The doctrine behind this word finds plenty of support in the East. If you wish to deny it, you'll have to throw out any notion of union in faith not only with all of the West prior to 1054, which I'm sure you will plainly admit upheld this from Apostolic antiquity, but also many illsutrious men of the East. Certainly, none in the east thought the west heretical for using it for hundreds of years until Patriarch St. Photius (who died in union with Rome), stirred up trouble with the help of this word.
The only-begotten Holy Spirit has neither the name of the Son nor the appelation of Father, but is called Holy Spirit, and is not foreign to the Father. For the Only-begotten Himself calls Him: "the Spirit of the Father," and says of Him the "He proceeds from the Father," and "will receive of mine," so that He is reckoned as not being foreign to the Son, but is of their same substance, of the same Godhead; He is Spirit divine, ... of God, and He is God. For he is Spirit of God, Spirit of the Father, and Spirit of the Son, not by some kind of synthesis, like soul and body in us, but in the midst of Father and Son of the Father and of the Son, a third by appelation
....
The Father always existed and the Son always existed, and the Spirit breathes from the Father and the Son; and neither is the Son created nor is the Spirit created.
-St. Epiphanius of Salamis, The Man Well-Anchored, 8 & 75 (374 AD)Since the Holy Spirit when he is in us effects our being conformed to God, and he actually proceeds from the Father and Son, it is abundantly clear that He is of the divine essence, in it in essence and proceeding from it.
-St. Cyril of Alexandria, The Treasury of the Holy and Consubstantial Trinity, Thesis 34, (423-425 AD)The peculiar relationship of the Son to the Father, such as we know it - we will find that the Spirit has this, to the Son. And since the Son says, "everything whatsoever that the Father has is Mine," we will discover all these things also in the Spirit, through the Son. And just as the Son was announced by the Father, Who said, "This is my beloved Son, in whom I am well-pleased," so also is the Spirit of the Son; for the Apostle says, "He has sent the Spirit of His Son into our hearts, crying 'Abba! Father!'"
-St. Athanasius, 3rd Letter to Serapion of Thmuis, 1 (360 AD)Those of the Queen of cities (Constantinople) have attacked the synodic letter of the present very holy Pope, not in the case of all the chapters that he has written in it, but only in the case of two of them. One relates to the theology (of the Trinity) and, according to them, says: "The Holy Spirit also has his ekporeusis (ekporeuesthai) from the Son". The other deals with the divine incarnation.
With regard to the first matter, they (the Romans) have produced unanimous evidence of the Latin Fathers, and also of Cyril of Alexandria, from the study he made of the gospel of St. John. On the basis of these texts, they have shown that they have not made the Son the cause (aitian) of the Spirit - they know in fact that the Father is the only cause of the Son and the Spirit, the one by begetting and the other by ekporeusis (procession) - but that they have manifested the procession through him (to dia autou proienai) and have thus shown the unity and identity of the essence...
They (the Romans) have therefore been accused of precisely those things which it would be wrong to accuse them, whereas the former (the Byzantines) have been accused of those things of which it has been quite correct to accuse them (Monothelitism). They have up till now produced no defence, although they have not yet rejected the things that they have themselves so wrongly introduced.
In accordance with your request, I have asked the Romans to translate what is peculiar to them [the 'also from the son'] in such a way that any obscurities that may result from it will be avoided. But since the practice of writing and sending [the synodic letter] has been observed, I wonder whether they will possibly agree to do this. It is true, of course, that they cannot reproduce their idea in a language and in words that are foreign to them as they can in their mother-tongue, just as we too cannot. In any case, having been accused, they will certainly take some care about this.
-St. Maximus the Confessor, Letter to Marinus, PG 91, 136The Spirit of the Word is like a love of the Father for the mysteriously begotten Word, and it is the same love that the beloved Word and Son of the Father has for the one who begot him. That love comes from the Father at the same time as it is with the Son and it naturally rests on the Son.
-St. Gregory Palamas, Chapters, 36, PG 150:1144D-1145A
Do you wish to dig yourself deeper into this hole? The Latin Church says the Holy Spirit proceeds from the Father and the Son. We hold this equivalent in meaning to saying the Holy Spirit proceeds from the Father through the Son. The only thing we cannot abide is denying any role at all for the Son in the procession of the Holy Spirit. The Holy Spirit cannot be from the Father alone, for the He loses His distinction from the Son, who is from the Father alone. So we say the Holy Spirit proceeds ineffably from the Father and consubstantially through the Son.
When Constantinople said "proceeds from the Father", they did not say "does not proceed from the Son" or "proceeds from the Father alone". The entire modern Orthodox argument following Photius (which not all Orthodox do) is one from silence.
Of course, even more troubling for the Orthodox is the necessity of rejecting part of the 2nd Council of Constantinople to uphold the Filioque, which says:
We further declare that we hold fast to the decrees of the four Councils, and in every way follow the holy Fathers . . . Hilary . . . Ambrose . . . Augustine . . . Leo and their writings on the true faith.
-Constantinople II, Acta, Session I
Why? Because these Holy men taught the filioque! Either their writings were not on the true faith, in which case the Council of Constantinople is made false and lying, or they were, and Photius' polemical writings are seen for the nonsense that they are.
Concerning the Holy Spirit, I ought not to remain silent, nor yet is it necessary to speak. Still, on account of those who do not know Him, it is not possible for me to be silent. However it is necessary to speak of Him who must be acknowledged, who is from the Father and the Son, His Sources.
-St. Hilary of Poitiers, The Trintiy, 2, 29 (356 to 359 AD)Know, then, that just as the Father is the Fount of Life, so too, there are many who have stated that the Son is designated as the Fount of Life. It is said, for example, that with You, Almighty God, Your Son is the Fount of Life, that is, the Fount of the Holy Spirit.
-St. Ambrose of Milan, The Holy Spirit, 1, 15, 152 (381 AD)For whatever the Son has, He has from the Father, certainly He has it from the Father that the Holy Spirit proceeds from Him ... For the Father alone is not from another, for which reason He alone is called unbegotten, not, indeed, in the Scriptures, but in the practice of theologians, and of those who employ such terms as they are able in a matter so great. The Son, however, is born of the Father; and the Holy Spirit proceeds principally from the Father, and since the Father gives to the Son all that He has without any interval of time, the Holy Spirit proceeds jointly from both Father and Son. He would be called Son of the Father and of the Son if, which is abhorent to everyone of sound mind, they had both begotten Him. The Spirit was not begotten by each, however, but proceeds from each and both.
-St. Augustine of Hippo, The Trinity, 15, 26, 47 (400 to 416 AD)Why, then, should we not believe that the Holy Spirit proceeds also from the Son, when he is the Spirit also of the Son? For if the Holy Spirit did not proceed form Him when He showed Himself to His disciples after His resurrection He would not have breathed on them, saying: "Receive the Holy Spirit." For what else did he signify by that breathing upon them, except that the Holy Spirit proceeds also from Him?
-St. Augustine of Hippo, Homilies on the Gospel of John, 99,7 (416 and 417 AD)For while the Son is the Only-begotten of the Father, and the Holy Spirit is the Spirit of the Father and of the Son, not as any creature, which is also of the Father and of the Son, but as living and having power with both, and eternally subsisting of that which is of the Father and of the Son.
-Pope St. Leo I, Sermons 75,3 (ante 461 AD)
Are you going to read all of these Saints out of the Orthodox Church too? Will you no longer venerate them because they taught the filioque "heresy"? (And they aren't the only ones.)
I dare you to find the filioque clause in the original Nicean Creed--I double dare you.
With the addition of the filioque clause the Frankish Popes did the EXACT same thing Luther did, a heresy. No difference.