves. They also were able to "Protest" the Catholic Church with their own Bible. Finally the Bible began to come to every home.
Today, the language of the people is mobile internet - if they can't read it from their hand, they don't read it.