(On the Mystagogy of the Holy Spirit by Saint Photios, Patriarch of Constantinople, trans. by Holy Transfiguration Monastery [Studion Publishers, Inc: 1983], p. 71.) It is common to find those who are under the impression that the Filioque clause is a trifling matter—a mere “word-squabble” among theologians; or that the Filioque is really not a big issue because some theologians have figured a way to keep the clause but interpret it in an “Eastern Way” that is acceptable to the Orthodox. Such a person with an honest, truth-loving soul will not be able to hold these opinions any longer when they read the Mystagogy. There is no way to get around the fact that it is a grave heresy and has always been seen as such by the Orthodox Church.
From the Synodicon of the Holy Spirit (to be read on the second day of Pentecost)

Background: This is subtitled: “A confession and proclamation of the Orthodox piety of the Christians, in which all the impieties of the heretics are overthrown and the definitions of the Catholic Church of Christ are sustained. Through which the enemies of the Holy Spirit are severed from the Church of Christ.” This Synodicon (a decision, statement, or tome either originating from a synod or council or possessing conciliar authority) is attributed to Patriarch Germanos the New (1222-1240). It demonstrates how the theology of St. Photios the Great became the Church’s definitive voice on the subject of the filioque. There can be no doubt that the filioque was judged to be heretical by the Orthodox Church. What follows are just a few of the anathemas from the Synodicon. (This background information, and the anathemas, are taken from On the Mystagogy of the Holy Spirit, ibid.)

“So likewise do they who despise and disdain piety receive curses; wherefore, all we in unison, since we constitute the plentitude of piety, lay upon them the curse which they have put upon themselves.” [an excerpt from the Synodicon of Orthodoxy, read on the Sunday of Orthodoxy during Great Lent].’ To those who do not deign to consent to the unaltered and unadulterated holy Symbol confessed by the Orthodox, that one, I mean, which was evangelically formulated by the First and Second Holy Councils and confirmed by the rest, but who rather amend it and distort it to support their own belief, thereby not only corrupting the conciliar traditions of the holy fathers and of the holy and God-instructed apostles, but also the definitions of our true God and Savior, Jesus Christ, ANATHEMA.” …

“To those who in any way undertake investigations into new doctrines concerning the divine and incomprehensible Trinity and who search out the difference between begetting and procession, and the nature of begetting and procession in God and who increase words and do not abide and persist in the definitions handed down to us by both the disciples of Christ and the divine fathers; and who thereby uselessly strive to dispute over things not delivered to us, ANATHEMA.”

“To those who scorn the venerable and holy ecumenical Councils, and who despise even more their dogmatic and canonical traditions; and to those who say that all things were not perfectly defined and delivered by the councils, but that they left the greater part mysterious, unclear, and untaught, ANATHEMA.”

“To those who hold in contempt the sacred and divine canons of our blessed fathers, which, by sustaining the holy Church of God and adorning the whole Christian Church, guide to divine reverence, ANATHEMA.”

“To all things innovated and enacted contrary to the Church tradition, teaching, and institution of the holy and ever-memorable fathers, or to anything henceforth so enacted, ANATHEMA.”
St. Mark of Ephesus, one of the three “pillars” of Orthodoxy

“We have cut the Latins off from us for no other reason than that they are not only schismatics, but heretics. For this reason it is wholly improper to unite with them…. The Latins are not only schismatics but heretics as well. However, the Church was silent on this because their race is large and more powerful than ours… and we wished not to fall into triumphalism over the Latins as heretics but to be accepting of their return and to cultivate brotherliness.” (1439)
SIGILLION of the Patriarchal formulation of an encyclical to Orthodox Christians throughout the world not to accept the modernistic Paschalion, or calendar of the innovated Menologion, but to keep what was once for all and well-formulated by the three hundred and eighteen Holy God-bearing Fathers of the First Ecumenical Council, under penalty of penance and anathema.

Background: In 1583, the Pope of Rome, Gregory XIII, who changed the Julian calendar, repeatedly pressured the Patriarch of Constantinople, Jeremias, who was called Illustrious, to follow him in the calendar innovation. The Patriarch repeatedly refused with letters, and finally in the same year, 1583, he convened a council in Constantinople…

“To all the genuine Christian children of the Holy Catholic and Apostolic Church of Christ of the East residing in Trigovysti and throughout the world, be grace and peace and mercy from God Almighty.

“No small turbulence overtook that ancient Ark, when, violently beset by billows, it floated upon the surface of the waters, and had not the Lord God remembered Noah and seen fit to still the water, there would have been no hope for it at all. Thus also in regard to the New Ark of our Church, against which misbelievers have launched an implacable war upon us, by means of these presents we have decided to leave a note that you may have in what is herein written the means of upholding and defending your Orthodoxy against such enemies more safely and surely.

But, lest the composition as a whole be weary to the simpler folks, we have decided to embody the matter in common langauge, wording it as follows: “In Common Language “From old Rome have come certain persons who learned there to wear Latin habits. The worst of it is how, from being Romans of Rumelia bred and born, they not only have changed their faith, but they even wage war upon the Orthodox dogmas and truths of the Eastern Church which have been delivered to us by Christ and the divine Apostles and the Holy Councils of the Holy Fathers.

Therefore, cutting off these persons as rotten members, we command:

1) That whoever does not confess with heart and mouth that he is a child of the Eastern Church baptized in Orthodox style, and that the Holy Spirit proceeds out of only the Father, essentially and hypostatically, as Christ says in the Gospel, shall be outside of our Church and shall be anathematized.

2) That whoever does not confess that at the Mystery of the Holy Communion the laity must also partake of both kinds, of the Precious Body and Blood, but instead says that he will partake only of the body, and that that is sufficient because therein is both flesh and blood, when as a matter of fact Christ died and administered each seperately, and they who fail to keep such customs, let all such persons be anathematized.

3) That whoever says that our Lord Jesus Christ at the Mystic Supper had unleavened bread (made without yeast), like that of the Jews, and not leavened bread, that is to say, bread raised with yeast, let him depart far away from us and let him be anathema as one having Jewish views and those of Apollinarios and bringing dogmas of the Armenians into the Church, on which account let him be doubly anathema.

4) Whoever says that our Christ and God, when he comes to judge us, does not come to judge souls together with bodies, or embodied souls, but instead comes to sentance only bodies, let him be anathema.

5) Whoever says that the souls of Christians who repented while in the world but failed to perform their penance go to a purgatory of fire when they die, where there is flame and punishment, and are purified, which is simply an ancient Greek myth, and those who, like Origen, think that hell is not everlasting, and thereby afford or offer the liberty or incentive to sin, let him and all such persons be anathema.

6) That whoever says that the Pope is the head of the Church, and not Christ, and that he has authority to admit persons to Paradise with his letters of indulgence or other passports, and can fogive sins as many as a person may commit if such person pay money to receive from him indulgences, i.e. licences to sin, let every such person be anathema.

7) That whoever does not follow the customs of the Church as the Seven Holy Ecumenical Councils decreed, and Holy Pascha, and the Menologion with which they did well in making it a law that we should follow it, and wishes to follow the newly-invented Paschalion and the New Menologion of the atheist astronomers of the Pope, and opposes all those things and wishes to overthrow and destroy the dogmas and customs of the Church which have been handed down by our fathers, let him suffer anathema and be put out of the Church of Christ and out of the Congregation of the Faithful.

8) That ye pious and Orthodox Christians remain faithful in what ye have been taught and have been born and brought up in, and when the time calls for it and there be need, that your very blood be shed in order to safeguard the Faith handed down by our Fathers and your confession: and that ye beware of such persons as have been described or referred to in the foregoing paragraphs, in order that our Lord Jesus Christ may help you and at the same time may the prayer of our mediocrity be with all of you: amen.

Done in the year of the God-man 1583 (MDLXXXIII), year of indiction 12, November 20 [O.S.]

Jeremiah of Constantinople
Silvester of Alexandria
Sophronius of Jerusalem

In the presence of the rest of the prelates at the Council.”

This is found in pages 13-15 of the Rudder, which goes on to add that at another Pan-Orthodox synod in 1593, the 8th canon, which was headed “For exclusion of the New Calendar decrees that “all those who dare to disturb the rules of the Great and Holy Ecumenical Council of Nicaea, as held in the presence of the pious and most God-beloved King Constantine in regard to the holy feast of man-saving Pascha, be excommunicated and excluded from the Church…”