" The first main work, To the Christian Nobility of the German Nation, was Luther’s trumpet blast of reformation against the defensive walls Rome had built around herself. There were three such walls, he said: Rome’s first defence was the claim that the pope was the supreme power on earth; the second, that only the pope may interpret the Scriptures; the third, that no one but the pope may summon a council and thus reform the church. "

― Michael Reeves , The Unquenchable Flame: Discovering the Heart of the Reformation