The heart of his protest was about the gospel. Luther ultimately believed that the Church of Rome had lost the gospel. Rome had come to believe that we are not justified by grace alone through faith alone in Christ alone. Rather, Rome believed and still believes we are justified by Christ and by our work through the sacraments. Christ does his part and we do our part and "voila"- we are justified.
Luther and the Reformers understood the Bible to teach that we are justified by faith apart from works of the law. Our works are actually an offense to God if we are seeking to be justified by them.
Rome's was no small error. Luther said that justification by grace alone through faith alone was the article by which the church stands or falls. This meant that if you denied this doctrine you no longer were a true church. John Calvin also agreed that this doctrine was the main hinge upon which everything else turns.
It is a frightening thing to deny justification by grace alone through faith alone in Christ alone. To do so is to drive off a cliff of Christian truth and explode on the rocks below. Be afraid.
The Roman Church stills denies this doctrine to this very day.
