There are actually four main versions of that argument. The first runs roughly as follows:
Scripture and/or Tradition are fully public and materially contain the full content of the deposit of faith. The Magisterium claims to "infallibly" hand on and clarify the doctrinal content of those two "sources" of transmission of divine revelation. But whether infallible or not, the Magisterium only does the sort of thing that any Spirit-guided Christian could in principle do, given the publicity and material sufficiency of the sources. Therefore, such a magisterium is in principle dispensable.The problem with that argument is that the conclusion does not follow from the premises. To get it to follow, one needs at least the following, additional premise:
(i) Some methodology other than binding and (allegedly) infallible interpretation by ecclesiastical authority enables the Spirit-guided Christian, at least in principle, to attain sufficient knowledge of the deposit of faith from Scripture and/or Tradition.
Many people, mostly Protestants, believe (i) either because their personal religious experience leads them to believe they've attained such knowledge without the Church, or because they believe that otherwise there would be no way to assess the orthodoxy of any self-proclaimed magisterium, Catholic or otherwise. But the problem with (i) is that there is no good reason to believe it.
The only good reason to believe (i) would be to hit upon a methodology, ecclesiologically neutral in itself, which objectively suffices to render a particular hermeneutic of Scripture and/or Tradition doctrinally comprehensive and rationally compelling. But if nearly two millennia of exegesis and theology show anything at all, they show that there is no such methodology. Eastern Orthodoxy, for example, has never claimed there is such a methodology; it has always insisted, like Catholicism, that authentic interpretation of the sources can only be conducted in conformity with the mind of "the Church." And Protestants who claim there is such a methodology often disagree about which doctrinal results are thereby secured. That's why there are Protestant "denominations." Rather few of the Protestant participants in that debate can be charged with outright irrationality; with more or less plausibility, they just disagree among themselves as well as with Catholicism and Orthodoxy. Absent appeal to an infallible interpreter, that leaves the question who is right as a matter of opinion rather than of binding doctrine. But that does not suffice for identifying the entire content of the deposit of faith as an object of the assent, precisely, of faith in God the infallible Revealer. All it does is present Scripture (and a fortiori Tradition, of which Scripture is the uniquely normative written record) as raw material for the forming of more or less plausible opinions. Many such opinions are doubtless logically equivalent to doctrines that are of faith; but as opinions, they neither constitute nor express assent by faith.
That points up the fundamental difficulty with the argument in question: there is no ecclesiologically neutral methodology for determining who interprets Scripture correctly, and who thus knows their interpretations to be binding and irreformable for the whole Church's assent of faith as distinct from tentative opinions. Some Christians appeal to a "burning in the bosom" or to their holy people of choice to confirm their interpretations; but such inherently subjective arguments can yield nothing that is rationally compelling and authoritative for the Church as a whole, without an ad hoc and doctrinally front-loaded limitation on who counts as "the Church."
In view of such difficulties, some people argue against the Magisterium's claims in a narrower way. Thus:
The Magisterium enables Christians to know the full content of the deposit of faith as an object for the assent of faith only if the doctrines it presents as binding and irreformable can be demonstrated to belong to the apostolic faith. But the most distinctively Catholic doctrines, including the Magisterium's claims for itself, are precisely those which cannot be thus demonstrated. Therefore, the Magisterium does not help Christians know the full content of the deposit of faith as an object for the assent of faith.The difficulty with that argument is that it begs the question at the outset. How? If the first sentence is true, then we can assess the Magisterium's claims for itself only if we can reliably know the content of the deposit of faith "given once for all to the holy ones" without recourse to the Magisterium's claims for itself. Hence, the Magisterium as it understands itself is justifiable only if superfluous for knowing the rest of the deposit of faith. But if the Magisterium is superfluous in that way, then its claim to be the sole "authentic" interpreter of the sources is false. An argument that begs the question at the outset need not be taken seriously as an argument.
That's probably why many non-Catholic Christians prefer a more philosophical approach. For purposes of a blog post, a good example is the argument made by a commentator over at Called to Communion:
What good Protestant theologians actually believe is that a sincere believer, aided by the Holy Spirit, who approaches the Scripture with humility in the context of a living community of faith and the Christian Tradition will be able to find great confidence about those truths necessary to salvation and to grow, however slowly and fallibly, closer to the truth on more doubtful matters. This fallibility is inherent to our situation as human beings and is in no way mitigated by your Catholic position since you have fallibly determined that organizational and doctrinal continuity with the Apostles is a guide to doctrinal reliability, and you have fallibly determined that the Church of Rome exhibits such continuity. Finally, you fallibly interpret the Roman Church’s doctrinal proclamations. Adding the infallibility of the Church generally or the Pope specifically will not get you into a significantly better epistemic state than the agreed upon doctrine of the infallibility of Scripture.In other words: since the assent of faith is up to each individual, and each individual is fallible, then the assent of faith is itself fallible; and if so, then proposing some set of doctrines S with alleged infallibility gives people no more certainty of the truth of S than would holding S as a set of human opinions only.
John Henry Newman's well-known rejection of "private judgement" in religion is often criticized in such a manner. Thus if the assent of faith as an epistemic stance is fallible, given the fallibility of each of the assenters, then ultimately there is no reliable way to distinguish the objective content of the irreformable deposit of faith, as revealed by God, from fallible opinions, held collectively by members of "the Church," about the data handed down to us. If the purpose of the self-styled Magisterium is to afford us a reliable way to make that distinction, then the Magisterium is wasting its own and everybody else's time. For what it's after is something that cannot be had and therefore should not be sought.
Now if the Magisterium were offering its definitive judgments merely as products of academic research, or even of special religious experiences, that criticism would be perfectly justified. And such factors often play a important role in forming magisterial judgments, as well as an even more important role in defending them. Yet no matter how well they serve, they could not themselves be decisive without the Magisterium's claims for itself succumbing to the objection at hand. What's decisive among and for the Magisterium's claims is its claim that it is divinely authorized, to the same degree as the Apostles themselves, to teach doctrine which irreformably binds the whole Church and is, by that same divine authority, protected from teaching what is false when it does so. If that is true, then the inherent fallibility of believers who take the Magisterium at its word does not infect the truth of what they assent to when they make the assent of faith; it infects only their degree of understanding that truth. Assuming Christianity is true, the fact remains that no particular believer, not even the pope, can ever be absolutely certain that their own understanding of a particular doctrine is as free from error as the doctrine itself. Rather, and as a matter of fact, they trust implicitly that the doctrine is true and seek to conform their mind ever more closely with that of the Church, for which the Magisterium speaks, on the doctrine's subject matter. Of course, if the Magisterium's particular claims for itself are true, then "the Church" as a whole will enjoy, or in due course attain, as sound an understanding of the doctrine as the subject matter permits. But that doesn't guarantee that any believer in particular will do so. That is one reason why the Catholic Church tolerates a great deal of what is, objectively speaking, material heresy in her ranks. It is often humanly impossible to tell which errors are being made in good faith, by people who (mistakenly) believe they are conforming their minds to that of the Church, from those which arise from culpable refusal to so conform oneself. Although the content of the deposit of faith is not a journey, most of us know that the life of faith is very much a journey. Accordingly, the position of the believing, intelligent Catholic is rather similar to that which our CtoC commenter attributed to "good Protestant theologians." The only difference is that the Catholic acknowledges a living authority not merely for identifying the deposit of faith—for which inspired Scripture indubitably serves—but for definitively resolving, as they arise, certain questions that the sources either occasion or fail to address explicitly.
Nevertheless, the journey would be not just unavoidable, but irremediably deficient, if the fourth and final common argument against the Magisterium were sound. Thus:
The Magisterium claims to be the sole "authentic" interpreter of Scripture and Tradition, meaning that only its interpretations are divinely authorized for the assent and profession of the whole Church. But all language requires interpretation, especially when it's about such lofty subject matter; so, the Magisterium's interpretations, in the form of dogmas or other definitive teachings, themselves require interpretation by both individual believers equipped to conduct it and the Magisterium itself. But if that is the case, then given the subject matter, there's no reason to believe that magisterial judgments, offered as interpretations of the "sources," are any more perspicuous than what they interpret. That is why heresies are so frequent, even recurring in new forms, despite conciliar and papal definitions; and that's how the interpretation of certain doctrines, such as extra ecclesiam nulla salus, can changes over time. But if magisterial judgments set forth with alleged infallibility leave so much unclarity, then the Magisterium's claims for itself are idle.Fortunately, that is the easiest argument to rebut. Magisterial judgments rarely answer all important questions about their subject matter, any more than Scripture does; they answer only the questions that are, or were, pressing in their historical context. Hence, such judgments are ordinarily not the last word for understanding what they're about; they are merely interpretive steps deemed necessary for dispelling particular misunderstandings. Ordinarily they do that job well, even though sometimes they do not, and can even raise serious questions of their own—as, I believe, was the case with the filioque, whose que admits of heterodox interpretations as well as an orthodox one. The point is this: even though the Church's collective meditation on the deposit of faith does not exhaust the cognitive content of the subject, and could never come close to doing so, the words in which magisterial judgments are framed are typically clear enough, in the broader context of Tradition and history, to exclude problematic interpretations as they arise. The Magisterium itself is on a faith journey of sorts, and the history of doctrine may be seen as that of an ongoing conversation about which direction the journey should take. But once a certain direction is taken definitively, interpretive clarity is thus gained to some degree.