An old post from Roman Theo:
“An occult heretic, according to Bellarmine and everyone else, is a Catholic who is in formal heresy and hence lacks the virtue of faith, yet who has not publicly left the Church or been expelled from the Church. As long as the culprit remains externally united to the Church, he remains an occult heretic, even if his materially heretical acts are public.
In fact, in his chapter on occult heretics in De Ecclesia Militante (cap x), Bellarmine teaches that it is infallibly certain that everyone who is externally united to the Church is a member of the Body of the Church.
Fr. Joseph Cliffort Fenton describes what Bellarmine meant by an occult heretic in his article, Scholastic Definitions of the Church, that he published in the American Ecclesiastical Review.
"Thus Francis Suarez, who held exclusively to the formula congregatio fidelium as a definition of the Church, actually insisted that catechumens are members of this society, and that occult heretics are not (cf. Opus de Triplici Virtute Theologica [Lyons, 1621], Tract. I, Disp. 9, sect. 1, pp. 156-62). On the other hand, St. Robert Bellarmine, who held for the second type of definition, denied that catechumens are members, and taught that occult heretics are within the Church until they are expelled, or until they leave this organization through public apostacy (cf. De Ecclesia Militante, chaps. 3, 10).” (Fenton, Scholastic Definitions of the Church, Part III, American Ecclesiastical Review).
A "manifest heretic" is not someone who manifest heresy, which is what most sedevacantists have been led to believe. A manifest heretic is someone who has either been expelled from the Church, or has publicly left the Church of their own will.
None of the Conciliar popes have come remotely close to meeting Bellarmine's definition of a manifest heretic. It would have required a conviction of heresy by bishops at a council for any of them to be considered manifest heretics, or to be deprived of their pontifical dignity or authority, according to the teaching of Bellarmine.”