I do not doubt that whoever reads or hears these letters, if he is reasonable or has any conscience before God, will have to say that there was an extraordinarily great spirit in this man, John Hus. He wrote and taught in such a Christian manner. He fought so valiantly against the trials of death. He suffered everything so patiently and humbly, and in the end he so manfully accepted the most shameful death for the sake of the truth. There were so many great and powerful people of high estate gathered from all over the world, and he stood there among them alone, like a little lamb among many lions and wolves. If he is to be considered a heretic, then surely there has never been a true Christian on earth. For by what fruits will one recognize a true Christian, if not by these fruits of John Hus?
He had done nothing worse than to teach that if the pope is not righteous, then he is not head of the holy Church. He allowed the pope to remain a head of the church, but not of the holy Church, just as a wicked pastor is a pastor, but not a member of the true saints in his parish. John Hus also said, "If the pope is a scoundrel, then he is not righteous even though he is supreme in the church." It is just as if you or I were to say, "If Judas is a thief and traitor, then he is not righteous, even if he is an apostle." But he ought to have said, "Even if the pope is a scoundrel and a villain, he is nevertheless holy and cannot err, and whatever he does or says is holy and right, nothing less than an article of faith." That is what the Lord's in the Council of Constance wanted to hear, no matter that they themselves surely condemned and deposed three popes as scoundrels, and yet no one was allowed to burn for that. But when John Hus said it, he had to submit [to the flames].
For the source of the conflict was that the pope had announced indulgences to the world and appointed a golden year at Rome in order to build the church of St. Peter, etc. And among other Roman and papal innovations the pope in his bulls promised all those who died on their way to Rome that they would immediately ascend to heaven and, in addition, that he (as an earthly god and God's vicar) had power to command the angels to lead such souls of the dead to heaven straightaway, just as Tetzel, who was selling the cardinal of Mainz’s indulgences, also taught that whenever a penny clinks in the chest, straightaway a soul flies from purgatory to heaven. Then they put away those pipes, and still keep them stored away, until they are able to pipe up such a dance again. . . .
. . . John Hus took a stand against these things in Prague . . . and he reproved them, saying that the pope did not have the power to do these things and had acted unjustly in these and other matters. . . . And when he was being led to the fire, he always had on his lips: "O Jesus, Son of God, have mercy on me." And when he saw the stake, upon which he was to be burned, he fell to his knees and called out: "O Jesus, Son of God, who suffered for us, have mercy on me.”. . .
. . . However, if [Hus] is not a great martyr of Christ – a man able in death to call earnestly upon the Lord Jesus, God’s Son, who suffered for us, and to go into the fire for such a cause and with such a faith and confession – then no one can be saved. For [Jesus] says, "Whoever confesses Me before the world, him will I confess" [cf. Matt. 10:32]. In sum, the pope makes many saints, and who knows whether they are in hell? And he has thrown this man into hell, who certainly must be in heaven. So let the devil be your saint, and you be the saint of the devil, dear pope!
Martin Luther, “Preface and Afterword to John Hus, Three Letters Written to the Bohemians from Prison at Constance by the Most Holy Martyr , 1536/1537.” Pages 126-128, 132, 133 in vol. 60 of Luther’s Works, American Edition. Edited by Christopher Boyd Brown. St. Louis: Concordia, 2011.