Martin luther’s posting of the ninety-five theses is considered by many to be a turning point in history because

Well, it marked the beginning of the Reformation, which was a time when people began questioning the Roman Catholic Church. Up until that point, if you were a Christian, you were in the Catholic Church, nowhere else. Because there was nowhere else to go. The Catholic Church was in control of the government at that time and so if you didn't agree with Catholic Church teachings, you were out of luck. Martin Luther's teachings later became the foundation of the Lutheran Church.