I'm not sure either, but I think the idea that sedevacantism cannot be regarded as an opinion forms the basis for some arguments against attending an R&R Mass. Thus the perceived need to preach and write against opinionism, which would be a small matter if not for its practical consequences for Mass attendance. Everything that can be proved is objectively a necessary conclusion, not an optional one, so if that's all that's being said in the anti-opinionist material, then why bother to say it, and why is it controversial?
I think the anti-opinionist material is wrong in two ways:
(1) It says that one cannot remain in doubt about the legitimacy of the revolutionist “popes”.
Bp. Sanborn says that we are obliged to make a diligent inquiry to resolve our doubt about the identity of the true Roman Pontiff. Fair enough, but what if someone thinks there are unresolvable contradictions in both the sedevacantist and the sedeplenist points of view? The proponents of each view will say that there are only
mysteries, not contradictions, but if someone doesn't see it, he is faced with an apparent dilemma. He has a perplexed conscience, and he cannot consult an authority for practical guidance because the perplexity concerns the would-be authorities themselves. In this situation, it makes sense for him to defer his decision on the speculative question and to take whatever practical steps seem reasonable and necessary in the circumstances. It would make sense for a person in this situation to regard the pope question as a doubtful matter.
I think it's especially clear that those who lack the knowledge, ability, and resources to investigate the pope question for themselves may form an opinion about it without regarding it as certain, as I said in this comment a while back:
Also, I wonder if there is a time limit on the diligent inquiry into the pope question that Bp. Sanborn says is required in case of doubt. Obviously one could remain in doubt for one day while making such an inquiry. Why not for one week? One month? … and so on.
For a demonstration of the mental state I am trying to describe, have a look at this mathematical fallacy (
link). It is a proof of 1=2. If someone can't find the error in the proof, he would say he must be missing something, because there must be an error somewhere. Likewise, the situation we see today could be proposed as a proof that “the Church has defected.” We know by the supernatural virtue of faith that this is false, but there is room for human minds to disagree about how to use evidence and reasoning to show that it is false. People can say some very stupid things without necessarily being guilty of sin, or at least not of heresy or schism, if they do so because they see no other way to deny that the Church has defected.
It might be relevant to ask, do the anti-opinionists say that it is absolutely certain that Angelo Roncalli was not a legitimate pope? I think there's a good case for each side of that question, which would make it an open question – a matter of opinion.
(2) It can easily be understood as claiming authority to direct the conscience of others.
It's one thing to say, I am certain that X has been proved; it's quite another to say, you must be certain that X has been proved. The sedevacantist position has not been taught by ecclesiastical authority, so it is only obligatory upon those who can see it for themselves. But it is beyond human power to make others to see the truth of sedevacantism – one can offer evidence and arguments, but one cannot create conviction in the minds of others. Nor, without authority, can one demand that others hold the sedevacantist position, however certain it may be in itself and in one's own mind.
Here's an analogy. I can say it's certain that the cube root of 343 is 7, which is true. If I were to tell someone, “you must be certain that the cube root of 343 is 7,” then he might say, “I know the cube root of 343 is 7, but who are you to tell me what I must be certain about?” Or he might say, “What the heck is a cube root?” Or, “Yes sir, I am certain about that if you say it, because you know about that kind of thing.” In the latter case, the person may firmly believe that the cube root of 343 is 7, but if he can't verify this for himself, then his belief rests on human testimony – but human testimony can be mistaken, so a conclusion that depends upon it is an opinion. If the testimony is very credible, then it can produce in others a moral certainty, but not an absolute or metaphysical certainty. In the case just given, the proposition itself is absolutely certain and can be known as such by reason alone, and yet some people may believe it as an opinion without any offense against reason, morality, or good sense.
Perhaps every adult should see for himself that the cube root of 343 is 7, once he knows what a cube root is. But there are truths of reason that most adults cannot be expected to verify for themselves, such as the formula for the surface area of a sphere (SA=4pi*r^2). It is absolutely certain by the light of reason, but most people can't attain that degree of certainty about it for lack of mathematical training. They are free to hold it as an opinion, more or less firmly depending on the confidence they have in the source from which they heard it.
Bp. Sanborn has admitted that most people cannot figure out the Vatican II crisis for themselves. From Bickering Priests (1994):
If this is true, and I think it is, then there's no basis for telling people that they must be absolutely certain about sedevacantism. So then, what's the point of giving sermons about how sedevacantism is absolutely certain in itself? Unless the corollary, spoken or unspoken, is that people are not free to hold it as an opinion, i.e. to be in doubt about it pending further information.
Well, what if a lay person has confidence in some priest or lay theological writer (who have the same authority as traditionalist priests, i.e. none) who holds that the illegitimacy of the revolutionist popes can and should be known with certainty, but is technically a matter of opinion and should not be made a litmus test of Catholicity in these confusing times. According to the passage just quoted from Bp. Sanborn, there is no basis for telling such a person that he must stop being opinionist and be certain.