Actually, that is the ONLY subject that should occupy space on any CI thread about EENS. All else is a waste of time and beating around the bush.
Yes, the Cushingites ALWAYS deflect the discussion to BoD because they can find support for it in St. Thomas, St. Robert Bellarmine, St. Alphonsus. They then try to imply that the positions of these Doctors regarding BoD support their own broader implicit-faith ecclesiology ... which they most certainly do not.
Problem with that ecclesiology for a Traditional Catholic is that, if you hold it, you must say that there's no error in Vatican II and therefore no justification for the Traditional movement. That's why EENS is THE burning issue for Traditional Catholics. If I accepted the ecclesiology held by most Traditional Catholics, I would be forced to renounce Traditional Catholicism and accept Vatican II. I could apply a hermeneutic of continuity to V2 that makes V2 look like Trent ... again, assuming their ecclesiology.
We can quibble that various catechumens or catechumen-like people have
in voto membership in the Church or else have an imperfect membership in the Church due to their profession of the faith (one of the criteria for membership as laid out by St. Robert Bellarmine) combined with
in voto reception of Baptism. That's the line along which St. Robert himself tentatively argued that catechumens could possibly be saved. We can go back and forth on that, but it's a moot discussion.
When, however, the likes of +Lefebvre, +Fellay, +Sanborn, +McKenna, and the majority of Traditional bishops and priests have said that people like Hindus and Muslims could be saved without having to profess the faith (and without conversion before death) and intending to receive Baptism, then the ecclesiological implications of this are mind-blowing.
So if these can be saved, it must mean that they are within the Church before they die. Consequently, the Church now consists not only of true actual Catholic members of the Church (whether catechumens fall into this category somehow or not is a purely hypothetical discussion) ... but also of various Protestants, Hindus, Muslims, etc. Now you clearly have subsistence ecclesiology, where the central visible core of the Church consists not only of the actual/public (and perhaps
in voto) members while you have various invisible members, not co-extensive with the Body of the Church, who are yet within it ... to varying degrees. Now suddenly you have various degrees of partial communion with this subsistent core depending on how close doctrinally you might be (materially) to the fullness of Catholic doctrine. Now these are truly separated brethren, brethren because they are within the Church (if they can be saved) and separated because of their material separation from the visible Church. Now, when their intention to do the will of God itself pleases God and becomes salvific, since they have a right to please God and save their souls, they clearly have a right to follow their even erroneous consciences (since doing so pleases God and saves their soul).
I simply cannot comprehend how Traditional Catholics cannot see this ... except due to some cognitive dissonance on their part.
If this implicit-faith ecclesiology is tenable, then Vatican II does NOT teach error or heresy but simply adopts this opinion as that of the Church. Since when is adopting a probable opinion tantamount to heresy? In fact, the Church adopted the minority opinion of Abelard when she embraced the doctrine of Limbo for infants as her own.