Eh, I think Theodulf of Orleans saw it and argued for it. He was the first Latin to make the case for an ontological distinction. He was actually the first Latin to really make an argument in favor of the Filioque at all. So I would push it back another century. The reason we don’t often hear about him is because the Opus Caroli was shelved. But I think it makes the argument very clearly.
There are numerous western fathers that were already teaching an ontological procession before Theodulf of Orleans
I look forward to the quotations. But I think they Eastern theologians for the most part used “through” in a different sense. By this, I mean in an economic mediary sense. This is in part why Theodulf argued against them using the word “through,” because he understood what sense the Eastern theologians were using, and disagreed with it. And I think that in large part is why the Council of Florence refrained from using the word “through” as well.
No a lot of then taught of the the procession in an eternal/ontological manner. I already showed you a few fathers but let me focus on eastern fathers :
St Athanasius of Alexandria :
“David sings in the psalm [35:10], saying: 'For with You is the font of Life;
'because jointly with the Father the Son is indeed the source of the Holy Spirit.”
“…Everything the Spirit has, He has from the Word (para tou Logou).”
St Basil the Great :
“Even if the Holy Spirit is third in dignity and order, why need He be third also in nature?
For that He is second to the Son, having His being from Him and receiving from Him and announcing to us and being completely dependent on Him, pious tradition recounts; but that His nature is third we are not taught by the Saints nor can we conclude logically from what has been said.”
At the council of Florence, Metropolitan Mark Eugenikos of Ephesus, struck by the unmistakable Filioquism of this passage, which is not compatible with his narrow Photian theology, was forced to maintain that it is not genuine. However, he was wrong, according to the Rev. Reuben Parsons, D.D. of pious memory :
the archbishop of Nicea tells us that out of six codices of St. Basel’s works brought by his countrymen to Florence, five gave this passage in its entirety; while the one that wanted it “was defective in some parts, and had many additions, according to the pleasure of the corrupter.” When he returned to Constantinople, Bessarion searched the libraries, and he found some new codices, written after the Council of Florence had terminated, and in which the above passage was wanting; whereas in other ancient MSS. which he consulted it was given.
St Epiphanios of Salamis :
"But someone will say, “Therefore we are saying that there are two Sons. And how then is He the Only-begotten?” Well then. “Who art thou that repliest against God?” [Rom 9:20].
For if he calls the one Who is from Him the Son, and the one Who is from both (παρ᾽ ἀμφοτέρων) the Holy Spirit, which things we understand by faith alone, from the saints— full of light, givers of light, they have their operation full of light…
St. Epiphanios distinguishes the hypostases of the Son and the Holy Spirit, Who share the two-person description “from the Father,” by adding that the Son is from the Father alone, whereas the Holy Spirit proceeds hypostatically from the Father and the Son.
St Cyril of Alexandria
“the Son is God, and from God according to nature (for He has had His birth from God the Father),
the Spirit is both proper to Him and in Him and from Him, just as, to be sure, the same thing is understood to hold true in the case of God the Father Himself.”
In 429 St. Cyril says in Thesaurus 34 :
“Thus, Paul knows no difference of nature between the Son and the Holy Spirit,
but because the Spirit exists from Him and in Him by nature, He calls Him by the name of Lordship.”
In the same part of the same work St. Cyril says,
“Therefore, when Christ lays down the law, **He lays it down that His Spirit naturally exists in Him and from Him.” **