For one thing, he translated the Bible into German. Before then, it was typically only written in Latin. This was the first time anyone tried to make the words of the Bible available for anyone to read in their own native language. He also defied the Roman Catholic Church in the he allowed members of the church who had committed suicide to be buried in church cemeteries. And finally, he was an ordained Catholic priest, but he didn't hold to his vows of chastity. Apparently he didn't think that a man couldn't serve God if he was also married to a woman. Further information: The main way in which Luther challenged the Roman Catholic church was by directly challenging its teachings. Luther struggled with the Roman Catholic church's demands that one could only earn favour with God through good works. Through his in-depth study of the Scriptures, he reached the realisation that salvation is a gift of God's grace, received by faith alone and by trust in Christ's death on the cross as the only means to that salvation.
It was this that led him to openly question the teachings of the Roman Catholic church, in particular, the nature of penance, the authority of the pope and the usefulness of indulgences. The Reformation of the church began on 31 October 1517, with Luther's act of posting his Ninety-Five Theses on the door of the Castle Church in Wittenberg, Germany. The document contained an attack on papal abuses and the sale of indulgences by church officials.
Controversy raged over the posting of the 95 Theses. Luther was excommunicated several years later from the Roman Catholic church for his attacks on the wealth and corruption of the papacy, and his belief that salvation would be granted on the basis of faith alone rather than by works. His translation of the Bible into German occurred many years later, while he was hiding from the church authorities at Wartburg, the castle of Elector Frederick III of Saxony. This was a significant step because, prior to the Bible being made available to the common people, Christians relied almost entirely on church leaders to tell them what the Bible said. For centuries, the Bible was out of reach of most Christians. The only copies that existed were in Latin, which most people could not read or understand, and it was left to the clergy who were educated in the Latin language to mete out their own explanations - a practice which tended to be subjective, rather than objective.