Why it is heterodox
If you were to attend a Roman Catholic Mass, an Anglican or Lutheran service and listened to the recitation of the Nicene Creed you would hear a strange three-word addition in their altered profession of Faith.
The seeming but not so innocent phrase [and the Son] was placed after these words in the Church's Symbol of Faith “And in the Holy Spirit who proceeds from the Father [and the Son”] This three-word appendage is nowhere to be found in the original confession coming out of the ancient ecumenical councils.
A little over a thousand years ago (1014 A.D.) the Roman Catholic Church succumbed to a temptation that had been unsettling her for several hundred years previous and officially inserted the innovative Filioque clause in their recitation of the of the Creed for the first time in a papal Mass.
This event marked what many consider the hard beginning of Rome’s schism which would reach fruition in 1054 A.D. Rome had now officially allowed a new understanding regarding the relationship within the Godhead to be inserted into the Creed in violation of the doctrinal formulations of the Great Ecumenical Councils. It also tacitly asserted the authority of the Roman Pontiff to override the Church’s firmly established understanding and ecumenical consensus. Consequently, Rome estranged herself from the other four great Patriarchal Catholic Sees and departed Orthodox Catholic consensus.
The genesis of this innovation can be traced back to two isolated local Council of Bishops in Toledo Spain 633 & 653 A.D. A regional group of western bishops unilaterally authorized the novel addition which wrongly states the Holy Spirit’s eternal procession is not uniquely from the Father but this procession is also from the Son thereby contradicting the Catholic Church’s witness and teaching expressed by the Ecumenical Nicene and Constantinopolitan Councils of 325 and 381 A.D. which teaches the eternal procession of the Holy Spirit as emanating from the Father.
The Holy Spirit’s procession from the Father, who is the fountainhead of the Godhead is the source and cause of The Holy Spirit's eternal procession. Likewise, the Son's being eternally begotten of the Father. Should we also add another Filioque stating the Son is eternally begotten of the Father "and the Holy Spirit"? To do either hopelessly confuses the proper distinction of persons in the Godhead within what is ultimately an unfathomable mystery. We have no more business saying the Son is eternally begotten of both the Father and Holy Spirit as we would confessing a dual procession of the Holy Spirit being from both the Father and Son.
Beside the problems this presents theologically the Church’s third Ecumenical Council held in Ephesus in 431 A.D. to deal with the Nestorian heresy made an emphatic point to uphold the two previous councils and forcefully condemned any tampering with the wording of the Creed with solemn anathemas.
The Bishop of Rome Sixtus III concurred and accepted the absolute authority of the Church’s third ecumenical council and for many years thereafter so did his successors.
Statements and practice over the subsequent centuries would illustrate Rome’s troubling inconsistency in the matter. Sometimes agreeing with the ancient Catholic Tradition and ecumenical consensus, other times wrongly defending the indefensible corrupt addition. Eminent Latin Churchmen like Alcuin, foreseeing the problems with such an interpolation said the following, “Beloved brethren look well to the sects of the Spanish error; follow in the steps of the holy Fathers; insert nothing new into the Creed of the Catholic Faith, and in religious functions be not pleased with traditions unknown in ancient times” He wrote this in 804 A.D.
Pope Leo III, five years later agreeing with and echoing Alcuin’s sentiments commissioned an engraving of the Creed upon two tablets of silver in both Greek and Latin hanging them in St. Peter’s Church minus the novel filioque and inscribed beneath the tablets, “I Leo have put up these tablets for the love and preservation of the Orthodox Faith.
The Holy Gospel of St. John definitively affirms the ancient Apostolic witness of the unalterable dogma which the Holy Orthodox Catholic Church steadfastly upholds and which no local synod or single bishop has the authority to amend. “But when the Counselor comes, whom I shall send even the Spirit of truth, who proceeds from the Father, he will bear witness of me” John 15:26
While arguments from Roman Catholic theologians have often tried to justify the insertion of the “Filioque” clause as an attempt to further clarify an understanding or even resist heresy it is clearly shown to be neither useful let alone helpful. Instead it became the primary catalyst for Rome’s ongoing estrangement from Catholic Tradition and her ongoing departure from The Faith once for all delivered to the saints Jude 1:3
Abbe Guette’ in his excellent book, [The Papacy: It’s Historic Origen and Primitive Relations With the Eastern Churches] speaking the mind of Catholic Tradition says, “The dogmatic truths of Christianity relating to the very essence of God—that is, of the Infinite—are necessarily mysterious; hence no one should presume to teach them of his own authority. Even the Church herself only preserves them as she has received them. Revelation is a deposit confided by God to His Church and not a philosophical synthesis which may be modified."