Material heresy, which is not heresy strictly so called but only error, ("if he is not obstinate, he is no longer in heresy but only in error" - St. Thomas
link and "such men are not to be counted heretics" - St. Augustine
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Thus far we have been discussing Catholic teaching. It may be useful to add a few points about purely theological opinions – opinions with regard to the pope when he is not speaking ex cathedra. All theologians admit that the pope can make a mistake in matters of faith and morals when so speaking: either by proposing a false opinion in a matter not yet defined, or by innocently differing from some doctrine already defined. Theologians disagree, however, over the question of whether the pope can become a formal heretic by stubbornly clinging to an error in a matter already defined.
As is obvious, we are not discussing the possibility of the Pope being in material heresy. No one denies, that mistakenly or by inadvertence, the Supreme Pontiff can fall into material heresy, as a private person.
Only public and formal heretics (this is what St. Robert means by "manifest heretic" for he explains "the manifest heretic is not in any way a member of the Church, that is, neither spiritually nor corporally, which signifies that he is not such by internal union nor by external union") lose the pontificate. The sin of heresy, publicly manifested, is necessary to lose interior supernatural faith, membership in the Church, and the papal power. Someone who is only in error, even grave error, but holds what he does only because he sincerely believes it to the be the teaching of the Church is not a heretic but a Catholic who is gravely mistaken.