CathInfo.com

Dogmatic non una cuм is straight from the pit of Hell and is a tool used by Satan to keep traditional Catholics from comingling or from the sacraments altogether.

I've said this before, but, there is absolutely no historical or Magisterial basis for non una cuм. At all. It is a novelty dreamed up by Bp. Guerard des Lauriers and later catapulted by Fr. Cekada into the only "Catholic" position. Citing an early 19th century English Missal, John Daly noted that the King of England is also prayed una cuм in the Mass following the Pope and the Ordinary; proving that the intention is that the Mass is offered through the office of the one named, not the person. Otherwise, all English Masses would have been mortally sinful to assist at because the Anglican King of England was named una cuм.

Furthermore, we have the Western Schism to look at as well. Where saints, such as St. Vincent Ferrer, supported the later-declared anti-Pope over the true Pope; therefore, meaning that he offered Masses una cuм an anti-Pope, and, by the logic of this position would have been offending God by committing a grave sin. Yet, as we know, St. Vincent was a great saint who was merely mistaken on the identity of the Pope, and therefore, was not at fault. One could retort that anti-Pope Benedict XIII was "orthodox" therefore nullifying any such comparisons between anti-Pope Francis and Benedict XIII. But this, too, is nonsense, as it either way, by their logic, the Masses said by St. Vincent would still be gravely sinful because a false Pope was named in the Canon.

Therefore, to dogmatically declare that there is not only sin attached to assisting at a Mass una cuм Francesco, but even a mortal sin, is beyond the authority of those proclaiming it: namely, SGG and MHTS. It is correct to form an opinion on the Pope question, or even act on it individually, but to teach the laity that it would be a sin to attend an SSPX, or even SSPX-Resistance, Mass because they say it una cuм Francesco is divisive, diabolism.

"And if a house be divided against itself, that house cannot stand."
[Mark 3:25]

That's all well and good, but, how can any of that apply if Francis has never been formally declared a heretic by the Holy Office? I can believe that someone is a heretic, but, that remains in the realm of my opinion, especially when pertaining to clergy or the Pontiff, until the Church makes a declaration.

Further, those instances you cite apply to communion with formal heretics, such as the Greeks or Anglicans etc. While many suspect the Novus Ordo is not the Catholic Church, this, again, has not been defined by the Holy Office. So I fail to see how that falls into Communicatio in Sacris?

Any kind of Communicatio in Sacris/Divinis with a heretic and/or schismatic or Communicatio in Sacris/Divinis with those in communion with a heretic or schismatic was unanimously held to be forbidden by all of the Church Fathers & Church Councils as well as the vast majority of theologians, Popes, and Doctors of the Church.

The logic is simple.

a) If Bergoglio is a heretic then one cannot be in communion with him, directly or indirectly, (knowingly and intentionally) under pain of heresy and mortal sin.

b) The CMRI & similar groups believe Bergoglio is a heretic.

c) The CMRI and similar groups are in indirect communion with Bergoglio. i.e. They are in communion with those who are directly in communion with Bergoglio such as the SSPX which offers the Una cuм Mass.

d) The CMRI and similar groups are heretics themselves for being in intentional indirect communion with a man they consider a heretic.