This is an attitude that I have run across quite often, actually: the idea that Roman Catholics, in recognizing a source of authoritativeness beyond themselves, are either unwilling or unable to think critically about the semantic content emanating from the three great pillars of our faith--Scripture, Tradition, Magisterium. That this attitude is mistaken has been shown, quite ably, by folks such as Dr. Michael Liccione, who has presented a persuasive philosophical argument against the attitude in his many posts on the issue that one can find throughout his blog, Sacramentum Vitae, so I won't attempt to address that issue here. I'm rather more interested in the obverse of this attitude: the idea that "thinking for oneself" is somehow inherently superior to submitting to an external source of authority. That is, the idea that private judgment is to be privileged in some sense.
In the standard sort of scenario, the person who objects to the Catholic understanding of authoritative teaching will present various cases where the Church, as an institution, has been perceived by the objector to err, whether in point of doctrine (usually the examples that are cited in this category are such things as Papal infallibility, the Immaculate Conception, or some other point of doctrine that is not accepted by some small minority of Christians), or in some matter of discipline or doctrinal development (for example, the perceived "change" in understanding regarding usury, chattel slavery, marriage of the clergy, etc.). The objector will often have a firm opinion of what the "correct" view in each of these cases is, and will argue, drawing upon both Scripture and history, that his (i.e., the objector's) own view of the matter is more rational and historically accurate than the Church's view of the matter, hence the Church's view of the matter is clearly mistaken and must be rejected. Often times the objector does not rely entirely on his or her own private judgment to the exclusion of all other rational deliberation; sometimes the objector will cite sources from within the Protestant tradition, such as Calvin or Luther or Barth, that support his view and that provide additional arguments against the traditional understanding of the Church. In the case of this "external" evidence, though, it is essential to note that it never fails to rely upon some element of private judgment, either Luther's or Calvin's or whoever else's own opinion, or their agreement with the opinion of some other person, or their agreement with some synod or other group of divines with whom they happen to agree. Even when the objector relies upon the analysis of another, such as Luther or Calvin, the objector, in the end, is in fact relying upon his own private judgment to the effect that Luther or Calvin got it right, so that his appeal to an authority is really no more than an appeal to his own agreement with some other person or group.
The Roman Catholic who is confronted with an official statement regarding a matter of faith or morals with which he does not agree is, of course, perfectly free to reason about such matters himself, but the point at issue is not really about freedom to engage in thoughtful contemplation about a given judgment, the point at issue that provokes some critics to charge Catholics with "checking their brains at the door" is the alleged "lack of freedom to judge for oneself" whether a particular judgment is true or false. Here there is a difficulty, because of course any Catholic is also free to judge for himself whether the judgments coming from the Church are true or false--it simply is not the case that Catholics are mere automata. The real sticking point for the objector is the fact that the faithful Catholic is willing to acquiesce, that is, submit to, some matter or other with which he may not agree. That is, the Catholic is willing to defer to the authority of the Church in certain matters, matters where it is required by the Church herself. There are, of course, plenty of Catholics who do not so defer; these Catholics are basically Protestants in Roman clothing, but I've griped enough about them already in other places. Here the discussion has to do with how Protestants differ from real Catholics, not how Protestantizing Catholics are like Protestants. The genuine Catholic will grant intellectual assent even to doctrines that he does not fully understand or agree with, provided that said doctrines are authoritative. This is obviously not the same thing as "checking one's brain at the door", it is rather an act of humility, but is it also wise? Is there any reason to think that it is better to defer on certain occasions, or worse to privilege private judgment instead?
Relying on private judgment assumes that one's own judgment is the best guide to what is true or false given the available evidence. Now I would imagine that in just about any other sphere of human affairs it would be widely recognized that it is, in fact, not a good idea to rely on the testimony of a single, perspective-bound witness to establish the facts of a given case, but in the domain of religion it is important to remember that Protestantism was born of the enlightenment, a movement in which every individual act of reasoning by a rational agent was thought to be carried out in a kind of value-free environment. It was the beginning of the privileging of the individual in general, not only in the domain of reasoning but in the domain of law, economics, politics, and many other social structures. This movement reached its zenith, as it were, in the American experiment, where the sanctity of the individual as the principle bearer of rights and duties was actually made a matter of positive law in the Constitution of the United States. It is no surprise, then, that Americans in general tend to assume that every opinion is sacred and that one man's view of things is as good as any other, just so long as he can give a rational account of himself in front of his peers. As a consequence of this, I think, it should come as no surprise either that there are more Protestant denominations in the United States than anywhere else in the world. Here, if you think your neighbor is doing things the wrong way, it is a relatively easy matter to just dash off on your own and start a new church where they do things your way.
Now, I don't think that very many reasonable people are seriously going to suggest that all of these folks who have run off to start their own denominations are equally right in their views of things. Surely most folks will agree that, given a particular judgment p, where p has to do with a matter of faith or morals, either p or ~p is the case, that is, there is some fact of the matter that determines whether p or ~p is true. How is any individual to determine which of this proposition-pair is the true one? According to the person who privileges private judgment, the individual is to determine it by means of his own powers of reasoning and a careful, thorough, and value-free examination of all of the relevant evidence. So it would seem that this has either not occurred, since so many very different groups have arisen as a consequence of approaching matters this way, or else it has occurred only in a very limited number of cases. Indeed, it is possible, of course, that everybody is still completely wrong about everything, but it is not clear why anybody would commit themselves to a denomination in which they thought it quite possible that all of the commitments were as likely to be false as true. I think that, as a general rule, people adopt propositions that they believe to be true, not propositions that they think are likely to be false.
So let us take a couple of proposed teachings and see how matters stand. Here are two teachings:
 The Holy Spirit is to be worshiped and glorified with the Father and the Son.I have chosen these two teachings in particular because I imagine that few, if any, mainline Protestants would question the truth of , while most, if not all (along with some Anglicans and Orthodox), would outright reject the truth of . The Church teaches both as matters that are to be believed de fide, but presumably those mainline Protestants who accept  do not accept it because the Church teaches it authoritatively. Some may accept it because it emanates from a Conciliar decree, but I imagine that even among those there will be some who accept it not merely because it is a Conciliar decree, but because something moves them to accept it. Indeed, it seems to be, in some sense, an essential element of the non-Catholic position, that matters of faith are to be believed principally because of a subjective judgment that the matter is worthy of belief.
 Mary was, by the grace of God, free from all stain of original sin from the moment of her conception.
So in the case of , a non-Catholic may believe it because he has investigated the Scriptures and found to his own satisfaction that they largely support the doctrine, or at worst do not outright contradict it, or he may believe it because, after weighing the matter carefully in his heart and examining all the evidence, he feels moved by something--call it the Spirit of truth or whatever--to accept the truth of it, or he may believe it for some other reason, but it seems that if he accepts it as true at all it will not be simply because of some authoritative source external to himself telling him to believe it, but because of a private judgment of his own that it is true.
Similarly in the case of , the non-Catholic will say, if he rejects the proposition, that he finds it inconsistent with the Scriptures, or that he finds it contrary to human experience, or that he finds it to be impossible to confirm given the available evidence, or for some other reason, but again it seems that if he rejects it as true it will not be simply because of some authoritative source telling him to reject it, but because his private judgment is simply that it is false.
Now, in each case, there will be an implicit source of authority that is external to the non-Catholic, namely Scripture. This is rather intriguing, because the Scriptures themselves are the product of the Church's teaching authority. As I have shown elsewhere, this is a fatal problem for the sola Scriptura principle, but we may put it aside for the purpose of the present discussion, since there is another matter at stake here that is equally important. So let us assume that all sides agree to the following principle:
 Every proposition to which we give our assent must at the very least be logically consistent with the teachings of the Scriptures.Now, on the one hand, we may probably regard the acceptance of this principle--again, just for the purposes of discussion here--as separate from the question of whether the source of authoritativeness is internal or external to the believing subject. On the other hand, it is possible to render  relevant to the current discussion by pointing out that if one is a non-Catholic, it takes an act of private judgment to accept the truth of the Scriptures, hence there is a sense in which the non-Catholic does not even accept the Scriptures themselves as a source of authoritative truth external to themselves.
So then, why would a non-Catholic believe not only that the Scriptures are authoritative, but that they are the only source of authoritative teaching? How is it that the non-Catholic comes to make this judgment? It cannot be simply by reading through the Scriptures, because any fool can do that and come away not only thinking that they are not true, but thinking that they are downright silly. It seems that there are a variety of different answers given to this problem, but two stand out. One of them is simply that the Scriptures "ring true", that is, they cohere pretty well with what the reader already believes to be the case and after an examination of what (very little) historical evidence there is they appear to be more or less reliable accounts. Personally, I don't see how any reasonable person could come to this conclusion, but I accept that there are those who draw the inference. The second answer will tend to invoke some causal principle of belief that they label "faith" in one form or another. Perhaps they have in mind the influence of the Holy Spirit, God's gift of grace to believe in the truths of revelation. If this is what is intended, it is identical to what the Catholic would invoke. Let's examine these two approaches.
The person for whom the Scriptures "ring true", as it were, who accepts them because he feels that he has read them very carefully, checked them against the available evidence, perhaps compared them with the writings from other religious traditions, and then on the basis of this decides that the Scriptures are, in fact, true, is on the most tenuous possible ground. Other, equally if not more intelligent readers have done precisely the same exercise and found the Scriptures to be completely unbelievable. Indeed, there is something of a cottage industry among non-believers in highlighting those instances where the Scriptures contradict themselves, or the historical record, or the archaeological evidence, or the sociological facts. The only way to get around these objections to the veracity of the Scriptures when one is relying on the fact that they appear to one to "ring true" is to engage in a method of interpretation that is so very ad hoc as to be absolutely useless. It amounts to saying something along the lines of "Well, if you just read the Scriptures the way I do they will make more sense to you." This is puerile, and obviously will not do.
The person who claims that faith leads him to accept the truth of the Scriptures is on slightly better grounds, and in fact it is faith that leads the Roman Catholic to accept the truth of the Scriptures, too. The difference between the Catholic and the non-Catholic does not lie in the causal principle that prompts them to accept the truth of the Scriptures. So, both the Catholic and the non-Catholic agree that the Scriptures are true, and that they can be known to be true. So where, in point of fact, to do they differ? Well, the Catholic believes that both  and  are consistent with, and follow of necessity from, the Scriptures, but the non-Catholic, if he accepts this idea of consistency and entailment at all, will accept it only with regard to , not with regard to . Why does the Catholic accept  while the non-Catholic rejects it? The oft-heard accusation from the non-Catholic is that the Catholic accepts it because the Church tells him to, and to a certain extent this accusation may be within bounds, because some Catholics may have difficulty understanding why  is true, even when they are willing to assent to its truth on the authority of the Church's teaching charism. The typical non-Catholic who rejects , however, rejects it because he cannot believe it. As I alluded to above, his reasons for rejecting it may vary but they are reducible to the plain and simple fact that he does not believe it.
Now, a belief may be either true or false, but knowledge is always of what is and, therefore, always true. The non-Catholic believes that  is false, but does he know it to be false? I think some non-Catholics do believe that they know it to be false, principally because they are already committed to the principle that all human beings save Jesus of Nazareth bear the guilt of original sin, so Mary can be no exception no matter how clever the exegesis. But I believe that folks such as these are relatively few in number, because the principle that all human beings bear the guilt of original sin is not, in point of fact, to be found in the Scriptures. St. Paul writes that "in Adam all have died", but to turn this into an endorsement of the principle at stake here requires some rather fancy footwork. As it happens, there is no expression anywhere in the Old or New Testaments that could fairly and straightforwardly be translated as "original sin". So an act of interpretation is required to get the relevant principle. So most folks who are intelligent enough to understand this fact about the Scriptures will see that it is at least possible that Mary was born free from the stain of original sin, if we agree that it is possible that God can will such a thing. Those who would deny that it is possible for God to will such a thing need to double check their understanding of omnipotence.
So we find that  is at least possibly true, but the non-Catholic denies that it is true. Given that it is at least possibly true, the denial that it is true must be based on a belief that it is false, rather than on certain knowledge that it is false. (This is not a straightforward move; it requires a special sense of "possibly true", but the required sense is present, so the move is warranted.) If one had certain knowledge that  is false, it would not be the case that  is "possibly true" in the relevant sense of "possibly true" (it could still be logically possible, obviously, even if it is certainly false). The Christian who does not believe  has a choice. He can insist that he is right about , or he can admit the possibility that he is wrong about . Whoever would insist that he is right about  needs to double check on what it means to be certainly right in a case such as this. Even if the person believes that the Holy Spirit himself descended from heaven and revealed to him personally that  s false, the person would have to ask himself how he could know that this revelation really happened when confronted with the fact that others claim equally solid foundation for believing  to be true. There is a genuine epistemic puzzle here that has no adequate solution, though there are, of course, folks who claim that "they just know" that the Spirit has told them certain things, and there are others who claim that they don't actually know it, they just have faith, etc., but in each case we wind up with one or the other of the two possibilities we've already seen: either the person is trusting in his own powers of reasoning or he is acting from what he believes to be a faith-warranted knowledge claim. The former is useless and the latter does not differ from what the Catholic has, and there is a kind of stalemate.
Only there isn't, really. The non-Catholic who trusts in a causal principle like faith must admit that others who disagree with him also claim to have similar causal principles behind their views and it's not just Catholics who differ with him but other Protestants, and we might as well toss in the Jews and Muslims for good measure. Suppose that our putative non-Catholic were the only person on the planet to arrive at a certain belief about a matter of Christian faith. Suppose that this person, acting in good conscience, found that he could not alter his belief on this matter, even though it was at odds with what every other Christian believed on the matter. While such an outlier is highly unlikely it is perhaps more probable than many people think, given the bizarre state of Americanized religion. Be that as it may, there is a very real question here as to what point there would be to Christ's impassioned prayer for unity among believers in the High Priestly Prayer of the Gospel of John if he really didn't think that it mattered what his followers believed. He wanted them to be one in some sense, but in what sense? Perhaps in the sense that they all love and follow him, but it is difficult to imagine what that would mean if there is constant bickering about the content of the faith that claims to be a manifestation of what it means to love and follow him. In addition, given the possibility of heresy it is clearly important to determine which propositions are true and which false, and historically it has been of tantamount interest to the orthodox Christian community to establish "what must be believed in order to be saved".
The historical record is quite clear that it is the community of Christian believers who determine this, not any single individual. Never in the history of the religion has it been the case that a particular individual, relying on his own private judgment, determined for everybody else what the content of the faith ought to be. Famously, Sts. Peter and Paul disagreed about certain points, and in no case were decisions made by either one or the other fully independently of what the rest of the community thought. Even today, it simply is not the case that  was laid down as law by a single man against the overall consensus fidelium: both the dogma of the Immaculate Conception and the dogma of the Assumption of Our Lady were promulgated only after a thorough review of the historical record, the theological arguments, and the opinions of the bishops of the world. To deny the validity of this process of communal establishment of doctrine is to claim that a single individual may determine for himself what the content of the faith is to be, and if he can do that, then there is no reason in principle that he could not lay it down as a law for everyone else as well, on the grounds that his own private and personal authoritativeness establishes the truth of the matter and to deny it would be heresy. And yet none of the non-Catholics who reject the Church's charism to teach authoritatively claim this power for themselves, even though they must have it if they have the authority that they claim they have. They claim only to be able to establish what they themselves ought to believe, and with unintended irony they assert that it is up to each individual to determine for himself what he ought to believe!
To make the claim that the non-Catholic makes is to deny the possibility of the unity that Our Lord prayed for so earnestly and, hence, is a sin against that unity. It is not merely a matter of humility to defer to the consensus fidelium as it is made manifest in the three pillars of our faith, it is a matter of following Our Lord.