Peter Has Spoken Through Leo: The Council of Chalcedon

By Thomas V. Mirus ( bio - articles - email ) | Oct 19, 2026 | In Lives of the Popes

Previous installment

In the aftermath of the Robber Council of Ephesus (449), the faction of Eutyches and Dioscorus came to dominate the Eastern churches, uniting them in open opposition to Rome and to Leo’s Tome, while that document was accepted in the West. Dioscorus would even dare to excommunicate the pope himself.

We closed the last installment with the dramatic escape of the papal legate St. Hilary, who brought back to Rome the written appeal of the martyred St. Flavian. This was not the only appeal to reach Pope Leo; he also received the complaints of other bishops deposed by the council, notably Eusebius of Dorylaeum and Theodoret of Cyrus.

(To give a sense of the shifting sides in the theological controversies of these centuries: Eusebius was one of the chief opponents of Eutyches, though before that he had, together with Eutyches, been one of the chief opponents of Nestorius. Theodoret, whose account of the dialogue between Pope Liberius and Emperor Constantius was quoted at length earlier in this series, had been condemned by the Robber Council in part because of his criticism of the theology of St. Cyril of Alexandria. He would be exonerated as orthodox at Chalcedon, but then even later, after his death, his writings against Cyril would be condemned by a legitimate council, Constantinople II.)

St. Leo himself called the Robber Council “an insult to the faith, an injury to all the churches of the world,” and he wrote to Emperor Theodosius calling for a new ecumenical council in Italy to set things right. In particular, the new council was to redress the injustice against St. Flavian, who had been denied his fundamental right as a bishop to appeal to Rome against his sentence. In calling for this new council, Leo was honoring what were virtually the dying wishes of the saintly bishop of Constantinople, as well as the appeals for justice from the other deposed bishops. But Emperor Theodosius ignored Leo’s letters—after all, he was the one who had called and confirmed the Robber Council.

In the meantime, the Western Emperor, Valentinian III, moved his imperial seat from Ravenna to Rome. He was accompanied by his mother, Empress Galla Placidia, and his wife Eudoxia, who also happened to be the daughter of Theodosius. When this imperial family came to St. Peter’s Basilica to attend the liturgy for the feast of the Chair of St. Peter, the pope asked them all to write to Theodosius on his behalf, and he also wrote to Theodosius’s sister Pulcheria asking her to do the same. After these interventions, Theodosius finally replied to Leo, only to say that he was perfectly satisfied with the results at Ephesus and saw no need for another council.

But by divine providence, the situation was about to change radically. To replace St. Flavian as bishop of Constantinople, Theodosius had appointed one Anatolius. Leo wrote to the emperor saying that he would not confirm Anatolius until Anatolius publicly and in writing accepted the decrees of the First Council of Ephesus (431) and Leo’s Tome. But while this letter was on its way to the emperor, another letter was on its way back to Rome announcing that Theodosius had died in a horse-riding accident. His sister Pulcheria had succeeded him as empress, and she had married a senator named Marcian and had made him emperor.

Marcian was an orthodox Catholic, and it was he who responded to the letter Leo had intended for Theodosius. He affirmed the “principate” of the pope over all bishops, and requested that Leo’s desired council be held in the East with the pope present in person. Meanwhile, Empress Pulcheria deposed and executed Eutyches’s nephew, the court chamberlain who had been such a malign influence on Theodosius.

The change in imperial policy brought in its wake a tide of good tidings. The body of St. Flavian, who had died in exile, had been returned to Constantinople and buried alongside his predecessors. The new bishop, Anatolius, had accepted the Tome according to Leo’s wishes. The other bishops exiled by the Robber Council had returned. The bishops who had succumbed to Dioscorus’s pressure wanted to be received back into communion with Rome—and so they would be in time, with the exception of Dioscorus himself and the bishop of Jerusalem.

With such good news, St. Leo changed his mind, thinking a fresh ecumenical council was no longer necessary. But since the emperor had already sent out the summons for bishops to assemble at Nicaea, the pope approved the holding of a council, provided that it would affirm the faith of the Apostles.

St. Leo himself did not attend, but sent his legates, choosing Bishop Paschasinus of Lilybaeum (a city in Sicily) to preside over the council in the pope’s name. He sent instructions saying that his letter to St. Flavian, the Tome, had already defined the true doctrine of the Incarnation. However, he would allow the council to make its own decisions about the reinstatement and reconciliation, respectively, of the orthodox bishops returning from exile and the repentant bishops who had followed Dioscorus.

In the event, the council was not held at Nicaea but at Chalcedon, conveniently closer to the imperial capital of Constantinople. And so, on October 8, 451, a little over two years after the Robber Council of Ephesus, the Ecumenical Council of Chalcedon was opened. Over 500 bishops were present—more than at any previous ecumenical council, and far more than the roughly 130 who had participated in the Robber Council.

The conclusion of the second day of Chalcedon is justly famous. The emperor sent a message suggesting that the fathers draft a definition of the Church’s doctrine of the Incarnation. But the council deemed this unnecessary, for this time, unlike at the Robber Council, not a single bishop supported Eutyches or Dioscorus (though not all voted to condemn them). One of them said, “The pope has given us a ruling about Eutyches, we follow the pope, we have signed the letter,” meeting with general approval. Instead of writing a new creed, the council read out the creeds of previous councils along with other orthodox documents, climaxing with the Tome of Leo. And then the famous exclamation of the bishops: “This is the faith of the fathers, this is the faith of the Apostles. So we all believe, thus the orthodox believe. Anathema to him who does not thus believe. Peter has spoken thus through Leo. So taught the Apostles.”

Finally, Dioscorus was condemned by Paschasinus in the name of Pope Leo, who “through us and through this holy council, in accord with the thrice blessed apostle Peter, who is the foundation stone on which the Catholic Church is built, the foundation of the orthodox faith, has stripped him of his rank of bishop and of all his episcopal functions.” Having been degraded, Dioscorus was banished by the emperor.

There was to be further debate over whether the council should put forth a new creed. A draft formulary was presented which was designed to avoid offending either extreme, and the majority of bishops would have accepted it—except that the papal legates and the council president totally rejected any attempt to replace the definition already given as the official teaching by Leo in his Tome. Paschasinus made the ultimatum: “If you will not accept the letter of the blessed pope, Leo, make out our passports, that we may return to Italy and the General Council be held there.”

I will quote Fr. Philip Hughes on what followed, for it illustrates how much Pope Leo had the final say even in absentia:

The [imperial] commissioners proposed that a committee be chosen to revise the formulary. “No! down with Nestorians,” from the bishops. “Mary is Theotokos, Christ is God.” Then the commissioners, “Do you accept Leo’s letter?” “All of us; we have signed it.” “Then add to the formulary what Leo has written.” “No! no change in the formulary. The formulary is sufficient; it confirms the letter. Leo says what Cyril says. Celestine, Sixtus, agree with Cyril.” And the baffled officials sent for direction to the emperor.

Marcian came down on their side—and the side of the legates. If the bishops refused to have the formula amended, the council should go forward in Italy. Even now a nucleus still clung to the text as it stood. “Those who don’t like it can go back to Rome,” bishops from Illyricum cried; “they are Nestorians.” The commissioners cut to the root of the matter, asking point blank, “Do you follow Leo, or Dioscoros?” “We believe what Leo believes” was the answer. “Then add to the formula what Leo says, namely that, according to the decision of our most holy Leo, in Christ there are two natures united, inconvertible, inseparable.” The bishops agreed. The committee was chosen. A new formulary was written.

It is a lengthy statement, for, acknowledging belief in the teaching of Nicaea and the council of 381 it repeats these two creeds verbatim; it passes to the two famous letters of Cyril, and then to the Tome, which it greets with a world of compliment enshrining the statement that the letter “is in harmony with the confession of the great apostle Peter, and is for all of us a landmark against ill thinkers, a protection for orthodox teaching.” And then the formulary comes to the point, a statement of faith on the point in dispute. It is written in the terminology of Leo’s Tome.

This was the famous “Definition of Chalcedon,” and I will quote from its most important section, which reiterates the doctrine of Leo. The fathers confessed:

…one and the same Christ, Son, Lord, Only-begotten, to be acknowledged in two natures, inconfusedly, immutably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ.

The Reception of the Tome

But what about the Tome itself? Did it have definitive authority on its own, or only because a council approved it? Does the fact that some people at the time discussed and debated it prove that it was not authoritative? St. Leo clearly believed his Tome was infallible, as Erick Ybarra demonstrates thoroughly in his book on the papacy.

Writing to Theodoret of Cyrus, Leo gave his perspective on the reception of the Tome. While the Tome was in fact the definitive word on the matter, and ought not have been disputed, God had used the debate to enhance its authority in the end:

Wherefore we make our boast in the Lord, singing with the prophet: “our help is in the name of the Lord, who has made heaven and earth”: who has suffered us to sustain no harm in the person of our brethren, but has corroborated by the irrevocable assent of the whole brotherhood what He had already laid down through our ministry: to show that, what had been first formulated by the foremost See of Christendom, and then received by the judgment of the whole Christian world, had truly proceeded from Himself: that in this, too, the members may be at one with the Head.

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For lest the assent of other Sees to that which the Lord of all has appointed to take precedence of the rest might seem mere complaisance, or lest any other evil suspicion might creep in, some were found to dispute our decisions before they were finally accepted. And while some, instigated by the author of the disagreement, rush forward into a warfare of contradictions, a greater good results through his fall under the guiding hand of the Author of all goodness. For the gifts of God’s grace are sweeter to us when they are gained with mighty efforts: and uninterrupted peace is wont to seem a lesser good than one that is restored by labours. Moreover, the Truth itself shines more brightly, and is more bravely maintained when what the Faith had already taught is afterwards confirmed by further inquiry. And still further, the good name of the priestly office gains much in lustre where the authority of the highest is preserved without it being thought that the liberty of the lower ranks has been at all infringed. And the result of a discussion contributes to the greater glory of God when the debaters exert themselves with confidence in overcoming the gainsayers: that what of itself is shown wrong may not seem to be passed over in prejudicial silence.

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For that God has dispelled all calumnious fallacies, we attribute to the blessed Peter’s wondrous care of us all, for after sanctioning the judgment of his See in defining the Faith, he allowed no sinister imputation to rest on any of you, who have laboured with us for the Catholic Faith: because the Holy Spirit adjudged that no one could fail to come out conqueror of those whose Faith had now conquered. (Letter 120)

Writing to the emperor, St. Leo compared the debate over the Tome to St. Thomas’s doubting of the Lord’s Resurrection. Out of mercy, Jesus allowed Thomas to test His living reality by putting his hands in the sacred wounds, and thus the faith of all the Apostles and all believers down the ages has been strengthened. But if St. Thomas had continued to doubt after this, he would have sinned gravely.

In his book The Popes and the Church of Rome in Late Antiquity, John Moorhead describes the reputation of Leo’s Tome through the following centuries:

By far the most powerful theological statement of any pope in this period, the Tome would never be challenged in the West, where it was always viewed as a touchstone of correct teaching. Indeed, in the late sixth century people in Rome believed that a draft of Leo’s Tome had been corrected by Peter himself before being sent to Flavian. As an abbot in Gaul lay dying in the seventh century he asked to have the Tome read out, for it constituted a full summary of the Catholic faith. In the late seventh century it was thought in Constantinople that Peter had dictated it to Leo.