Based on the lost text of “Passio Marcellini” (The Passion of Marcellinus, also called Acts of St. Marcellinus), Pope Marcellinus, living under the persecution of Diocletian, was called upon to offer an incense sacrifice to the Roman idols. He repented and confessed his faith in Christ only to suffer martyrdom with his companions.
Later, in the fifth century, the Donatist Bishop Petilianus of Constantine claimed in a letter that St. Marcellinus and his priests Melchiades, Marcellus, and Sylvester (his papal successors) had given up the sacred books, and offered incense to the pagan gods. In doing so, these men saved their lives.
It was never proven, but because of these acts, Marcellinus, in his day, was considered to have lost his papacy. He wasn’t universally acknowledged as a pope. Some other documents even explain a defection from the pope. The Formula of Hormisdas in 519 A.D. from the East specifically states that “in the Apostolic See the Church has been preserved without blemish.” Either the Eastern Patriarchs didn’t believe the acts ever occurred; they didn’t think his acts constituted a blemish on the papacy; or they didn’t recognize Marcellinus as a true pope.
St. Augustine appears to have demonstrated that the whole event never happened. However, he did so to protect the papacy presuming (falsely so) that such acts would indeed entail the loss of the papacy even when done under duress as in the case of Marcellinus.
The Roman Breviary reads on April 5, “During the cruel persecution of the Emperor Diocletian, Marcellinus of Rome, overcome with terror, offered incense to the idols of the gods. For this sin he did penance, and wearing a hairshirt, went to the Council of Sinuesso, where many Bishops had assembled, and there he openly confessed his crime.”
Presuming the acts occurred, they wouldn’t have constituted the loss of the papacy, since they were clearly done under duress at the time and affirmed with the confession of Marcellinus. We’re not certain the “Passio” was presenting actual historic facts, but it doesn’t matter. Even a myth can present the truth.
The point is that Pope St. Marcellinus didn’t actually become an apostate. Acts of apostasy, heresy, and schism done under duress does not make one an apostate, heretic, or schismatic.
This is vastly different in comparison to the Vatican 2 popes who’ve never been under duress for their acts of apostasy and heresy.
Pope Leo XIII declared in his Encyclical, Satis Cognitum (On the Unity of the Church), June 29, 1896:
“But he who dissents even in one point from divinely revealed truth absolutely rejects all faith, since he thereby refuses to honour God as the supreme truth and the formal motive of faith.”
Rejecting “all faith” is apostasy, not mere heresy.
All the Vatican 2 popes have denied the divinely revealed truths on the four marks of the Church.
Christ’s literal descent into hell is a divinely revealed truth. Yet, John Paul II denied this truth when he taught in 1989 it was metaphorical rather than literal by saying “the primary meaning” of decent into hell means “experience of death,” “placed in a tomb,” and “separation of body and soul.”
Of course, this is not the primary meaning at all as all good Catholic know.
It just so happens that Pope St. Pius X declared:
62. The principal articles of the Apostles’ Creed did not have the same meaning for the Christians of the earliest times as they have for the Christians of our time. CONDEMNED as an error of the Modernists, by Pope St. Pius X in Lamentabili, July 3, 1907.
John Paul 2 is giving a different meaning when he surely knew what he taught wasn’t what it meant in earlier times.
As for Francis, he professes his faith openly in deeds and words, too. He approves of the LGBTQ by placing openly pro-homosexual bishops in office and keeps them there as Bishop John Stowe of Lexington, KY. He encourages others to stay in their false religions and not convert, condemns proselytism, condemns the death penalty as intrinsically evil, and declares that God permissively willed the diversity of sex.
There’s no way Pope St. Marcellinus can be likened to the Vatican 2 popes. Those Vatican 2 apologists who try to use St. Marcellinus against sedevacantism are either extremely ignorant or dishonest.
Speray's Catholicism in a Nutshell 
You make good points and preserve common sense unlike the new religion of VATII. They can never justify their actions nor will they ever reap the graces necessary for salvation.
Can a priest deny Sacraments to a Catholic when they have different theological opinions on sth?
A priest thinks sth is a grave sin and a Catholic thinks not (and sth is not established by a Pope)?
Speray: An opinion is an opinion, not a err against the Faith.
Can a priest deny Sacraments to a Catholic who receives “abp Thuc” Sacraments?
Speray: Not lawfully.
When a priest unjustly deny Sacraments e.g. a Confession is it a grave sin of him? Is he a public sinner?
Speray: Depends on the situation.
What other priests should do? Warn Catholics about him or pretend that nothing happened?
Speray: Admonishing sinners is a work of mercy, right?
Can a Catholic vaccinate using a vaccine made out of aborted murdered child or is it a mortal sin?
IF you know better, than don’t do it.
There are usually popes parrotted about to get us to believe a pope can teach, officially, error: Marcelinus, Honorius, and John XXII.
All are simple glosses, just snippets of what actually happened, so the average lay-nothing believes that JPII was simply mistaken, or Bergoglio is just a “bad pope”.