The Challenge to Rome
Luther's idea of grace (salvation coming directly from God) implied a less important role for clergy, the group that had traditionally led others in prayer and who made up the Catholic Church.
From Luther's point of view, the clergy's leading role was unjustified -- nothing man could do (including the sacraments) could bring one closer to God; therefore, nothing the clergy could do -- other than reading Scripture to the people -- would justify it having authority over others.
To protest what he considered the excessive authority of the clergy over lay believers, Luther wrote in 1517 a formal complaint with 95 different items. Following standard practice of the day for making a public announcement, he posted these 95 theses on the door of the cathedral in the town where he was then living, in Wittenberg.