Here's the bottom line ... thanks to the article linked to by Decem.
We can argue until the cows come home about what Bellarmine meant ... and it doesn't really matter, since this question has not been definitively settled by the Church.
Even St. Robert's opinion regarding the requirements for membership in the Church have not been authoritatively settled. There's another school of thought, which, although it has become less common these days, which holds, among other things, that occult heretics are not fully members of the Church. I think that the position has a lot of merit. And there are a tremendous number of permutations regarding these views.
So, in other words, of someone wanted to hold even that occult heretics would lose the papal office, there's nothing to stop them.
So we're wasting our time debating this issue.
Any theory must abide by things that the Church HAS settled, such as that Councils cannot formally depose popes (Lateran V), and that the Holy See is judged by no one, and that the Pope has supreme absolute authority in the Church (Vatican I). Any theory that does not contradict these principles can be held by a Catholic. What we're arguing about is whether some distinction or another is legitimate so that some theory which APPEARS to contradict one of these principles really doesn't.