I don't think the lack of the formal motive of faith necessarily makes you a formal heretic. That might be an area of discussion. Fr Michael Mueller would agree with St Thomas that heretics do not possess divine faith. But on the other hand he says that there is a distinction between a material heretic and a formal heretic. So if there is a distinction AND it is not possible for anyone outside the visible communion of the Catholic Church to have the formal motive of faith then it follows that not all those who lack the formal motive of faith are formal heretics.
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I should also point out that a Catholic is never a material heretic. I know it is popular nowadays to say that some Catholics are material heretics but where do theologians teach that? I've not seen it. Catholics may be in error about certain doctrines but as long as they have the formal motive of faith, they are not heretics, neither material nor formal.
These are both somewhat slippery terms and are variously defined by many theologians. I've seen a discussion somewhere by a more recent (pre-V2) theologian speaking about various definitions held by different theologians.
Yes, there are those who say Catholics cannot be formal heretics, due to the etymology of heresy (which entails pertinacity). Others use it. I like the term because it means that the Catholics are in material error about a matter that is dogmatic, i.e., where it would be "heresy" in the strict etymological sense to reject the truth. So material heresy, in that sense, means material error about a matter that would contradict a dogmatic truth ... to distinguish it from being in material error regarding a truth of a lesser theological note. I think that the disagreement there would be semantic ... except for ...
This is how they shifted the notion of material heresy into people who are heretics "in good faith" or "with sincerity".
I agree that not everyone who lacks the formal motive of faith is a formal heretic. There are people to whom the term heresy doesn't even apply because they're infidels. This term "formal heresy" specifically refers to those who are NOT infidels (in that they believe materially in the core truths of the Holy Trinity and Incarnation) but who do not hold these truths with the formal motive of faith, i.e. at least implicitly reject the formal motive behind all dogma. So, for instance, take an Old Catholic, who might believe most dogmas but then rejects papal infallibility. Since this teaching was delivered through the authority that underlies all dogmas, the rejection of this single dogma entails a rejection of the rule behind them all, replacing the correct formal motive of faith with a false one.
Where the issues started is with equating "formal" with "sincerity." I am a well-meaning Protestant who sincerely believes my errors, so I am only a material heretic (as per Xavier's position). Heck, who IS a "formal" heretic then? I am sure that most Old Catholics and Protestants are convinced that they're right. This limits formal heresy to those guys who are like, "Yeah, I know what the Church teaches, and I recognize that the Church teaches infallibly, but I don't like that particular dogma, so I refuse to accept it." It's similar to the issue of saying that those who KNOW the Church to be the True Church but REFUSE to enter it anyway. That would effectively render the term "formal heresy" meaningless, a matter only of the internal forum where God alone can judge TRUE sincerity or insincerity. It leads to invisible Church (internal forum) ecclesiology. It leads to all the errors we've been discussing here.
So, by formal heresy, I mean heretics who at least implicitly reject the rule of faith (the Church's teaching authority), and material heretics as Catholics who adhere formally to the rule of faith (acknowledge the Church's authority) but are mistaken about WHAT the Church teaches on a matter that would have the theological note of dogma.