In the revised version, I add this argument, that if Popes could commit heresy, we could never be sure which teachings to believe. We would not even know which ideas were heresy. If any Pope might be a heretic, we would not be able to trust any particular Pope’s teaching. Then the teaching of Councils would also be in doubt. If a Pope commits formal heresy, he ceases to be a member of the Church and therefore ceases to be Pope. But an Ecumenical Council is only a true Ecumenical Council if it has a true Pope as its head. You would never be sure which Popes might be secret heretics, having lost the faith interiorly. Therefore, you would never know which Ecumenical Councils were valid.
Having lost all confidence in Popes and Councils, we would be left with Sacred Tradition and Sacred Scripture. But we require an authoritative interpreter to understand Tradition and Scripture. Otherwise, we would end up disagreeing and dividing into different factions, as has happened to the Protestants. So even by considering Tradition and Scripture, we could not be sure which Popes were heretics, since it would depend on interpretation.
The section of the book examines this argument in greater depth. But essentially, if any Pope could teach heresy, or could commit formal heresy, the surety of the faith is lost. We would never be sure which Popes and Councils to trust and which teachings are truly of the Magisterium. The only solution to the problem is that God prevents every Pope from teaching material heresy and from committing formal heresy.
The final point is whether God might permit a Pope to teach mere material heresy, inadvertently, only as a private theological opinion. This would not seem to harm the Church in the ways described. My response is this: God does not do things by half measure. Since He has decided to give the Church the gift of a Pope who cannot commit formal heresy, nor teach material heresy under the Magisterium, there would be no reason for Him to permit material heresy as a private opinion. For the private opinions of Popes are given more weight than the private opinions of ordinary theologians. And God does not give flawed gifts. Since He decided to give the Church the gift of Popes who are free from teaching or committing heresy, He would not leave this gift with the flaw of the accidental teaching of mere material heresy.
The book will have some new chapters, to consider documents and controversies since the book was published in 2015.
Please take a look at this list of my books and booklets, and see if any topic interests you.