Once a Catholic, always a Catholic just as surely as once a priest always a priest, even for all eternity.
That is incorrect. If you renounce the Catholic doctrine and embrace Protestant heresies, you are no longer member of the Church (as Pope Pius XII teaches). Baptismal character does not keep you Catholic.
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But a manifest heretic is not a Christian, as St. Cyprian and many other Fathers clearly teach. Therefore, a manifest heretic cannot be Pope." (St. Robert Bellarmine,
De Romano Pontifice)
"Now the fifth true opinion, is that a Pope who is a manifest heretic, ceases in himself to be Pope and head,
just as he ceases in himself to be a Christian and member of the body of the Church: whereby, he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics soon lose all jurisdiction(...) The foundation of this opinion
is that a manifest heretic, is in no way a member of the Church; that is, neither in spirit nor in body, or by internal union nor external(...)" (St. Robert Bellarmine,
De Romano Pontifice)
St. Robert teaches that manifest heretic is not even a Christian. Clearly he knows nothing about supposed "once Catholic, always Catholic" rule.
Now, tell me who was Martin Luther after his excommunication and after his rejection of Catholicism and embracing Protetand doctrines - was he still a Catholic? Both Catholic and Protestant at the same time? No, as Pope Pius XII teaches in
Mystici Corporis Christi, he separated himself from the Church and was no longer her member. Therefore, he was not a Catholic.
This is the understanding that sedevacantists harbor, they also believe that heresy is something other than mortal sin as well - what that something is no one knows
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Heresy, schism and apostasy are mortal sins, but different from all other ones, because they deprive one of membership in the Church:
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For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy." (Pope Pius XII, Mystici Corporis Christi)
but the fact remains that in the case of the pope, even if he incurs the censure, there is nothing anyone can do about it. It is precisely because there is nothing anyone can do about it, that he is still the pope and because of that, we must still be subject to him unless he wants us to do something sinful, otherwise, we will never see heaven - that is the dogma.
St. Robert Bellarmine does not agree with you:
"Now the fifth true opinion, is that a Pope who is a manifest heretic, ceases in himself to be Pope and head, just as he ceases in himself to be a Christian and member of the body of the Church:
whereby, he can be judged and punished by the Church."
No serious theologian taught that heretical Pope cannot be deposed - they (St. Robert Bellarmine, Suarez, John of St. Thomas and others) only differed in what way does that happen and whether the Church merely announces that a heretical Pope already lost his office.
As for the rest of your post, I've already explained to deaf ears what is being condemned by the council of Constance, no sense in explaining it another time.
You have been refuted numerous times on that - Constance speakes about sinful, not heretical Pope. Heresy is different than other mortal sins (see Mystici Corporis Christi), because it removes one from the Church. Someone who is outside the Church cannot excercise any authority in the Church.
I posted this already, but am interested to hear your reply........If you totally lost the faith tomorrow and turned into another heretic like Billy Grahm or Father Martin Luther, and for decades you preached only Protestantism, you would be a heretic and excommunicant. According to you, you would have been outside the Church and no longer a Catholic for decades.
Correct. You did not bother to reply to the quote from Mystici Corporis Christi which An even Seven brought up:
"Actually
only those are to be included as members of the Church who have been baptized and profess the true faith,
and who have not been so unfortunate as to separate themselves from the unity of the Body, or been excluded by legitimate authority for grave faults committed."
If someone separates himself from the Body of the Church, he is no longer member of the Church - clearly Pope Pius XII knew nothing of supposed "once Catholic, always Catholic" rule.
However, suppose that in your last hour or if you believed that your own death was imminent, you finally (Deo Gratias!) had a change of heart and sought repentance.
As quoted above per Trent, in danger of death, you could do what no prots can do, you could do that which ONLY Catholics are permitted to do and which Catholics actually practice since it is essential - namely, you could walk into the confessional, confess your sins to the priest and be absolved of all your sins, including those sins of heresy, apostasy and schism - as Trent says: "from every kind of sins and censures whatever".
Please explain how a non-Catholic is able to be absolved from his sins in the sacrament of penance.
If I were to repent and return to the Catholic faith, I am back a Catholic and no longer a Protestant.
The other side is that if you were a priest who lost the faith and became a heretic and excommunicant, you could still absolve penitents, something non-Catholics are incapable of, something only a Catholic priest can do, even if a heretic and excommunicant.
Untrue - Eastern Orthodox priests can also validly absolve. Being able to validly absolve has nothing to do with whether one is a Catholic or not, but whether one is validly ordained priest (and, in case of Catholic priests, whether they have jurisdiction).