St. Thomas Aquinas:

Quaestiones quodlibetales III, q. 4

Article 2: Whether those listening to different teachers of Theology who have contrary
opinions are excused from sin if they follow the false opinions of their teachers.

Sic: As for the second article, the case for an affirmative answer goes as follows: It seems that those listening to different teachers who hold diverse opinions are excused from the sin of being in error if they follow the opinions of their teachers. For at Matthew 23:2 the Lord says, "The scribes and pharisees sit upon the chair of Moses: do everything and observe everything they tell you." It follows that those things which are taught by doctors of Sacred Scripture are all the more to be respected; so those who follow their opinions do not sin.

Sed contra: But opposed to this is what is said at Matthew 15:14, "If one blind man leads another, they will both fall into the pit." But anyone who is in error is blind insofar as he is in error. Therefore, whoever follows the opinion of a teacher who is in error falls into the pit of sin.

Response: It should be said that if the differing opinions of the doctors of Sacred Scripture do not pertain to faith or good morals, then the listeners can follow either opinion without danger. For in that case what the Apostle says in Romans 14:5 applies: "Let each abound in his own understanding."

But in those matters that pertain to faith and good morals no one is excused if he follows the erroneous opinion of some teacher. For in such matters ignorance does not excuse; otherwise, those who followed the opinions of Arius, Nestorius and the other heresiarchs would have been immune from sin.

Nor can the naivete of the listeners be used as an excuse if they follow an erroneous opinion in such matters. For in doubtful matters assent is not to be given easily. To the contrary, as Augustine says in De Doctrina Christiana III: "Everyone should consult the rule of faith which he gets from the clearer texts in the Scriptures and from the authority of the Church."

Therefore, no one who assents to the opinion of any teacher in opposition to the manifest testimony of Scripture or in opposition to what is officially held in accordance with the authority of the Church can be excused from the vice of being in error.

As for the argument on behalf of the contrary position, then, one should respond that the reason he first said "The scribes and pharisees sit upon the chair of Moses" was so that what he then added, viz., "Do everything and observe everything they tell you," might be understood to apply to those things which pertain to that chair. However, things which are contrary to the faith or to good morals do not pertain to that chair.

This article would strike fear into my heart if I were in SSPX

Why?

The reason I posted it is pretty obvious.  St. Thomas speaks of those who hold false opinions regarding faith or morals.  Yet, he says simply do not follow them, without mentioning anything regarding their forfeiture of membership in the Church.  This is strange considering the idea of certain modern Catholics who seem to think that as soon the intellect grasps such an erroneous opinion, or the terms of a proposition, the immediate and necessary, even self-evident, fact follows that they cease to be members of the Church.  Recourse to the concept of publicity is futile as the context indicates these false teachers are well-known.  Your point regarding the Pope is again irrelevant considering that he is no more of a member of the Church than you or I.  Membership does not admit of a "more" or "less" it is a juridical fact.

Read my post to Telesphorus in his strange and ironic thread regarding cult-like behavior; it applies to you as well.    

Membership does not admit of a "more" or "less" it is a juridical fact.

Do you believe it can be forfeited within the order of fact before its forfeiture is recognized within the order of law?