I’ll break the quote down piece by piece, something I usually don’t do, because I think it will help in displaying why I think St. Gregory of Nyssa’s point is so relevant.
If, however, any one cavils at our argument, on the ground that by not admitting the difference of nature it leads to a mixture and confusion of the Persons
He’s responding to the objection that having three persons share a single nature leads to Sabellianism. It’s an understandable objection on its face, but it misses the point of how we distinguish Divine Persons.
we shall make to such a charge this answer;— that while we confess the invariable character of the nature, we do not deny the difference in respect of
cause, and that which is caused, by which alone we apprehend that one Person is distinguished from another;
The Divine Nature is singular, and not different between the Divine Persons, so we don’t distinguish between the Divine Persons because of a difference in Divine Nature, but rather we distinguish them by reason of relations of origin. One Nature is shared, but this Person is the Cause, and that Person is the caused, and that suffices to distinguish between Divine Persons without at all needing a distinct Nature for each individual.
— by our belief, that is, that one is the Cause, and another is of the Cause; and again in that which is of the Cause we recognize another distinction. For one is directly from the first Cause, and another by that which is directly from the first Cause;
This is where we run deep into filioque territory. We have the distinction of Cause and Caused, but the distinctions don’t stop there; if it did we’d have two “Sons” (as addressed next in the quote). Instead we have a distinction within the “Caused”, one being directly from the Cause, and another being “by” that one who is directly from the Cause. In other words, the distinction between the Holy Spirit and the Son, in terms of origin, is that the Son is only from the Father, while the Holy Spirit has His being in some way “by the Son” as well as from the Cause (the Father).
so that the attribute of being Only-begotten abides without
doubt in the
Son, and the interposition of the
Son, while it guards His attribute of being Only-begotten, does not shut out the Spirit from His relation by way of nature to the Father.
Here we have an explicit affirmation of the filioque, as understood in Latin theology at least. The Son is “Only-Begotten” and not “first-Begotten” precisely because He is interposed in a certain sense between the Father and the Holy Spirit. If the Holy Spirit were directly from the Father alone, just as the Son is, then the Son could not be called “only-Begotten”. We would then have two perfect Persons, direct images of the Father, with no distinction between them (no distinction of Nature, because they have an identical, and really singular, Divine Nature along with the Father, and no distinction with relation to the Cause, as they would both simply be “from the Cause” with no modifier added to either).
St. Gregory of Nyssa is interposing the Son, not as an obstacle between the Father and the Holy Spirit, and not to remove the Father from alone being the ultimate Cause, but in order to establish the third “identifier” in terms of origin. We have the Divinity which is Cause (Father), the Divinity which is from the Cause (Son), and the Divinity which is by that which is from the Cause (that is, the Holy Spirit is by the Son).
I don’t have the Greek version of this text, but I do find it noteworthy that in the English translation it is not said that the Holy Spirit is “***from ***that which is from the Cause”, but rather “
by that which is from the Cause”, which could indicate a different term than “ek” being used there. If so, the Greek “from”, with its implications of Source, is being reserved for the Father, but the Son is still put in a position of putting forth the
Person of the Holy Spirit in some manner (St. Gregory even uses the term “interposed”, at least in the translation, which is stronger than the Latin theology would typically put it). Indeed, St. Gregory of Nyssa is using this as
the manner of distinguishing Divine Persons, so it can’t be argued that he’s not refering to the Personal procession of the Holy Spirit (he even says that it is only by these relations that the Divine Persons themselves are distinguished).
I’m not saying that this is the quote that locks up the debate on the filioque, but I am saying that it shows that there is plenty of room for filioque-like theology in Greek Patristic thought, even as it regards hypostatic procession. In fact, I would argue that this quote shows that the later Scholastic Latin theological formula of “relations of opposition” defining the Divine Persons actually has its basis in Greek theological tradition, as opposed to the Augustinian model of “Lover, Loved, and Love”, or “Thinker, Thought, and Communication”.
continued…
Nah, wishful thinking. 
), then I recommend getting his take on it as well.
) place existence with hypostasis, not nature. The Father exists as Father, the Son exists as Son, and the Holy Spirit exists as Spirit. For this reason, if the Holy Spirit were to proceed from the Son hypostatically then one of two things is compromised: 1) the Father as sole source/origin of the Trinity because of duel procession; or 2) the distinction between Father and Son because they would be un-hypostatic.
), it is revealing to see what they would have accepted in the Greek text. They would have know the meaning of ekporeusis, and so how it is used in the text will be important. (Yes, it does appear in the text). Also, it appears that, for the sentence where it is said that the Son is to “be signified, according to the Greeks indeed as cause,” the word “aitían” is used in Greek (“causam” in Latin if you are curious). I will examine the grammar to the best of my ability to make sure that that word aitían] is linked, as I believe it is, to the Son.
Who were the Eastern Bishops or representatives who came into an agreement with the Latins in this text; A bunch of Latins posing as Easterners?