Q: Today’s situation is unforeseen. No one can tell us with certainty, that does no fear error, which way to take regarding sedevacantism.
We can and we have to find some kind of direction, a true root of it in the past teaching and in the past Church’s praxis. It means also that under any circumstances we are not allowed to contradict, to go against, the pre-Vatican II Church’s teachings in order to explain today’s situation. If we are aware of the VII apostasy, we have to stick to the teaching which was before (for example we can not negate Vatican I and Mystici Corporis) ,and draw conclusions out of that.
Similar times in Church history must be studied moiré carefully, long vacancies, Church Law in relation to Protestantism, the French revolution, the rise of communism, modernism and others. We should study these past situations and apply true ecclesiology, Vat. I, papal encyclicals to our conclusions.
In the case of new ecumenism, we can start from the First Commandment. We see that this is not so “new” and that God always forbids any esteem of false gods. The Catholic Church always forbade any active participation in false cults (even non-active, if the reason for it isn’t sufficient enough). In the case of new ecumenism we can judge it with metaphysical certainty to be a certain apostasy.
“Judgements are metaphysically certain, when error is absolutely impossible, the opposite of what is held by the mind being a contradiction in terms which omnipotence itself could not make true. Example: the judgements that the same, identical act cannot be both good and bad, that good is to be done and evil to be avoided, that God is to be honoured, are metaphysically certain, since they result immediately from the very concepts of being, of goodness, and of God.”(Moral Theology: A complete course based on St. Thomas Aquinas and the best modern authorities; John McHugh, O.P., Charles Callan, O.P., N.Y., 1929; Vol. I, 643.)
It is certain that the N.O. (ecumenical church) is heretical. Could it be Catholic at the same time? To say “yes” we would go against Catholic teaching, and then against the nature of and the first purpose of the Catholic Church.
“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, June 29th , 1943, (23))
“But we must not think that He rules only in a hidden or extraordinary manner. On the contrary, our Divine Redeemer also governs His Mystical Body in a visible and normal way through His Vicar on earth. (…) Nor against this may one argue that the primacy of jurisdiction established in the Church gives such a Mystical Body two heads. For Peter in virtue of his primacy is only Christ’s Vicar; so that there is only one chief Head of this Body, namely Christ, Who never ceased Himself to guide the Church invisibly, though at the same time He rules it visibly, through him who is His representative on earth. After His glorious Ascension into heaven this Church rested not on Him alone, but on Peter too, its visible foundation stone. That Christ and His Vicar constitute one only Head is the solemn teaching of Our predecessor of immortal memory, Boniface VIII, in the Apostolic Letter ‘Unam Sanctam’, and his successors have never ceased to repeat the same. They, therefore, walk in the path of dangerous error who believe that they can accept Christ as the Head of the Church, while not adhering loyally to His Vicar on earth.” (op. Cit. 40-41)
To conclude: to say that a heretic is a visible head of the Mystical Body of Christ is the same as to say that Christ is preaching against Himself, that God has changed His mind so at the same time changed His teaching – a heretical blasphemy.