“I demonstrate in this volume that the proposition, that ‘a pope actually can fall into formal heresy’, is proximate to heresy, but it is not de fide; and the first Vatican Council, as the Gasser Relatio states quite unequivocally, did not intend to define on this point. On the other hand, the proposition that a pope can ‘teach false doctrines by way of the authentic papal Magisterium’, has always been generally accepted by theologians, even by Don Pietro Ballerini; and even after the definition on papal infallibility by the First Vatican Council, not only theologians, but even documents of the supreme magisterium admit that pronouncements of the authentic papal magisterium are not infallible, such as Lumen Gentium 25, which distinguishes between ex cathedra pronouncements which are infallible, and “the authentic magisterium of the Roman Pontiff, even when he is not speaking ‘ex cathedra’ – which is not infallible.”

Kramer, Paul. To Deceive the Elect: The Catholic Doctrine on the Question of a Heretical Pope (Kindle Locations 3514-3521). Kindle Edition.

“The question of whether a given pope has lost his office on account of heresy is hypothetical, since it has never been proven that a pope can actually fall into formal heresy. The opinion, that a pope cannot be a heretic, (the first opinion outlined by Bellarmine) is the one that is most commonly taught as the most probable by the majority of theologians and Doctors: St. Robert Bellarmine, St. Alphonsus de Liguori, Francisco Suárez, Melchior Cano, Domingo Soto, John of St. Thomas, Juan de Torquemada, Louis Billot, Joachim Salaverri, A. Maria Vellico, Charles Journet, Cardinal Tommaso de Vio ‘Cajetan’, Francesco Bordoni, Pedro de Simanca, Domingo Bañez, and Martino Bonacina – and Bonacina cites others who were of the same opinion. For roughly a century this nearly unanimous opinion has been the most common, even among those who admit only the hypothetical possibility of a pope falling from office due to public defection into heresy.”

Kramer, Paul. To Deceive the Elect: The Catholic Doctrine on the Question of a Heretical Pope (Kindle Locations 3385-3392). Kindle Edition.