Martin Luther was a Catholic priest, if you remember, so he was NOT attacking from the "outside". ALL of the areas he tried to reform were intended to be from within and several faults actually DID get changed. For those the hierarchy refused to correct, Luther was commanded to recant and bow in submission to the Pope on all areas. He could not and remain honest to the word of God because the Lord had opened his spiritual eyes and he saw through the accursed gospel Rome insisted on clinging to. For that, he was cast out, though this was not his intention.
During the time of the Reformation, there were numerous depraved and wicked popes and bishops who recognized what they would have to personally give up to get right with God and they refused. They chose to persecute and hunt down those who wanted to restore the church to what Christ intended it to be. They left the reformers with no choice BUT to leave and continue to lead others to saving faith in Jesus Christ. What many Catholics today don't know is that their church leaders had perverted the Gospel and the teachings of Jesus and the Apostles and used their offices for personal enrichment and power - something the early church was forbidden to do.
The Reformation in the sixteenth century was NOT the first time the Catholic Church had to face the correction of God. It won't be the last either. I have a strong sense that Martin Luther has already been welcomed into heaven with a hearty, "Well done my good and faithful servant.", and he is enjoying being in the presence of the Lord for eternity.