In short, the answer is that nowhere did the Fathers of Florence - that is the context of Cantate Domino
- define what it means to be "outside the ecclesiastical unity" of the Catholic Church.
The article Steve linked to demonstrates that there is, in fact, a difference between "being inside the Catholic Church" and "being a visible member of the Catholic Church." The former is absolutely necessary for salvation, while the necessity of the latter for salvation is not
As the article puts it:
That last assertion, then, is the key: the maxim “outside the Church there is no salvation” does not imply, as it seems to at first sight, that “only inside the Church is there salvation”.
In support of this argument,
Harrison takes another look at the quote from Cantate Domino
and basically asks, "What about catechumens?"
They have not yet been received into the Church; indeed, they've not yet received any Sacraments. They are not
yet "members of the Church," yet they're not "outside" the Church either. Conspicuously
, Cantate Domino
list "catechumens" in its list of people who cannot benefit from grace. It lists, rather:
The fact that "catechumens" are omitted suggests that the document does not intend to exclude from them the possibility of salvation. Since they are not yet members of the Church, therefore, it follows even by the standards of Cantate Domino itself
that "visible membership in the Church" is not
necessarily a prerequisite for salvation.
Likewise with Unam Sanctam:
one can be "subject to the Roman Pontiff" in other ways than visible membership in the Church. Say someone, if he knew of the papacy, would
desire to submit to the pope. It seems Tradition says that can be sufficient.
Harrison makes this very point in this article
: if in danger of death, it is lawful to baptize someone even if they only know about and profess (1) the Trinity and (2) the Incarnation. Such a baptized person on his deathbed would certainly be "subject to the Roman Pontiff" without knowing it.
In short, I now feel I know how Cantate Domino
fits within Sacred Tradition.
If anyone is still confused or uncertain, then for the sake of context and comprehensiveness I wholeheartedly urge you to read the entirety of Harrison's article
How does everyone else feel? Are you satisfied by Harrison's explanation?