4] Byzantine and Eastern fathers and monks testify that the spiration of the Spirit from the Father does not exclude but rather is mediated through the Son.
St. Basil the Great states that the Holy Spirit is united through the Word in the eternal unity of the Holy Trinity: “Through the Son, who is one, he is joined to the Father, who is one, and by himself completes the Blessed Trinity” . The Son is One, the Father is One, the Spirit is One united to the Father through the Son.
St. Maximus the Confessor said: “By nature the Holy Spirit in his being takes substantially his origin from the Father through the Son who is begotten” . The Holy Spirit takes His being substantially from the Father through the Son, and this in such a way that the Father gave the Spirit to the Son in eternally begetting Him.
St. John Damascene is the sole saint cited as possibly denying the Filioque, yet even he does not deny that the Trinitarian Order has the Spirit always issuing from the Father through the Word: “I say that God is always Father since he has always his Word coming from himself, and through his Word, having his Spirit issuing from him” .
We have seen that St. Tarasius dogmatized such a profession at the Second Nicene Council, the seventh ecumenical council. This is the true tradition of the fathers.
 The Latin fathers are absolutely unanimous in teaching the doctrine of the Filioque. Bishops and several councils do the same.
This is a fact so clear that it will hardly be doubted. It is explicitly stated by St. Maximus , and further evidence for the same can be read in Dr. Henry Barclay Swete’s monumental work on the subject . The evidence documented in point  already establishes this, and in St. Robert’s treatise, the doctor explicitly cites much proof; but we will cite the Athanasian Creed, which even secular scholars do not doubt was the widely accepted faith of the Western Church by at least the 5th century.
As St. Robert adduces it, “blessed Athanasius who says in his Creed, ‘The Holy Spirit is not made nor created nor generated by the Father and the Son, but proceeds.'”
To this testimony an objection might be made — namely, that this creed is not really from Athanasius. This is easily refuted, both by Nazianzen, where he says in praise of Athanasius that he composed a most perfect confession of faith that the whole West and East venerate, and also from Augustine, who by name cites Athanasius, bishop of Alexandria and adduces a complete section of this creed, and he uses whole sentences from it, with the name of Athanasius, as if it were well known in the Church.
The Third Council of Toledo (589) is also evidence of the universal acceptance of this doctrine: “Credo in Spiritum Sanctum qui ex patre filioque procedit” (I believe in the Holy Spirit Who Proceeds from the Father and the Son). Both Archbishop St. Leander of Seville, who presided, and his brother, St. Isidore, teach the Filioque dogma.
 The Greek Fathers are unanimous in teaching the doctrine “per Filium” (through the Son). This fact has been found embarrassing by deniers of the Filioque.
Philip Schaff, in History of the Christian Church, says, “Photius and the later Eastern controversialists dropped or rejected the per Filium, as being nearly equivalent to ex Filio or Filioque, or understood it as being applicable only to the mission of the Spirit, and emphasized the exclusiveness of the procession from the Father . “The teachings of St. Basil and St. Maximus shown earlier, and especially the profession of St. Tarasius at Nicaea II, demonstrate that per Filium is dogma.
 The Roman pontiffs, the successors of St. Peter, have unanimously taught the Filioque explicitly for millennia. There is clear unbroken tradition present here.
Pope St. Damasus, quite likely in a synod before the year 380 A.D., used the Filioque in a response to the Macedonian heresy: “We believe … in the Holy Spirit, not begotten nor unbegotten, not created nor made, but proceeding from the Father and the Son, always co-eternal with the Father and the Son” .
Note the special value of this ancient testimony of the 4th-century Roman Church, world-renowned for its Catholic orthodoxy and defense of St. Athanasius contra mundum under Pope St. Julius, et al. It is incidental and undesigned. It presupposes the dogmatic truth of the Filioque in a controversy against Macedonian heretics (who blasphemed against the Divinity of the Holy Spirit). And it shows that the dogma of the Holy Spirit’s divinity is no less certain than the dogma of the Filioque.
Another 4th-century Roman synod states: “The Holy Spirit is not only the Spirit of the Father, or not only the Spirit of the Son, but the Spirit of the Father and the Son. For it is written, ‘If anyone loves the world, the Spirit of the Father is not in him’ (1 Jn. 2:15). Likewise, it is written, ‘If anyone, however, does not have the Spirit of Christ, He is none of His (Romans 8:9).’ When the Father and the Son are mentioned in this way, the Holy Spirit is understood, of whom the Son Himself says in the Gospel, that the Holy Spirit ‘proceedeth from the Father (John 15:26)’ and ‘He shall receive of mine and shall annuonce it to you (Jn. 16:14)'” .
Are there more such testimonies from the ancient orthodox Roman Church? Yes: Pope St. Leo the Great, in the 5th century, says, “And so under the first head is shown what unholy views they hold about the Divine Trinity: they affirm that the person of the Father, the Son, and the Holy Ghost is one and the same, as if the same God were named now Father, now Son, and now Holy Ghost: and as if He who begat were not one, He who was begotten another, and He who proceeded from both yet another” .
This letter of Pope St. Leo I is cited in the Catechism of the Catholic Church. Is there another Pope, saint, and great who teaches Filioque? Yes: Pope St. Gregory the Great in the 6thcentury shows the dogmatic Roman and universal tradition when he confesses, “We can also understand His being sent in terms of His divine nature. The Son is said to be sent from the Father from the fact that He is begotten of the Father. The Son relates that He sends the Holy Spirit[.] … The sending of the Spirit is that procession by which He proceeds from the Father and the Son. Accordingly, as the Spirit is said to be sent because it proceeds, so too it is not inappropriate to say that the Son is sent because He is begotten” . This statement shows that, contra the Greeks, sending reveals hypostatic relation. That is why, throughout the Holy Scriptures, we never read that the Father is sent. The Father does not proceed from anyone. The Son proceeds from the Father alone, by generation, therefore He is said to be sent by the Father. The Spirit proceeds from the Father and the Son, therefore the Son explicitly says many times, “But I tell you the truth: it is expedient to you that I go: for if I go not, the Paraclete will not come to you; but if I go, I will send him to you” (Jn. 16:7) that we may understand the eternal relation implied here.