This Stepanich is a heretic. His understanding of 'invincible ignorance' is entirely heterodox, and takes the teachings of Pope Pius IX out of context. And Pius XII was neither saintly nor a pope, but a heretic.
Pope Pius IX, Singulari Quadem, December 9, 1854: "For it must be held by faith that outside the Apostolic Roman Church, no one can be saved; that this is the only ark of salvation; that he who shall not have entered therein will perish in the flood; but, on the other hand, it is necessary to hold for certain that they who labor in ignorance of the true religion, if this ignorance is invincible, are not stained by any guilt in this matter in the eyes of God."
Ignorance itself will not excuse a person from the guilt of infidelity to the true religion, such as subjecting oneself to a heretical antipope, but invincible ignorance alone. Notice also that the following teaching of Pius IX does not state that such persons will be exonerated from the guilt of original sin or other sins against morality. Pope Pius IX never taught that a person in the state of invincible ignorance of the true Faith could be saved, but that their just punishment would not include punishment for the sin of infidelity.
Let us take this one step further.
Pius IX does not teach that salvation can be attained by invincible ignorance alone. In fact, no pope ever has. In contrast however, no pope has ever explicitly stated that those, though invincibly ignorant of the Truth that is Catholicism, are condemned to hell. If invincible ignorance will not be punished for, then what lay in the way of salvation?
Original Sin. It's remedy? Baptism, most certainly.
Let us investigate Justin Martyr...
St. Justin Martyr, 1.46 (c. 150 AD): "We have been taught that Christ is the first-begotten of God, and we have declared him to be the Logos of which all mankind partakes [John 1:9].
Those, therefore, who lived according to reason [Greek, logos] were really Christians, even though they were thought to be atheists, such as, among the Greeks, Socrates, Heraclitus, and others like them. . . .
Those who lived before Christ but did not live according to reason [logos] were wicked men, and enemies of Christ, and murderers of those who did live according to reason [logos], whereas those who lived then or who live now according to reason [logos]are Christians. Such as these can be confident and unafraid" And likewise, Clement of Alexandria...
Clement of Alexandria (Stromata 7.17, 1.5)"From what has been said, I think it is clear that there is one true Church, which is really ancient,
into which those who are just according to design are enrolled."
How can reason alone justify a man? There would have to be two factors accounted for...
First would be obviously an invincible ignorance of Catholicism, such as the Greeks would have enjoyed. This ignorance would also apply today to those without the reason to understand Catholicism, such as the handicapped, the young, the uneducated, and those without access to the resources to learn.
Secondly, we can safely assume both Justin and Clement believed in Baptism of Desire, as we understand no one can be justified without baptism. Hence, if the Greeks could be justified, it must be admitted they had somehow received a valid baptism.
So, as CM says, Ignorance itself will not excuse, however, invincible ignorance coupled with a valid baptism of desire, would be sufficient in terms of what is 'required' for salvation.
St. Alphonsus Liguori 1691-1787 Moral Theology - (Bk. 6):
"But baptism of desire is perfect conversion to God by contrition or love of God above all things accompanied
by an explicit or implicit desire for true Baptism of water, the place of which it takes as to the remission of guilt, but not as to the impression of the [baptismal] character or as to the removal of all debt of punishment. It is called 'of wind' ['flaminis'] because it takes place by the impulse of the Holy Ghost Who is called a wind ['flamen']
This in no way means 'Everyone saved', as ultimately there must still be a response to the Lord's grace, but the possibility exists and we should ultimately pray that God's will be done with such individuals, and be our lord's hands and feet in bringing to them the truth of Catholicism.