Read Aquinas on authority:
http://www.newadvent.org/summa/3104.htm
Legitimate authority is confined to its proper sphere. If a person in authority steps outside that sphere, his authority does not bind.
It is not in any Pope’s power to modify the doctrine of the faith. He cannot command what Christ didn’t give him to command. So if he does so, it is proper to resist him.
If he stays within his own authority, then he should be obeyed. Say Francis put my parish under the interdict tomorrow because he doesn’t like the Latin Mass: told us we could not receive any sacraments. I would be bound to obey it, because that is a proper (though insanely unjust) exercise of authority.
Aquinas i not the pope, not is everything he taught Catholic doctrine, and can be subject to interpretation, while the one duty of the multitude is to allow themselves to be led, and, like a docile flock, to follow the Pastors.
It is not in any Pope’s power to modify the doctrine of the faith.
But what constitutes modifying versus clarifying is not to be determined by you, which leads to schism, as exampled by the SSPV in the light of what they see as some contradictions in part of V2 with certain historical RC teaching.
If he stays within his own authority, then he should be obeyed.
That too sees different interpretations. RCs are enjoined,
to suffer themselves to be guided and led in all things that touch upon faith or morals by the Holy Church of God through its Supreme Pastor the Roman Pontiff, who is himself guided by Jesus Christ Our Lord. ( CASTI CONNUBII, ENCYCLICAL OF POPE PIUS XI)
...when we love the Pope, there are no discussions regarding what he orders or demands, or up to what point obedience must go, and in what things he is to be obeyed ; when we love the Pope, we do not say that he has not spoken clearly enough, almost as if he were forced to repeat to the ear of each one the will clearly expressed so many times not only in person, but with letters and other public documents ; we do not place his orders in doubt, adding the facile pretext of those unwilling to obey – that it is not the Pope who commands, but those who surround him; we do not limit the field in which he might and must exercise his authority ; we do not set above the authority of the Pope that of other persons, however learned, who dissent from the Pope, who, even though learned, are not holy, because whoever is holy cannot dissent from the Pope. (Pope Saint Pius X, Allocution Vi ringrazio to priests on the 50th anniversary of the Apostolic Union, November 18, 1912, as found at http://www.christorchaos.com/?q=content/choosing-ignore-pope-leo-xiii-and-pope-saint-pius-x )
Also, there are different classes of magisterial teaching, and which level they belong to (a well as their meaning) can be subject to interpretation, and each require different degrees of assent relative to their certainty.
As asked before, do you instead affirm ascertaining the veracity of anything that is taught by examination of the warrant for it from your chosen authoritative documents?