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St Robert Bellarmine was born a few years before the start of the Council of Trent. In his work De Ecclesia, Book III: On The Church Militant, Chapter III On the Unbaptised, he teaches this:

"... it is said outside the Church no man is saved, and this ought to be understood on those who are neither in fact nor in desire within the Church, just as all the Theologians commonly teach on Baptism. Moreover, if the Catechumens are not in the Church de facto, at least they are in the Church in desire, therefore they can be saved. This is not opposed to the similitude of the Ark of Noah (outside of which no man was saved), even if he were in it by desire since similitudes do not agree in all things. For that reason, 1 Peter 3 compares Baptism to the ark of Noah and still it is certain that some are saved without Baptism in fact.

"But one might say, Augustine says that Catechumens are in the Church; it is true, but in the same place he separates them from the faithful. Therefore, he meant that they are in the Church not by act, but by potency, which he explains in the beginning of the 2nd book on the Creed, where he compares Catechumens to men who are conceived but not yet born."

It is certain that some are saved without Baptism in fact. It is certain. Immediately following the Council of Trent. Saint, Bishop and Doctor of the Church, St Robert Bellarmine.

But it must be believed without doubt that true conversion supplies for Baptism of water when, not of contempt, but of necessity some die without Baptism of water.  It is expressly stated, Ezech. 18, if the wicked do penance for all his sins, I will not remember all his iniquities.  Thus also Ambrose clearly teaches in his oration on the death of Valentinian the younger:  Whom I was, he says, about to regenerate, I have lost; but he did not lose the grace which he had hoped for.  Thus also Augustine lib.4. de baptism, cap.22. & Bernard epist.77 & after them Innocent III. cap. Apostolicam, de presbytero non baptizato, whence also the Council of Trent, Session 6, Chapter 4 says that Baptism is necessary in fact or in desire.

St Robert "believed" that catechumens might have BOD.  Ok, fine.  St Thomas talks about the same thing.

The heresy begins when you start to apply the word "catechumen" incorrectly.  When you say that Jєωs, muslims, pagans, unbaptized protestants, etc can have BOD.  They cannot and that is grave error and a denial of Trent.  Why?  Because these groups of people cannot be catechumens, in the proper sense.

If one is a formal catechumen, this means they have 1) rejected their former false religion, 2) are formally learning the Faith at a Catholic Church, 3) have an express desire to become a Catholic.

If someone does not fulfill the above general requirements, then they aren't a catechumen and the St Thomas/St Robert's writings don't apply.