A council took place near/in Constantinople in 754 AD, aka the Council of Hieria. It was well-attended, ~338 bishops, and called itself an Ecumenical Council.
So...why isn't it an official EO Ecumenical Council? Ultimately, because the church came to reject its conclusions; ie, the more modern church looked back and picked and chose what traditions from church history would become Sacred Tradition. The point is so obvious as to be beyond reasonable dispute, but that doesn't mean that our friendly neighborhood sold-out on-fire EOx are going to be reasonable.
Here are some notes on this council that I thought are interesting.
This would appear when the enquiry is made, of which of the two Natures was the image of Christ the representation? Of the human nature or of the human and divine conjoined? If it was the picture of the human nature only, then did not the worshippers become practical Nestorians, worshipping the humanity apart from the divinity? But did they, on the other hand, assert that the image of Christ was an image of His human and divine nature conjoined — what was that but the error of Eutyches?
This part of their discussion they conclude by showing that no other image of Christ was needed than that which He Himself had left us — namely, the bread and wine of the Lord's Supper, true and real images of His body and blood.
Satan misguided men, so that they worshipped the creature instead of the Creator. The Mosaic law and the prophets cooperated to undo this ruin; but in order to save mankind thoroughly, God sent his own Son, who turned us away from error and the worshipping of idols, and taught us the worshipping of God in spirit and in truth. As messengers of his saving doctrine, he left us his Apostles and disciples, and these adorned the Church, his Bride, with his glorious doctrines. This ornament of the Church the holy Fathers and the six Ecumenical Councils have preserved inviolate. But the before-mentioned demi-urgos of wickedness could not endure the sight of this adornment, and gradually brought back idolatry under the appearance of Christianity. As then Christ armed his Apostles against the ancient idolatry with the power of the Holy Spirit, and sent them out into all the world, so has he awakened against the new idolatry his servants our faithful Emperors, and endowed them with the same wisdom of the Holy Spirit. Impelled by the Holy Spirit they could no longer be witnesses of the Church being laid waste by the deception of demons, and summoned the sanctified assembly of the God-beloved bishops, that they might institute at a synod a scriptural examination into the deceitful colouring of the pictures (ὁμοιωμάτων) which draws down the spirit of man from the lofty adoration (λατρείας) of God to the low and material adoration (λατρείαν) of the creature, and that they, under divine guidance, might express their view on the subject...
After we had carefully examined their decrees under the guidance of the Holy Spirit, we found that the unlawful art of painting living creatures blasphemed the fundamental doctrine of our salvation—namely, the Incarnation of Christ, and contradicted the six holy synods. These condemned Nestorius because he divided the one Son and Word of God into two sons, and on the other side, Arius, Dioscorus, Eutyches, and Severus, because they maintained a mingling of the two natures of the one Christ.
Wherefore we thought it right, to shew forth with all accuracy, in our present definition the error of such as make and venerate these, for it is the unanimous doctrine of all the holy Fathers and of the six Ecumenical Synods, that no one may imagine any kind of separation or mingling in opposition to the unsearchable, unspeakable, and incomprehensible union of the two natures in the one hypostasis or person. What avails, then, the folly of the painter, who from sinful love of gain depicts that which should not be depicted—that is, with his polluted hands he tries to fashion that which should only be believed in the heart and confessed with the mouth? He makes an image and calls it Christ. The name Christ signifies God and man. Consequently it is an image of God and man, and consequently he has in his foolish mind, in his representation of the created flesh, depicted the Godhead which cannot be represented, and thus mingled what should not be mingled. Thus he is guilty of a double blasphemy—the one in making an image of the Godhead, and the other by mingling the Godhead and manhood. Those fall into the same blasphemy who venerate the image, and the same woe rests upon both, because they err with Arius, Dioscorus, and Eutyches, and with the heresy of the Acephali. When, however, they are blamed for undertaking to depict the divine nature of Christ, which should not be depicted, they take refuge in the excuse: We represent only the flesh of Christ which we have seen and handled. But that is a Nestorian error. For it should be considered that that flesh was also the flesh of God the Word, without any separation, perfectly assumed by the divine nature and made wholly divine. How could it now be separated and represented apart? So is it with the human soul of Christ which mediates between the Godhead of the Son and the dulness of the flesh. As the human flesh is at the same time flesh of God the Word, so is the human soul also soul of God the Word, and both at the same time, the soul being deified as well as the body, and the Godhead remained undivided even in the separation of the soul from the body in his voluntary passion. For where the soul of Christ is, there is also his Godhead; and where the body of Christ is, there too is his Godhead. If then in his passion the divinity remained inseparable from these, how do the fools venture to separate the flesh from the Godhead, and represent it by itself as the image of a mere man? They fall into the abyss of impiety, since they separate the flesh from the Godhead, ascribe to it a subsistence of its own, a personality of its own, which they depict, and thus introduce a fourth person into the Trinity. Moreover, they represent as not being made divine, that which has been made divine by being assumed by the Godhead. Whoever, then, makes an image of Christ, either depicts the Godhead which cannot be depicted, and mingles it with the manhood (like the Monophysites), or he represents the body of Christ as not made divine and separate and as a person apart, like the Nestorians...
The evil custom of assigning names to the images does not come down from Christ and the Apostles and the holy Fathers; nor have these left behind them any prayer by which an image should be hallowed or made anything else than ordinary matter...
Whoever in future dares to make such a thing, or to venerate it, or set it up in a church, or in a private house, or possesses it in secret, shall, if bishop, presbyter, or deacon, be deposed; if monk or layman, be anathematised, and become liable to be tried by the secular laws as an adversary of God and an enemy of the doctrines handed down by the Fathers. At the same time we ordain that no incumbent of a church shall venture, under pretext of destroying the error in regard to images, to lay his hands on the holy vessels in order to have them altered, because they are adorned with figures. The same is provided in regard to the vestments of churches, cloths, and all that is dedicated to divine service...
(4) If anyone does not confess one Christ both God and man, etc.
(5) If anyone does not confess that the flesh of the Lord is life-giving because it is the flesh of the Word of God, etc.
(6) If anyone does not confess two natures in Christ, etc.
(7) If anyone does not confess that Christ is seated with God the Father in body and soul, and so will come to judge, and that he will remain God forever without any grossness, etc.
(8) If anyone ventures to represent the divine image (χαρακτήρ) of the Word after the Incarnation with material colours, let him be anathema!
(9) If anyone ventures to represent in human figures, by means of material colours, by reason of the incarnation, the substance or person (ousia or hypostasis) of the Word, which cannot be depicted, and does not rather confess that even after the Incarnation he [i.e., the Word] cannot be depicted, let him be anathema!
(10) If anyone ventures to represent the hypostatic union of the two natures in a picture, and calls it Christ, and thus falsely represents a union of the two natures, etc.!
(11) If anyone separates the flesh united with the person of the Word from it, and endeavours to represent it separately in a picture, etc.!
(12) If anyone separates the one Christ into two persons, and endeavours to represent Him who was born of the Virgin separately, and thus accepts only a relative (σχετική) union of the natures, etc.
(13) If anyone represents in a picture the flesh deified by its union with the Word, and thus separates it from the Godhead, etc.
(14) If anyone endeavours to represent by material colours, God the Word as a mere man, who, although bearing the form of God, yet has assumed the form of a servant in his own person, and thus endeavours to separate him from his inseparable Godhead, so that he thereby introduces a quaternity into the Holy Trinity, etc...
The holy synod cried out: Thus we all believe, we all are of the same mind. We have all with one voice and voluntarily subscribed. This is the faith of the Apostles. Many years to the Emperors! They are the light of orthodoxy! Many years to the orthodox Emperors! God preserve your Empire! You have now more firmly proclaimed the inseparability of the two natures of Christ!
To forestall possible objections:
-Remember that I am a Sola Scripturist and subject everything, even voices from the earlier church that agree largely with me, to Scripture. I can identify what is right and wrong out of those voices, praise God for what is good and true, and reject what is false. Thus I obey Jesus' commands and example from Mark 7:1-13.
-In citing all this I do not seek to establish my own position. Rather I am showing how the EO (and RC) illusion of an unbroken consensus throughout history for their pet dogmas is bogus. It is an internal critique.
-Further, I am showing how the modern EOC and RCC beg the very question they're supposed to be arguing for when they cite early church writings. When you claim to be reproducing and believing what the early church believed, or what the church throughout history believed, ANY dissenting voice from that history proves the claim to be false. The problem is not with history; the problem is with the claims made by these groups. They are so overgeneral and so sweeping that it takes only a little digging in history to disprove and embarrass such ridiculous claims as they make.
-"But that council has long been called a 'robber council'!" our EO and RC friends might object. Thus they'll simply demonstrate what I've said. These people lived in earlier times and were members of the church. They disagreed with you. Going back later and slandering them or refusing to accept their conclusions does not change the fact that these disagreements in these earlier times actually existed.