The argument for patristic denial of the Filioque, which has never struck me as particularly strong, relies quite heavily on an argument from silence concerning St. Cyril's response to John of Antioch. In particular, it asserts that Theodoret of Cyrus made an accusation against Cyril of disagreement with what Theodoret perceived to be the patristic tradition that the Spirit derived his existence from the Father alone, and that Cyril's lack of response to the charge confirms that he accepted the validity of Theodoret's critique. The use of the argument is summarized nicely by Brian Duong at 29:00 of this video, but I believe that there may be an even worse problem in relying on Theodoret's position: it is based on Theodoret's Nestorian philosophy.
Ed Siecienski presents a much-cited account of this argument at p. 49 of The Filioque, so let's use his version. First, let's start with his background on the situation.
Yet in none of these passages, or anywhere in his writings, does Cyril say that the Spirit proceeds (ekporeuesthai) from the Father and the Son. Rather he consistently maintains that the Spirit progresses or flows forth (proienai, procheitai) from the Son, which is something rather different. That Cyril intends to retain an important distinction between the two concepts becomes clear in his exegesis of John 15:26, where he writes:
"Jesus calls the Paraclete 'the Spirit of Truth,' that is to say, his consoling Spiriti, and at the same time he says that He proceeds from the Father [para tou patros ekporeuesthai]. Thus as the Spirit is naturally proper to the Son, who exists in Him and progresses through him [di autou proion], yet he is at the same time the Spirit of the Father." [quoting the Commentary on John]
This distinction between ekporeuesthai and proienai allows Cyril, like Gregory of Nyssa before him and Maximus after him, to establish both a temporal and eternal relationship between the Son and the Spirit, yet one that does not involve the Son in the Spirit's ekporeusis. In Cyril's theology the Spirit proceeds from the Father [ekporeuetai ek tou Patros] but [from the Commentary on John] "is not a stranger to the essence of the only Son because he progresses naturally from him [proeisi de physikos ex autes]." Even if he never fully explicates the exact nature of this progression, Cyril is clear that the Spirit does not derive his ekporeusis or personal existence from the Son, a fact that becomes apparent in his debate with Theodoret of Cyrus.
There is actually nothing in Cyril's writings to support this distinction. The same is true of Gregory of Nyssa or Maximus, but Cyril is acknowledged in the scholarship for his interchangeable use of these terms. First, there is Brian Duong's citation of Epistle 55 that uses ekporeusis as a synonym of procheitai. This use is synonymous with how Latins use the term procedere for the Spirit. Moreover, Siencienski's own source on Cyril's pneumatology, The Theology of Cyril of Alexandria, also shows Cyril using ekporeuesthai in the same way as procedere. In interpreting Cyril's use of ekporeutai for the Son in the Commentary on John, Brian Daley in his chapter "The Fullness of the Saving God":
Cyril seems to be deliberately using the now-canonical terminology for the Spirit's origin to denote the Son's origin, as well, so as to identify both in terms of unity of substance and equality of status within the divine Mystery. In doing so, he shows concern about the negative implications of what would later be called a 'monopatrist' position on the origin of the Spirit: in the terms of the debates in which he was engaged, it could be taken to suggest that the Son and Spirit participate in different degrees in the one saving Mystery of God, which flows from the Father.
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While not being a 'filioquist', then, in the precise sense of the later controversies, Cyril does show a tendency, unusual in the Greek theological tradition, to stress the Son's role, alongside that of the Father, in being genuinely the source of the Holy Spirit. The reason for this role of the Son, Cyril often repeats, is his unity of substance with the Father, a fully divine status which the Son himself received in being begotten. Nor is it helpful to apply to Cyril's thought a distinction often found in Greek theology since Photius: that the Spirit can rightly be said to come 'from the Son' with regard to his mission in sacred history. As we have seen repeatedly here, Cyril avoids and even outright rejects any way of thinking or speaking that might appear to drive a wedge between God's being in itself and God's action in history, through Christ and the Spirit, to create, to save, and to sanctify.
I follow Fr. Thomas Crean in concluding that Daley's assertion here that Cyril is not a "filioquist" is simply wrong. But we don't even need to reach that issue in order to show that Siecienski is mistaken in his assertion about the technical use of ekporeusis. Cyril has been shown to have used ekporeutai in exactly the same way that Latins use procedere: to broadly indicate procession and not as a technical term for the Spirit's mode of procession. Furthermore, he specifically seems to have used the term in this broader way to respond to Nestorian monopatrism. Even if you believe, as Daley and Siecienski do, that Cyril believes in some sort of eternal relation other than a relation of origin between the Son and the Spirit (which I believe to be completely wrong), the point is that Cyril considers the Nestorian denial of this relationship to be at least dangerous if not outright heretical.
But Siecienski, by contrast, seems to think that Cyril is conceding Nestorian monopatrism. Siecienski says the following:
Cyril, in his ninth anathema against Nestorius, had stated that the Spirit was Christ's own Spirit, which led Theodoret to question whether Cyril was advocating the idea that "the Spirit has his subsistence from the Son or through the Son" (ex Yiou e di Yiou ten hyparxis echon). For Theodoret this idea was both "blasphemous and impious ... for we believe the Lord who has said: 'the Spirit of Truth who proceeds from the Father.'" Cyril denied that he held this teaching, leading Theodoret to confirm the orthodoxy of Cyril's trinitarian theology, since the Church had always taught that "the Holy Spirit does not receive existence from or through the Son, but proceeds from the Father and is called the proprium of the Son because of his consubstantiality."
Siecienski seems to have absolutely no idea what Theodoret is actually saying here, because he doesn't understand Nestorian philosophy, which is evidenced here by the technical term proprium. That philosophy is helpfully summarized in Vasilije Vranic's dissertation "The Christology of Theodoret of Cyrrhus," which updates Paul Clayton's magisterial work on the same subject. Proprium in Nestorian philosophy is used to establish a distinction between hypostasis and prosopon, as Vranic explains (pp. 94-96):
P. Clayton argues that the fact that Theodoret used the term prosopon to indicate distinction in the Holy Trinity does not necessarily mean that he used it as a synonym for hypostasis, but that the Antiochene tradition preferred this term "insofar as it indicates the outward perceptibility of the concrete reality being referred to. In the case of the Trinity's distinctions, this is pointed to in the earlier use of God as 'known' in the Father, Son, and Holy Spirit." Clayton concluses that "the probable metaphysical assumption" underlying Theodoret's Trinitarian theology is the Stoic doctrine of being. "Inasmuch as prosopon is the outward countenance of a hypostasis, and is thus that by which human sensibility experiences the hypostasis, it would have been easy for this Antiochene to use the former as a term of preference for indicating distinctions within the Godhead."
Clayton is right to suggest that it would be an error to equate Theodoret's understanding of the term hypostasis with his understanding of the term prosopon. The two are not interchangeable, since, like the Cappadocians before him, Theodoret understood hypostasis to be a set of individuating characteristics belonging to a prosopon. However, Clayton's understanding of Theodoret's use of prosopon to mean "an outward countenance of of a hypostasis" reduces it to a mere mask, which sits very uneasily with how it is used in Chapter 3 [of the Expositio rectae fidei]. There the term hypostasis designates only a part -- the personal characteristics -- of a prosopon. Thus, hypostasis functions as a pars pro toto for a hypostasis. At the end of chapter 3, Theodoret says that the term "unbegottenness," "begottenness," and "procession" define the hypostasis of each of the persons of the Trinity. Theodoret affirms that each term designates only the property (to idikon) of the person (prosopon). Had Theodoret, in his Trinitarian theology, used the term prosopon for the merely outward expression of a hypostasis, as Clayton argued, it would be hard to see how he could escape a charge of Modalism, i.e., of teaching that the three prosopa in the Godhead are not actually three distinct personal entities but a single divine prosopon, while the differentiation among the Father, Son, and the Holy Spirit is a mere outward countenance, a mask. Such a blunder surely would not have escaped the attention of an astute theologian such as Cyril of Alexandria.
Vranic is, it seems to me, correct both in his metaphysical assessment of Theodoret and in his later assessment that the terminology originates from the Cappadocians. In other words, he is correct to say that what Theodoret means by prosopon is not the mere appearance (parsopa in Syriac). But he is demonstrably wrong if he tries to find the "part for whole" (mereological) account of hypostasis in the Cappadocians (pp. 118-19, inaptly citing Prestige and Turcescu concerning the definition of hypostasis). It is clear that the Cappadocian understanding of hypostasis in this sense was purely conceptual, while the reality of the hypostasis and the prosopon were the same thing, which I believe is what Andrew Louth, (who was also cited for his disagreement with Vranic) was actually saying. That real identity between the two is clearly the basis of the subsequent phrase "hypostasis or prosopon" in Greek and Latin theology. There is one reality, viewed as hypostasis, tropos hyparxeos, or prosopon conceptually, but they are not three things or parts in the metaphysical sense, but rather one individual or concrete existence.
The fact that Theodoret has a real distinction between hypostasis and prosopon seems clear enough from the Expositio rectae fidei 3:
In fact, they say, how - if what begets and what is begotten are differentiated, and what proceeds from that from which it proceeds (for the Father is uncreated, from whom the Son was begotten and the Spirit proceeded) - does the Son and the Spirit are the same as the Father? Because “uncreated”, “generated” and “proceeding” are not expressions of the essence, but rather modes of existence; The modes of existence characterize these expressions. For the manifestation of the essence is indicated by the name "God", since there is a difference between the Father and the Son and the Spirit according to the mode of existence, but they are the same by the definition of the essence. For by this the Father has being uncreated, the Son generatedly, and the Spirit proceedingly, the characteristics arising from the differentiation being visible; on the other the essential being of its substance is indicated, and is implied by the common name of “Divinity.” What I'm saying would be clearer this way. He who reflects upon the existence of Adam, the manner in which he was brought into being, will find that he was not begotten, for he was not born of some other man, but was formed by the divine hand. But the formation shows the mode of existence, since it indicates in what way he was created. In the same way, conversely, the mode of existence characterizes formation, since it is equally evident that it existed when it was formed. If you investigate the essence of him, by which he is united in common with the [men] who [have arisen] from him, you will find that the foundation of him is a man. For as the formation shows the mode of existence, the mode of existence characterizes the formation, and the definition of the essence shows that the foundation is a man, so we will also recognize it in God and Father. For if you inquire into his mode of existence, seeing that he has not been created by anyone else, you will call him “uncreated”; If you consider the name “uncreated”, you will recognize that it expresses the mode of existence. If you also wanted to know the very essence, by which he is united in common with the Son and the Spirit, you will explain it with the name of “God.” As “uncreated” and the mode of existence make each other known, the “God” is an indicator of the essence. For as Adam, although he had no birth, is united in common by the same essence with those who were begotten from him, in the same way no argument will be able to separate the communion, on account of his being uncreated, from the essence of the Father with the Son and the Spirit. Because “uncreated”, “generated” and “proceeding” are not indicators of the essence, but are designations of the substances; distinguishing the Persons and, particularly, showing the substance of the Father, the Son and the Holy Spirit is sufficient for us. For just as we said that the uncreated directly distinguishes the substance of the Father, so, when we hear the designation "begotten" we understand it as a sign of the Son, and on the other hand, by means of the sign "proceeding" we recognize the particular Person of the Spirit. And these things are sufficient to demonstrate that “uncreated,” “generated,” and “proceeding” do not indicate the essence itself, but are distinctions of substances (hypostatic being, hypostaseon einai), and thereby also indicate the mode of existence (hyparxeos tropon).
Theodoret is associating the hyparxeos tropon within the individual existence, the prosopon, which is "also" indicated by the part (the hypostasis or hypostatic being). But the hypostasis is not the identical reality with the prosopon, as it is in Cappadocian theology. Rather, it is the composition of hypostasis (the individuating mode) with qnoma (the expressed nature) that produces prosopon. While Theodoret is using the terminology of the Cappadocians, the underlying concept seems to come from his predecessor Theodore of Mopsuestia (translated by Fr. Thomas Crean in Vindicating the Filioque at p. 207):
The Spirit Himself bears witness, who proceeds from the Father. For if by the word "proceed" he had understood not a natural procession but some external mission, it would have been uncertain about which of the many spirits who are sent in mission he was speaking, concerning which the apostle Paul says: "Are they not all ministering spirits, who are sent in mission?" But here he notes something proper, from which he can be known to have alone proceeded from the Father.
[Commentary on John 15:26]
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We believe in the Holy Spirit, who is from the substance of God, who is not a Son, who is God by substance, being of the substance of which is God the Father, from whom according to substance he is. "For we have not," he says, "received the spirit of the world but the Spirit who is from God," separating him from all creation and joining Him to God, from whom he is in a proper manner beyond that of all creation; we consider creation to be from God not according to substance but by a creative cause; and we neither consider him a Son, nor as taking His being from the Son [oute dia Yiou ten hyparxis eilephos].
[Creedal statement attributed to Theodoret around the Council of Ephesus]
What Theodore seems to have in mind for "substance" here is the Syriac qnoma, the "real" existence of the nature as expressed, as contrasted with the abstract nature kyana. But he distinguishes this from the existence (hyparxis) of the prosopon, which is taken from the Father alone. So we see here the same composition between hypostasis (the idiomatic mode with its propria) and qnoma to produce prosopon, where the hypostasis/hyparxis/proprium comes from the Father alone. Nonetheless, the Spirit can properly be described as the proprium of the Son on account of the common qnoma from the Father, but not from [ex] the Son, since the Spirit does not receive existence from the Son.
The reason for the ambiguity between Theodoret's interpretation of the Cappadocians and the Cyrillian tradition seems to be what Johannes Zacchuber points out in The Rise of Christian Theology and the End of Ancient Metaphysics: concrete existence was barely even a subject of interest in pagan philosophy. As Zacchuber observes of the Cappadocian account, "[i]t also leaves unexplained the relationship between the 'substantial' and the 'accidental' component of the individual" (p. 69). It is that ambiguity that causes Theodoret to posit his own mereological account in which the hypostasis (propria) is a part of the prosopon, which is in turn composed of the hypostasis plus the concretely expressed nature (qnoma in Syriac, rather than parsopa). The hypostasis functions a bit like the property of haecceity in Scotist metaphysics, but it differs in including individuating characteristics (propria), which is unique to the Nestorian metaphysics.
In that respect, I think Vranic's attempt to exonerate Theodoret from the charge of Nestorianism is in vain; there can be no strict identity between hypostasis and prosopon required by Cyrillian Christology as endorsed by Chalcedon. If there is a real distinction, a real non-identity, between the hypostasis and prosopon, Nestorianism is unavoidable. I do believe that this error is philosophical rather than dogmatic, as evidenced by the Common Christological Declaration between the Catholic Church and the Assyrian Church of the East. But the denial of the real identity between hypostasis and prosopon as the concrete existence of both divine and human natures cannot be consistently maintained with orthodox Christology.
Yet this distinction between hypostasis and prosopon is exactly the basis by which Theodoret asserts that "the Holy Spirit does not receive existence from or through the Son, but proceeds from the Father and is called the proprium of the Son because of his consubstantiality." It is not even coherent to say that the Spirit proceeds from the Son according to essence (ousia) (as Cyril affirms) but not according to existence unless one accepts this distinction between the qnoma of the prosopon and the hypostasis (hyparxis). It is exactly this distinction that Chalcedonian orthodoxy rejects, so both the anti-filioquist argument and the assertion that Cyril was not a filioquist are based on Nestorian philosophy. For that reason, the rejection of the filioque based on Theodoret requires the implicit adoption of a Christological heresy.
