I think this has become needlessly heated. Catholics and Orthodox would agree in perhaps 95% to 98% of what we confess together, so I think there is no need to attack each other over slight disagreements. If both sides are open to it, the Bishops of both our Churches can come to an agreement over this, in a future Ecumenical Council. In discussing such a lofty doctrine, we should also keep in mind the guiding principle of St. Maximus in the disagreement between Greeks and Latins, the other side "cannot express their meaning in a language and idiom that are foreign to them as precisely as they can in their own mother-tongue, any more than we can do."
Dear Rohzek, the citation from Pope St. Gregory the Great is from Morals on the Book of Job 2:56:92 in PL 75:599A and that of St. Isidore is Etymologies 7:3 in PL 82:268A; these Fathers and Popes speak of the Filioque in the context of consubstantiality and the Triune unity of the divine Essence, as is clearly seen in the context of the works cited. If the procession of the Holy Ghost from the Father is mediated through the Son so that the Most Holy Trinity is One consubstantial and undivided Essence, as these Fathers teach, it is manifest that the doctrine of the Filioque is true.
Indeed, one finds many Fathers who say expressly that it is His being and eternal subsistence that the Holy Ghost receives in the eternal act of procession from the Father mediated through the Son, even St. Maximus, cited in the link Alpo mentioned above "By nature (jusei) the Holy Spirit in his being (kat’ ousian) takes substantially (ousiodwV) his origin (ekporeuomenon) from the Father through the Son who is begotten (di’ Uiou gennhqentoV)" (Quaestiones ad Thalassium, LXIII, PG 90, 672 C)." St. Hillary, "May I receive your Spirit Who takes His being from You through Your only Son", St. Cyril "the Spirit is from God the Father and, for that matter, from the Son, being poured forth substantially from both, that is to say, from the Father through the Son. [On Worship and Adoration in Spirit and Truth 1 in PG 68:148A] These texts clearly concern the nature/being/essence/substance of the Holy Spirit, thus they describe the mystery of the unity of the Trinitarian Essence. The like doctrine on the procession of the Holy Ghost is found in the Trinitarian Athanasian Creed and in the solemn confession of Faith by Pope St. Hormisdas, in the year of Our Lord 517, "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" (proprium Spiritus sancti ut de Patre et Filio procederet sub una substantia Deitatis)[PL 63:514B],