That is is, in fact, the central question in the debate at hand, however, is obvious. Without a normative criterion that is external to the individual believer, there is literally no means at all for determining whether a particular teaching is true or false. If, on the one hand, you should happen to believe that your own private judgment is as good as anyone else's, then you have reason to believe, perhaps, that others are as likely to screw up as you yourself are, but you have as yet no principle for distinguishing between true and false judgments. If, on the other hand, you should happen to believe that your own private judgment is better, at least in principle, than any other external judgment, on the grounds that your private judgments are aided by the direction of the Holy Spirit, pick up your phone and call Elvis right away, he needs you in his band.
Granted, if it were really the case that some particular individual really did have a private line to the Holy Spirit, that would presumably be a big help in deciding whether to endorse certain teachings rather than others. But the question remains: according to what criterion are we to judge the truth or falsity of the judgment that we are genuinely guided by the Holy Spirit in our private judgments? There is a very real danger of vicious circularity here.
Does the same danger exist for those who defer authoritativeness to judgments made by institutions outside of themselves? Clearly not. We may begin, perhaps, by noting that there is already a rather interesting substantive difference between someone who thinks of himself as the source of authority (albeit, with the help of the Holy Spirit), and someone who thinks of some institution as the source of authority (also with the help of the Holy Spirit). Since the names for that difference aren't always pretty, I'll simply point out that the danger of circularity is not present here because when the source of authoritativeness is within oneself, it will never be the case that you find yourself thinking the following:
I do not believe X, and yet I must, and do, believe X because my inner source of authoritativeness with respect to belief compels me to..When the source of authoritativeness is external to oneself, it becomes possible to wonder whether what is taught authoritatively must be believed, and pretty much for precisely the reasons that some Protestants reject the authority of the Church qua institution: they claim that they not only don't know for sure that the Church is teaching authoritatively, they go so far as to claim that they know it to be teaching what is false. A Catholic may say, "I know X to be true because the Church teaches it authoritatively", and in this sense the Catholic and the Protestant say the same thing, except that where the Catholic says "Church" the Protestant says "I and my inner source of authority". But a Catholic may also say "I find it very difficult to believe X, but the Church teaches it authoritatively and so I will submit my will to hers and give intellectual assent to it anyway." No Protestant can say this, even if he swaps out the word "Church" and replaces it with "I and my inner source of authority". If he leaves the word "Church" in, obviously, he fails to make any sense qua Protestant. If he swaps it out for the suggested replacement, he fails to make any sense qua epistemic agent. It is precisely because he believes X that he believes it to be authoritatively true, so it is simply not the case that he finds it difficult to accept; indeed, he fools himself if he thinks it true that he really finds it difficult to accept, for he has in fact already accepted it, and that is precisely why he says that he gives his assent to it. So the Protestant, in making any kind of claim about authority, must say something that is meaningless in one way or another.