So, this is a methodological question, seeking to arrive at (to the extent that the question is genuine, which it isn't) an understanding of how someone actually learns the faith. It's a good question! It's at the heart of the entire BoD "controversy", in fact.
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Fenton is a theologian. It's his job, as it is the job of any other theologian, to explain the ins and outs of dogma. Remember how St. Thomas Aquinas responded to the vision he received (which is largely believed to have been similar to St. Paul's when he was taken up to where "eye hath not seen and ear hath not heard," etc., etc.)? He called his work, which was (and remains) the singular greatest contribution to Catholic theology, straw. Again, as St. Paul says, who can comprehend the depth, breadth, etc.? The Catholic Faith is so incredibly-- and incomprehensibly-- vast that there's probably not a singular work-- in all of the scriptures, councils, and manuals-- that captures all of it. Saint John's Gospel ends on that effect, claiming that not all the books in the world could contain everything that Christ did. And when we look at councils, condemnations, etc. they are always very focused on a particular controversy. At Nicea it was Christ's divinity. At Trent it was the Church's authority. At Vatican I it was the pope's authority. So on and so forth. No Council has ever been called to just list all of the teachings of the Church, never mind to explain in definitive detail why they each exist. The theologians, though, help to satisfy this role and in scholastic fashion, they systemically produce proofs for the various dogmas, and in the process we find certain implied truths. To illustrate how this works, consider that there's no solemn definition that Jesus Christ was an only child. There's nothing in scripture to conclude this, in fact, scripture refers to relatives of Christ, and depending on the translation, one could conclude that He did have siblings, and only an intricate knowledge of Greek or an appeal to Catholic religious authority i.e., the Church and her translators as authoritative judges, would lead one to reject the idea of Christ had any siblings. Similarly, no council condemns this proposition. But it can be deduced clearly enough from other dogmas-- for instance, the perpetual virginity of the Mother of God tells us that whomever she bore, she bore miraculously. Combine with the knowledge that Christ alone was conceived and born miraculously, we know that she did not bear any other children miraculously, and knowing that she was perpetually a virgin, we know that she obviously bore no children naturally. As a matter of logical necessity, we see that Christ had no siblings. These are the sorts of things that theologians help with. You're not going to learn that Christ was the Blessed Mother's only son from primary texts. It's not there.
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Furthermore, inasmuch as the teaching of the theologians is properly incorporated into the Church's magisterium, some doctrine or another, or some explanation or another, really is part of the magisterium. Not on the authority of the theologians alone, but on the authority of the Church who incorporated their findings. This happened with St. Thomas, it happened with Bellarmine, Ligouri, Augustine, etc., and we see it especially with Bellarmine's ecclesiology which became the Church's in a real proper sense both in how she incorporates it into canon law (c.f. C. 18 ), and in how it became the boilerplate ecclesiological model for virtually all ecclesiologists after him.
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This all goes to show the value of the theologians. No one has ever said they, alone, are infallible. Fenton's value is two-fold. He is valuable in himself, as a learned man. He is a contributor to the sacred sciences. This value impacts serious discussions about ecclesiology among like minds (in other words, this isn't a value that most here would agree with, but it is a value nevertheless). At a broader level, he is valuable inasmuch as his voice contributes to the consensus of the theologians, which is indicative of what the Church teaches, and of what the Church allows to be taught. The Church cannot universally allow error to be published; her theologians are responsible for training her priests, so for every theology manual that breeches the subject (of BoD) to teach that BoD is Church teaching is tantamount to BoD being Church teaching. If it were not, and it were contained in all the manuals, the Church would be universally imposing error on this point. An impossibility. This is, of course, a point that some may disagree with; those who contend that the NOM is produced by the Church and is evil have of course already committed to believing that the Church is capable, indeed, is quite proficient in the destruction of souls. But for those who already apprehend and appreciate that the Church cannot impose universal error for belief, it follows neatly that the theologians cannot universally err, inasmuch as the Church approves and disseminates their work, on a point of morals or doctrine.
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Of course, the teaching of BoD is not something one only finds in the theologians, either. It's found in Trent's Catechism, which is how the Church taught Trent to the faithful. The Church didn't print of reams of Trent's canons in Latin for the laity, she published a catechism. Which was written by the same people who were at the council. Trent put the Summa up on the altar along with the Gospels, and the Summa teaches BoD. St. Alphonsus taught BoD, as did Bellarmine, who developed what is sometimes derisively called "Fenton's theory" (the soul of the Church). That's Bellarmine, not Fenton. Two doctor's of the Church, on top of popes (Innocents II & III, Pius IX, etc.). Cyprian, Basil, et al. When one looks at the fathers, one does not find a condemnation of BoD; one either finds reticence, or approval. When we look at Trent, we find that all the minds contemporary to it considered it to have taught BoD, we see that St. Thomas, the principal theologian resorted to at the Council, taught BoD, we find that Alphonsus, the premier moral theologian (and another Doctor of the Church) taught it, we find that Bellarmine, who was a veritable hammer of heretics and whose office was recipient of the most challenging protestant screeds (because the Church trusted Bellarmine more than anyone to overcome them) taught it. We find no dissent whatsoever, except after Vatican II, from certain traditionalists. So one can only conclude (on the hypothesis that BoD is an error) that Satan's forces have succeeded in concealing from the Church the fact that she has been teaching universal error for five hundred years. Now again, there are some who have no problem with that, which is lamentable. But for those who at least make a nominal effort to understand how the Church is unstained, this is an untenable position. BoD has clearly belonged to the ordinary magisterium since the earliest fathers, and it was taught at Trent, so it's part of the solemn magisterium, too (obviously a point of contention for Feeneyite interlocutors, but bears mentioning anyways).
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In the OP, it is implied that Fenton is untrustworthy because of his response to VII. This is a self-serving exception for a couple of reasons. First of all, with the exception of Home aloners who don't patronize any traditional clergy at all, every traditional Catholic in praxis extends a certain benefit of the doubt to those clergy who endured Vatican II. Lefebvre said the New Mass. He signed off on the docuмents. So did Thuc. Saint Benedict's Center pledges their allegiance to the Novus Ordo. The dogmatic sedevacantists were once heretics (by their own logic, because they at one point "recognized" Vatican II and the conciliar claimants). Fenton died before the new mass even came out. To whatever extent he approved of Vatican II, he didn't approve of its errors, but made an attempt-- as did virtually everyone else-- to understand Vatican II "in the light of tradition." Now the traditional clergy will smirk at such an idea, and we've learned over time that Vatican II is irredeemable, but their own pioneers and heroes once believed such a thing. If Fenton is untrustworthy due to this, so is Lefebvre and everyone else. At least Fenton didn't live through puppet masses and the rest and still try to reconcile it. God had mercy on him in that regard.
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Another challenge with the OP is that the question-- which again, is a question of how Catholics learn-- can be turned around in a hurry, and it looks much worse for those who deny BoD. The fundamental approach to Catholic learning taking by those who are broadly called "Feeneyites"-- generally those who believe that the gates of Heaven are absolutely and unexceptionally shut to those who did not receive water baptism-- is that Catholics learn the faith through primary texts, which include the scriptures and the docuмents of solemn councils, and whatever solemn papal definitions we have. The reasoning is that only such material is solemn magisterium, and as such, only such material is absolutely guaranteed to be free from all error. This excludes from Catholic learning pretty much every seminal text that traditionalists resort to in order to make their case; Pascendi, The Syllabus of Errors, etc. go into the wastebin.
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Now, of course, the interlocutor would reply by saying that such docuмents are cohesive with what's been received so far at a solemn level, so they're trustworthy. But this ruins the notion of ordinary magisterium (remember, magisterium means teaching office) because one never actually learns from it. What one does is, if one is so inclined, is check to see if there is any inconsistency between the ordinary and extraordinary. And in so doing, one fails to actually learn from it, because one can only accept whatever one apprehends as being consistent with the solemn magisterium. There's nothing in an ordinary text (according to this view) worthy of belief that wasn't already in a primary text. Now, in practice, the traditionalist who denies BoD is happy to apply the sort of learning methodology that he condemns when engaging with those who believe BoD is Church teaching, because he will eagerly show how a given ordinary text (be it Pascendi, Rerum Novarum, etc.) is consistent with Trent or whatever else, and naturally we would agree, but we agree on the grounds that ordinary teaching (when understood as the ordinary magisterium in its proper, strict sense) is already infallible, not because we checked it against Trent and found it legit.
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Summary: The theologians, taken singularly, are not infallible. But even taken singularly, they have value because it is their job-- not the job (necessarily) of solemn definitions, to explain the intricacies of a particular doctrine and its relationship to others. Taken collectively, they have the same value, with the additional value of being an indicator of what the Church teaches, allows to be taught, and condemns, because for all of the theologians to err on a particular issue (inasmuch as their work is approved and used by the Church) is tantamount to the Church teaching error universally. They further have value when their teachings are incorporated into the Church's ordinary or even solemn magisterium proper (e.g., Bellarmine's influence on ecclesiologists and the influence of that ecclesiological model on Mystici Corporis Christi, or St. Thomas' influence [including BoD] at Trent). At that point, there really is no difference between a particular theologian and the Church herself, inasmuch as the Church completely mirrors whatever that theologian taught. And whatever the case, BoD's inclusion in the deposit of faith is hardly limited only to the works of minor theologians; it finds no dissent from the fathers, it finds positive approval at Trent, in Trent's catechism, and then a similar approval with its inclusion in subsequent catechisms throughout the centuries. There is not only a failure on the part of the Church to condemn BoD, it is positively taught by her at every level. Obviously those who deny BoD will say that Trent condemns BoD when it says that Baptism should not be considered a metaphor-- and if they think that BoD is a metaphor for baptism, they clearly don't know what metaphors or BoD actually are. Or they'll say that Trent's condemnation of baptism as optional is a condemnation of BoD-- again, belying a misapprehension of what BoD is-- if a person desires it (i.e., they think "I'd better go get baptized") then they obviously don't think that it's optional. Thinking it's optional is thinking that I'll be just as good with or without it. BoD isn't that. Even when considered implicitly, like St. Alphonsus teaches.
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At any rate, these problems are faced because of the learning method that those who deny BoD employ. This learning method proposes that only that which is solemnly defined (or contained in scripture) is "really" infallible, i.e., that the learning Church can only trust whatever teaching is contained in them. The ordinary magisterium isn't really a teaching organ of the Church, because it is only accepted by virtue of the reader's apprehension of the degree to which it "gels" with a primary text. Which isn't a learning process at all; it's a critical process that presupposes that error can be contained in the ordinary magisterium. Of course, this method wreaks utter havoc not just on the Catholic faith (since it precludes from trustworthy scriptural commentary, catechisms, and anything that's translated into the reader's tongue [since the primary docuмents are not in English, or French, but Latin and sometimes Greek]), but it also wreaks considerable havoc on learning as a Catholic today, given that it precludes from the infallible realm of Church teaching such quintessential and seminal anti-modernist texts like Pascendi, the Syllabus of Errors, and similar docuмents.
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By way of a concluding thought, the question (when asked honestly) about how "we" differentiate ordinary teaching from error is a good question. Vatican II shows that an organ which proposes to be the Church may teach error. How, then, do we confidently throw out the ordinary teaching of Vatican II while remaining faithful to the ordinary teaching before Vatican II? Doesn't Vatican II and the modernist revolution show that ordinary teaching is far from infallible, and that Catholics who hope to remain faithful to the Catholic Church have a proper duty to vet ordinary teaching?
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I would say that traditional Catholics don't become traditional Catholics (unless they were raised so, of course) by vetting the ordinary magisterium. Rather, they become traditional Catholics precisely because they don't. They believe that the Church's ordinary magisterium is infallible, so they receive it. Then, they begin to see contradictions between what was taught "then" and what is taught "now." If one is already prepared to vet the ordinary magisterium, Vatican II really does not pose any serious difficulties, nor does it require any explanation. It's just "BoD all over again." The Church is teaching and believing error, just as she always has. It's par for the course, it's not unusual, it's not against her constitution, nor is it anything that we need to make any serious effort to understand. But, if we begin (as we should) from a position of docility to "believe these and all the truths the Holy Catholic Church teaches," only then do we come to a point where something requires explanation. Notice the operation here: traditionalists become traditionalists because they trust the ordinary magisterium, not because they don't.
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Unfortunately, what happens next for some is that they hastily conclude that Vatican II's teachings came from the Church, so they end up doubting Vatican I, or adopting unorthodox views about it, or (as we see in the case of the Feeneyite error) they end up abandoning the ordinary magisterium altogether. Rather, a better explanation can be found in apprehending that the infallibility of the ordinary magisterium is a diffusal of the Pope's infallible teaching, so it depends on a pope. With no pope, there really is no (living) ordinary magisterium, not to the extent that it enjoys any infallibility at any rate. Vatican II wasn't "not infallible" because it lacked some magic words, it was not infallible because there was no pope to protect it from error. Likewise, the new CIC, JPII's catechism, so on and so forth, are all beyond the actions of the ordinary magisterium because the ordinary magisterium's infallibility is, by definition, what the bishops teach in union with the pope. No pope=no infallibility.
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We live in an unusual time (those of us who were born after Vatican II, which is most of us, I gather) because we have never lived with a pope, nor have we ever lived at a time where there was a living magisterium that was infallible due (once again) to there being no pope. This can create the impression, at first glance, that ordinary teaching is liable to error. But it's a misapprehension of what ordinary teaching is, and of how Catholics learn. We are unfortunately bereft of an accessible authority (at least most of us anyways; by necessity they're somewhere) to assist in resolving many issues, but that is a result of the times, not a result of the Church's actual constitution, which protects both her extraordinary and ordinary teaching. So, we do the best we can with access to what ordinary teaching is available. This does not mean we must exclude council texts, but that we read them with the Church as our interpreter, and that means reading them with deference to the authoritative explanations handed down to us through commentaries, encyclicals, manuals, catechisms, etc. Just as a Catholic can say to a protestant, "show me where in the bible it says that the bible is the sole rule of faith," so too can a Catholic say, "show me where in a solemn definition we learn that only solemn definitions and scripture are the sole rule of faith." The Church didn't have a solemn definition until Nicaea. The blood of the martyrs, the seed of the Church, was spread without a single solemn definition. And you can be sure they didn't know how to read the scriptures, either. They believed in the ordinary teaching of the Church, which has always been infallible.