St. Leo the Great, celebrated, Feb 18, is also known for his Tome of Leo, a statement of faith which he sent to the 4th Ecumenical Synod in Chalcedon in AD 451. Here is an interesting historical anecdote concerning the Tome:

From St. Demetrius of Rostov, “The Eighteenth Day of the Month of February” , The Great Collection of the Lives of the Saints, Volume VI, February, Chrysostom Press, House Springs, MO, 2003, p. 206-207.

At that time, following the condemnation of the Nestorian heresy, two shameless blasphemers appeared: Eutyches, and archimandrite living in Constantinople, and Dioscorus [the contrivers of the Monophysite heresy].
These miscreants commingled the divinity and humanity of Christ our Lord into a single nature and will, greatly troubling the Church of God. At the Robber Council of Ephesus [AD 449] they unjustly condemned Saint Flavian, the champion of Orthodoxy and Patriarch of the Imperial City, so mistreating him that he died.

Learning that the heretics were persecuting the true believers, Saint Leo made every effort to restore peace to the Church. He wrote first to the Emperor Theodosius, then to his successor Marcian, requesting that an ecumenical synod be convened. Marcian and his consort Pulcheria agreed and convoked the Fourth Ecumenical Council at Calcedon. Six hundred and thirty holy fathers assembled to condemn Eutyches and Dioscorus. His All-holiness Pope Leo could not attend due to age, distance, and the incursions of barbarians, but he sent legates: the hierarchs Paschasius and Luncentius, and the presbyters Boniface and Basil.

In the course of the debates with the heretics, doubts concerning the truth arose in the hearts of many; whereupon, the holy fathers commanded that the Tome of Leo be read. This letter originally was sent by the Pope to Saint Flavian, the martyred Patriarch of Constantinople, when the latter convened a synod [AD 449] in the eastern capital to anathematize the misbelievers. It is said that the holy chief Apostle Peter himself edited the document. Thus, we read in The Spiritual Meadow [of John Moschos, Chapter 147], written by Saint Sophronius of Jerusalem:

“Abba Menas, superior of Salams, a coenobium near Alexandria, related that he heard this from Eulogius, Patriarch of Alexandria: ‘While staying in Constantinople, I was a guest in the house of my lord Gregory, archdeacon of the Church of Rome, [Saint Gregory the Dialogist, later Pope of Rome] a truly illustrious and virtuous. He told me a story recorded in the archives of the Roman Church about the most blessed and Most Holy Pope Leo. He said that Leo wrote a letter to Saint Flavian, Bishop of Constantinople, condemning the impious Euthyches and Nestorius, and put it on the tomb of Peter, the chief Apostle, “if I, as a man, have in this letter erred in any way or failed to explain the truth fully, do thou, to whom this Church and episcopal throne were entrusted, set it right.” Forty days later the Apostle appeared while Leo was praying. He said, “I have read your letter and corrected it.” The Pope took the epistle from the blessed Peter’s tomb, opened it, and found that it had been amended by the Apostle’s hand.’”

During the reading of the Pope’s Tome at the Fourth Ecumenical Synod, all the holy fathers exclaimed, “Peter the Apostle has spoken through Leo!” The council was guided in its deliberations by the Pope’s epistle, and the misbelievers were put to shame. This letter, which shut the mouths of the heretics, was held in the highest esteem by the fathers, both at the synod and later, and particularly by the aforementioned Eulogius, Patriarch of Alexandria [between AD 580-608] [and?] during the reign of Heraclius [AD 610-641]. By staunchly defending the Tome against the miscreants who rejected and reviled it, the blessed Eulogius won the favor of the divine Leo. As proof of this we cite the following account related by Saint Sophronius in The Spiritual Meadow: [by John Moschos, Chapter 148]

“While I was synkellos [advisor/counselor to a bishop or patriarch] of Eulogius, the holy Patriarch of Alexandria, I saw in a dream a radiant man of venerable appearance. He commanded, ‘Announce me to Patriarch Eulogius.” ‘Who are you, my lord?’ I asked. He replied, ‘I am Leo, Pope of Rome.’ I told Saint Eulogius, ‘The most blessed and Most Holy Leo, Primate of the Roman Church, wishes to pay his respects.’ Patriarch Eulogius rushed to meet the saint. The two men prayed, then exchanged the kiss of peace and sat. The divine Leo asked Eulogius, ‘Do you know why I am here?’ ‘No,’ answered the holy Patriarch. ‘I have come to thank you,’ said Leo, ‘because you have stoutly defended the letter I wrote my brother, Patriarch Flavian of Constantinople, refuting the impious Nestorian and Eutychian heresies. You have understood my teaching well and proclaimed it fearlessly, thereby silencing the misbelievers. Know, brother, that you have gratified not me alone by your godly zeal and labor, but the chief Apostle Peter as well, for he read my epistle and corrected it. Above all you have pleased Christ our God, Who is Truth Itself, preached by us.’ I had the dream not once or twice, but three times. Convinced by this, I related it to Saint Eulogius. He wept, stretched out his hands to heaven, and said, ‘I thank Thee, O Master Christ our God, that Thou hast vouchsafed me, the unworthy one, to proclaim Thy truth. In Thy great and ineffable compassion and by the intercessions of Thy servants Peter and Leo, Thou has deigned to accept my feeble efforts as Thou didst the widow’s mites.’”