Unless the world DOES END next week, I can't believe God would make it THAT hard for us to save our souls. I believe it would be morally impossible!
It's not a good idea to base your theology on what you theoretically think can't be happening because it's too awful to contemplate... That is how we got here. Burying our heads in the sand.
Since 90% of the world went along with the Arian heresy, how many people do you think had access to true Catholic sacraments in that time? How many people just happened to have an Athanasius in their town? I doubt there were many. Should those who were stuck with Arian priests have shared in the sin of a heretical priest's Mass, because for some reason they felt they were entitled to the Mass or that God wouldn't allow it to be taken away?
In times of emergency we can do what they did in Japan when they had no priests for centuries; confess to each other, while keeping in mind that we must confess again if we eventually do find a priest. I "offer" a Mass every Sunday with a spiritual communion. God knows our situation and will give you the actual graces. But as St. Thomas says, those who attend the Mass of a heretical priest become "sharers in his sin."
Don't be misled. I just found this on a site called Holy Spirit Interactive, trying to explain how we should not fear attending the Mass of someone who doesn't believe in transubstantiation:
"The issue of the disposition of the priest has arisen in the past. In the early 300s, the heresy of Donatism arose, which asserted that the validity of a sacrament depends upon the minister's orthodoxy and state of grace. For the Donatists, a priest who is a heretic or in a state of mortal sin cannot validly perform a sacrament; therefore, a person baptized by such a priest would have to be re-baptized."
He is now conflating baptism with offering a Mass; yet even a pagan can validly baptize.
St. Augustine (d. 430), one of the great opponents of Donatism, in his "In Ioannis evangelium tractatus," forcefully distinguished the action of Christ versus the action of the minister when performing a sacrament: Christ acts by His power, while the minister acts by his ministry entrusted to him by Christ. Therefore, " ... those whom Judas baptized, Christ baptized. So too, then, those whom a drunkard baptized, those whom a murderer baptized, those whom an adulterer baptized, if the Baptism was of Christ, Christ baptized" (5,18). Nevertheless, St. Augustine also sharply chastised the minister not properly disposed to perform the sacrament: "As for the proud minister, he is to be ranked with the devil. Christ's gift is not thereby profaned: what flows through him keeps its purity, and what passes through him remains clear and reaches the fertile earth. ... The spiritual power of the sacrament is indeed comparable to light: those to be enlightened receive it in its purity, and if it should pass through defiled beings, it is not itself defiled" ("In Ioannis evangelium tractatus," 5, 15).
"Defiled" does not mean heretical. The Donatists revolted against priests who were not heretics. They were cowardly and sold out their flock to the political authorities, many of whom ended up being thrown into prison or killed. These bishops essentially ratted out their people to save their own skin.
After the smoke had cleared, the cowardly clergy apologized for what they did but the Donatists did not accept the apology and didn't want to go back to Church under the same bishops who had sent off their family members to be killed; which is understandable, but not Catholic, as we are supposed to be forgiving.
This has nothing to do with attending the Mass of a heretic who is not even Catholic.
Therefore, the validity and efficacy of the sacrament do not depend upon the holiness or orthodoxy of the minister; rather the validity and efficacy are independent of the subjective constitution of the minister.
Holiness and orthodoxy are two different concepts. I see this so often, tragically, even among sedevacantist clergy. Like and unlike are brought together to create a misleading impression in the minds of the reader. We can accept the Host from an unholy or sinful priest; but it is another matter entirely to accept it from an unorthodox priest.
Therefore, in answering the question, two important principles govern: First, the sacrament must be performed validly with proper matter and form. Second, the minister must have the intention at least of doing what the Church intends, which is demonstrated by validly performing the sacrament, i.e. appropriately saying the specified Words of Consecration over the unleavened bread and wine. Therefore, if the priest in question is a heretic and has an identity crisis, but offers Mass validly, then the people indeed receive the Body, Blood, Soul and Divinity of our Lord in the Blessed Sacrament. Without this assurance, the people would always be left in a state of uncertainty as to whether they actually received a sacrament.
Heretics do indeed have the power to bring Christ down into the bread and wine, but not in a way that is pleasing to Him. Hence, those who attend the Mass of a heretical priest become sharers in his sin.
Summa Theologica, Q. 82, Art. 7, Reply to Objection 2:
"Baptism alone is allowed to be conferred by heretics, and schismatics, because they can lawfully baptize in case of necessity; but in no case can they lawfully consecrate the Eucharist, or confer the other sacraments."
Aquinas in the next article then quotes Gregory, who I assume is St. Gregory the great: "The faithless father sent an Arian bishop to his son, for him to receive sacrilegiously the consecrated communion at his hands. But when the Arian bishop arrived, God's devoted servant rebuked him, as it was right for him to do."
And here we come to my oft-used quote, on which I am staking my soul by the way, because I trust St. Thomas:
"As was said above, heretical, schismatical, excommunicate or even sinful priests, although they have the power to consecrate the Eucharist, yet they do not make a proper use of it. But whoever communicates with another who is in sin, becomes a sharer in his sin. Hence we read in John's Canonical Second Epistle that He that saith unto you, God speed you, communicateth with his wicked works. Consequently it is not lawful to receive Communion from them, or to assist at their Mass."
This may seem like St. Thomas is contradicting himself because before he said that sinful priests can confect the sacrament and that we can accept it from his hands, and now he's saying that such a sacrament is improperly used. This is because he says if we KNOW for a fact that a priest has a concubine ( or in our day, a gαy lover ) we should avoid his Mass. But if we don't know and he just happens to be living in sin, we still get grace from the sacrament.
The devil wants to involve everyone in sin by making them attend Masses that are blasphemous or given by heretical priests. The SSPX masses are a sacrilege offered una cuм with one of Christ's worst enemies; while NFP poisons the sedevacantist world, or at least the priests and bishops that I know about. This doesn't mean that no true Masses exist anywhere in the world, it only means that I personally do not know of any.