In 451, during the reign of the Sovereigns Marcian and Pulcheria, the Fourth Ecumenical Council was convoked in Chalcedon against Eutyches and those of like mind with him. After much debate, the Fathers who were the defenders of Orthodoxy, being 630 in number, agreed among themselves and with those who were of contrary mind, to write their respective definitions of faith in separate books, and to ask God to confirm the truth in this matter. When they had prepared these texts, they placed the two tomes in the case that held Saint Euphemia's relics, sealed it, and departed. After three days of night-long supplications, they opened the reliquary in the presence of the Emperor, and found the tome of the heretics under the feet of the Martyr, and that of the Orthodox in her right hand.
She also appeared to St Paisios (+ 1994, feast day 12th July) in his keli when he was having trouble advising bishops on a certain matter, once again illuminating the right path.
When hearing the Orthodox say the 4th Ecumenical Council was a misunderstanding and that we have the same faith as the Monophysites, he said, "They don’t say that the Monophysites didn’t understand the holy fathers—they say that the holy fathers didn’t understand them. In other words, they talk as if they’re right, and the fathers misunderstood them." He considered proposals to erase from the liturgical books statements identifying Dioscorus and Severus as heretics to be a blasphemy against the holy fathers. He said, "So many divinely enlightened holy fathers who were there at the time didn’t understand them, took them the wrong way, and now we come along after so many centuries to correct the holy fathers? And they don’t take the miracle of Saint Euphemia into account? Did she misunderstand the heretics’ tome too?"
Apolytikion of Great Martyr Euphemia in the Third Tone
O Euphemia, Christ's comely virgin, thou didst fill the Orthodox with gladness and didst cover with shame all the heretics; for at the holy Fourth Council in Chalcedon, thou didst confirm what the Fathers decreed aright. O all-glorious Great Martyr, do thou entreat Christ God that His great mercy may be granted unto us.
Kontakion of Great Martyr Euphemia in the Third Tone
You made fervent effort in your struggles of trial, in your struggles of faith for Christ your bridegroom. But even now, intercede with the Theotokos that the heresies and insolence of enemies be trod beneath the feet of our rulers, O All Praised, who received and kept the Decree of the 630 God-bearing Fathers.
Let us not undermine what an Ecumenical Council is and what authority it holds, and let us not undermine the Divinely illumined Holy Fathers, who were guided by the Holy Spirit, or the miracle of St Euphemia, or the wisdom of St Paisios. Let there be no doubt - even if these heretics hold the same faith as us today (which they don't by virtue of rejecting most of our Ecumenical Councils), they did not back then. Besides this, they also reject the saints we canonised, and won't let go of the heretics they canonised. The only thing stopping our unity is their stubbornness and refusal to accept they were and have been in error this whole time.
Holy 630 Fathers of the Fourth Ecumenical Council, Saint Euphemia, St Paisios, pray for us!