It is not possible to be partially '“faithful”, either one is in “full communion” or one is not.
The SSPX makes just this claim, using the ‘Eternal Rome’ justification - here are some extracts from an excellent article by Dr Mirus of Catholic Culture which exposes and debunks this dangerous myth:
Eternal Rome vs. the Magisterium: A Contemporary Myth
"One may still hope that the Society of Saint Pius X will seek to return to full communion with the Catholic Church. Bishop Bernard Fellay’s
comments following the General Chapter of the Society could indicate acceptance of an arrangement similar to that of the Fraternity of St. Peter, or they could indicate a continuing insistence on every aspect of their current identity, including the SSPX rejection of the Magisterium of the Second Vatican Council and the modern papacy (which would make full communion impossible).** But there is one very dangerous expression in these remarks which puts clearly on display a myth often perpetuated by Traditionalists—a myth which must be exploded if authentic reconciliation is to be achieved**. Let us call this the myth of “eternal Rome”.
“Bishop Fellay states: “It is not us [sic] who will break with Rome, the Eternal Rome, mistress of wisdom and truth.” But of course** the SSPX has already broken with Rome through a refusal of obedience, including the consecration of bishops without the consent of the Holy Father. And the reason for this breach is the myth which Traditionalists have concocted of “eternal Rome”**. Another name for this myth is “perennial doctrine” or “perennial teaching”.”
"I call this a myth because it is used by Traditionalists generally to create a false dichotomy between “eternal Rome” and the authority of the Magisterium today, or between “perennial doctrine” and what the Magisterium has taught since, say, 1960. The myth says that there can be a difference between these two things, and that the former is the rule of faith. But the truth is that there can be no difference between these two things, and that a proper understanding of the Catholic faith is achieved only by obedience to all of the relevant statements of the Magisterium of all times, including our own times."
“The key principle is simply this: The
Faith is unique among all religions in that it contains within it a divinely guaranteed principle of authority, namely the Magisterium of the successors of Peter, and their universal jurisdiction over the entire Church.
Against this authority, which comes from God Himself, it is impossible to appeal. And it is precisely her possession of this authority which makes Catholicism unique among all religions, the only faith which is guaranteed by God Himself to be true.”
"Moreover, a corollary is equally clear:
Those who reject the Magisterium in their own time, when it is teaching about the questions with which they themselves are intimately engaged, in fact reject the Magisterium whole and entire, the Magisterium of every age, the authority of Christ himself."
catholicculture.org/commentary/articles.cfm?id=541