Now Luther criticized the Church when the Church did not follow his criticism of the sacrament of penance. There is only one ultimate criterion for Christianity, namely the message of the Gospel. Therefore there is no infallibility of the Pope.
The Pope may fall into error. — Then his Catholic enemies showed him that it is not only the Pope but also some of the Councils which deserved to be attacked now.
Then he didn’t retire, but said: Then also the Councils may fall into error. — And this was actually the break, because this meant even if you go from the curialistic theory that the Pope in Rome alone is the monarch who decides… ; if you go then to the conciliaristic theory that the great Councils of the Church are absolutely infallible, even then Luther said: No, they are human, they may fall into error. The Pope could be tolerated, he says, if he were only by human law, by the law of expediency, as the chief administrator of the Church. But that is not what the Pope claims. He claims to be by Divine right, and that means he is an absolute figure in the Church. And here Luther said this cannot be stood, because no human being can ever be the vicar of the Divine power; the Divine right of the Pope is a demonic claim and actually the claim of the Antichrist. Of course, when he said this the break was clear. There is only one head of the Church, namely Christ, and the Pope as he is now is the creation of the Divine wrath to punish Christianity for its sins.
This was meant theologically, and not as name-calling; he meant it very seriously, theologically, when he called the Pope the Antichrist. It was not directed against a special man and his shortcomings – everybody criticized the behavior of the Pope at that time – but he criticized the position of the Pope, namely that the Pope is by Divine right the representative of Christ. In this way the Pope destroys the souls, because he wants to have a power which God alone can have.