Thanks for your reply. Your point is interesting. In summary, you indicate [the Church teaches] that if a person is recognized as the Pope then that person is in fact the pope. Is it possible for a non-Catholic to become pope in this manner? May I request a reference to material that supports this sedeplenism orientation?
Someone saw my post and sent me this via PM:
The citation is from Cardinal Billot, Tractatus De Ecclesia Christi, thesis XXIX, §3
“Finally, whatever one may think of the possibility or the impossibility of an heretical pope, there is at least one point absolutely clear which no one can put in doubt, and it is that the acceptance, the adherence, of the Universal Church to a pope will always be, by itself, the infallible sign of the legitimacy of such-and-such a pontiff; and consequently of the existence of all the conditions required for legitimacy itself. God can permit that at times a vacancy in the Apostolic See be prolonged for a long time. He can also permit that doubt arise about the legitimacy of this or that election. He cannot however permit that the whole Church accept as Pontiff him who is not so truly and legitimately. Therefore, from the moment in which the Pope is accepted by the Church and united to her as the head to the body, it is no longer permitted to raise doubts about a possible vice of election or a possible lack of any condition whatsoever necessary for legitimacy. For the aforementioned adhesion of the Church heals in the root all fault in the election and proves infallibly the existence of all the required conditions ... For this very reason, Alexander VI was not a false Pope, but a legitimate one. Therefore he was not a heretic"
This was posted on this forum a while back by Nishant. I'm sure others can find similar quotes from other sources.