The dogma of the Filioque is clearly taught in the Athanasian Creed, "The Holy Ghost is of the Father and of the Son; not created, not begotten, but proceeding". This is a dogmatic confession of the Catholic Faith necessary for salvation. He who does not believe it no longer has an orthodox and patristic understanding of the Trinitarian mystery. He who denies this dogma can never find life or salvation but will go into that eternal fire prepared for the devil and his Angels.
This site produces nearly 100 ancient patristic witnesses establishing the dogma of the Filioque against the later novel errors of Photius and the Greeks.
http://catholicpatristics.blogspot.in/2009/08/filioque.html1. St. Athanasius says, "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."
St. Cyril says, "For, in that 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."
St. Augustine says, The Father begot a Son and, by begetting Him, gave it to Him that the Holy Spirit proceeds from Him as well. If He did not proceed from Him, He would not say to His disciples, "Receive the Holy Spirit" [Jn 20:22], and give the Spirit by breathing on them. He signified that the Holy Spirit also proceeds from Him and showed outwardly by blowing what He was giving inwardly by breathing. If He were born, He would be born not from the Father alone or from the Son alone, but from both of Them; He would beyond any doubt be the son of both of Them. But because He is in no sense the son of both of Them, it was necessary that He not be born from both. He is, therefore, the Spirit of both, by proceeding from both."
St. Ambrose says, "The Holy Spirit also, when He proceeds from the Father and the Son, is not separated from the Father nor separated from the Son."
St. Isidore of Seville says, ""The Holy Spirit is called God because He proceeds from the Father and the Son and has Their essence ... There is, however, this difference between generation of the Son and the procession of the Spirit, that the Son is begotten of One, but the Spirit proceeds from Both."
St. Fulgentius says, ""Believe most firmly, and never doubt, that the same Holy Spirit, the One Spirit of the Father and the Son, proceeds from the Father and the Son. That He proceeds also from the Son is supported by the teaching both of Prophets and Apostles."
2. St. Leontios at Nicaea I says, "the Spirit proceeds from the Father, and is proper to the Son and gushes forth from Him."
Pope St. Leo the Great says, "as though there were not one Who begat, another Who is begotten, another Who proceeds from both"
Pope St. Hormisdas says, "Great and incomprehensible is the mystery of the Trinity. God the Father, God the Son, God the Holy Ghost, an undivided Trinity, and yet it is known because it is characteristic of the Father to generate the Son, characteristic of the Son of God to be born of the Father equal to the Father, characteristic of the Spirit to proceed from Father and Son in one substance of deity."
Patriarch St. Tarasius of Constantinople says at Nicaea II, "And in the Holy Spirit, Who proceeds from the Father through the Son, and Who is acknowledged to be Himself God."
Pope St. Leo III says, "the Holy Spirit, proceeding equally from the Father and from the Son, consubstantial, coeternal with the Father and the Son. The Father, complete God in Himself, the Son, complete God begotten of the Father, the Holy Spirit, complete God proceeding from the Father and the Son..."
Innumerable ancient ecclesiastical sources clearly establish this dogma.