Pope Celestine's letter regarding Nestorius completely backs up sedeprivationism.
St. Robert Bellarmine, quoting Pope St. Celestine:
Pope Celestine I, in an epistle to John of Antioch, which is contained in Volume One of the Council of Ephesus, ch. 19, says: “If anyone who was either excommunicated or exiled by Bishop Nestorius, or any that followed him, from such a time as he began to preach such things, whether they be from the dignity of a bishop or clergy, it is manifest that he has endured and endures in our communion, nor do we judge him outside, because he could not remove anyone by a sentence, who himself had already shown that he must be removed.” And in a letter to the clergy of Constantinople: “The Authority of our See has sanctioned, that the bishop, cleric or Christian by simple profession who had been deposed or excommunicated by Nestorius or his followers, after the latter began to preach heresy, shall not be considered deposed or excommunicated. For he who had defected from the faith with such preaching, cannot depose or remove anyone whatsoever.”
This pretty much closes the book in Siscoe and Salsa.
If you read carefully what Pope Celestine wrote, all he said is that the excommunications pronounced by Nestorius and his confreres were null and void, and that those they excommunication remained in communion with Rome. He never said Nestorius was ipso facto deposed before being judged by the Church. Before considering the historical facts of the case, let’s see what the Sedevacantist apologists say it proves: “The Remnant also contradicts Pope St. Celestine I and St. Robert Bellarmine who both taught that warnings are not necessary to prove defection of faith. Bellarmine put it this way: ‘And in a letter to the clergy of Constantinople, Pope St. Celestine I says: “The authority of Our Apostolic See has determined that the bishop, cleric, or simple Christian who had been deposed or excommunicated by Nestorius or his followers, after the latter began to preach heresy shall not be considered deposed or excommunicated. For he who had defected from the faith with such preachings, cannot depose or remove anyone whatsoever”.’ In other words, Nestorius lost his office immediately after he began preaching heresy, which is why he had no authority to depose or remove anyone. It happens by Divine law, not by sentence of Church law.” (Steve Speary)
“The Catholics who immediately rejected Nestorius, until then Patriarch of Alexandria (sic), when he began preaching heresy, were justified by the pope after the fact. Their excommunications were declared to have been null and void, because ‘…he who had defected from the faith with such preachings, cannot depose or remove anyone whatsoever.’ (Quoted by Bellarmine.) In other words, once he became a public heretic he lost his office, automatically and without any declaration by Rome.” (John Lane)
They both interpret Celestine statement that those who were excommunicated by Nestorius remained in communion with him, as meaning Nestorius automatically lost his office, the moment he began preaching heresy, without the need of any warnings, antecedent judgement or declaration.
The facts of the case prove otherwise, as we will now see.
Nestorius first professed his heresy in December of 428, and persecute those who resisted him. He was sent two warnings by the Patriarch of Alexandria, St. Cyril, and rejected them both. In spite of what Pius IX called “the hardened pertinacity of Nestorius,” St. Cyril did not consider him “ipso facto deposed,” or cut off communion with him, without first appealing to Rome for a judgment. In his letter to Pope Celstine, he wrote: “The ancient custom of the Churches admonishes us that matters of this kind should be communicated to Your Holiness… But we do not openly and publicly forsake his Communion before indicating these things to your piety. Vouchsafe, therefore, to prescribe what you feel in this matter so that it may be clearly known to us whether we must communicate with him or whether we should freely declare to him that no one can communicate with one who cherishes and preaches suchlike erroneous doctrine.”
In response, Pope Celestine convened a council in Rome in August of 430, and Nestorius was found guilty as charged. The Pope gave St. Cyril the task sending Nestorius a third and final warning, informing him that he had 10 days to renounces his errors and subscribe to the 12 article that had been drawn up by the council. If not, he would be cut off from communion with the Church and deposed.
Before receiving the final warning, Nestorius petitioned the Emperor to call a general council to decide the matter, which he did. When Cyril informed the Celestine, the Pope agreed to keep the sentence of deposition in abeyance until the council had rendered a judgment, in the hope that the additional time would allow Nestorius to renounce his heresies. Instead, Nestorius failed to even attend the council and was deposed.
St. Alphonsus relates all these events and ends by noting that Nestorius remained in possession of his See till the council judged and deposed him. The following is taken from his book, “History of Heresy”:
St. Alphonsus, History of Heresy, and their Refutation: “26. St. Cyril, in discharge of the commission to which he was appointed by the Pope, convoked a Council, in Alexandria, of all the Bishops of Egypt, and then, in the name of the Council, wrote a Synodical letter to Nestorius, as the third and last admonition; telling him that, if in the term of ten days after the receipt of that letter, he did not retract what he had preached, those Fathers would have no more communication with him, that they would no longer consider him as a Bishop, and that they would hold communion with all clergymen and laymen deposed or excommunicated by him (…) St. Cyril appointed four Egyptian Bishops to certify to Nestorius the authenticity of this letter, and two others one to the people of Constantinople, and another to the abbots of the monasteries, to give them notice likewise of the letter having been expedited. These Prelates arrived in Constantinople on the 7th of the following month of December, 430 (22), and intimated to Nestorius the sentence of deposition passed by the Pope, if he did not retract in ten days; but the Emperor Theodosius, previous to their arrival, had given orders for the convocation of a General Council, at the solicitation both of the Catholics, induced to ask for it by the monks, so cruelly treated by Nestorius, and of Nestorius himself, who hoped to carry his point by means of the Bishops of his party, and through favour of the Court. St. Cyril, therefore, wrote anew to St. Celestine, asking him (23), whether, in case of the retractation of Nestorius, the Council should receive him, as Bishop, into communion, and pardon his past faults, or put into execution the sentence of deposition already published against him. St. Celestine answered, that, notwithstanding the prescribed time had passed, he was satisfied that the sentence of deposition should be kept in abeyance, to give time to Nestorius to change his conduct. Nestorius thus remained in possession of his See till the decision of the Council. This condescension of St. Celestine was praised in the Council afterwards, by the Legates, and was contrasted with the irreligious obstinacy of Nestorius.”
Nothing about the case of Nestorius supports the Sedevacantist understanding of how a “manifest heretic” loses his office. Speray appealed to the case to prove warnings are not necessary, yet Nestorius was issued three. John Lane appeals to it to prove that a declaration from Rome is not necessary, when in fact the Pope himself judged the case before Nestorius lost his office. And they both appeal to it to prove a prelate who preaches heresy is immediately cut off from the Church, and as a result automatically loses his office by divine law; yet Nestorius was recognized by St. Cyril, Doctor of the Church, and the Pope himself as remaining “in communion with the Church” and retaining his See for years after he began preaching heresy. St. Alphonsus, another Doctor of the Church, who studied that case thoroughly, said in no uncertain terms that Nestorius remained in possession of his See until he was deposed by the Council of Ephesus in 431.
What the historical case of Nestorius does prove is that an excommunication imposed by a heretical prelate against those who resist his errors, is null and void, and will eventually be declared so. That, and it also proves that if a Patriarch – or higher - publicly preaches heresy he will not be ipso facto deposed, without first being warned multiple times and judged by the Church … and maybe not even then.
It is also worth noting that this is the only historical case Bellarmine used in an attempt to refute Cajetan’s opinion, yet if anything it does the opposite. And you can rest assured that if Bellarmine were aware of any better historical examples - such as any bishops who were ipso facto deposed during the Arian crisis - he would have used them instead.