“Upheld principally by the powers of this world, Arianism fell at the end of the century, having no root but its recently converted nations who had not had to shed their blood for the Divinity of the Son of God.
It was then that Satan produced Nestorius, crowned with a fictitious halo of sanctity and knowledge. This man, who was to give the clearest expression to the hatred of the serpent for the Woman, was enthroned in the See of Constantinople amid the applause of the whole East, which hoped to see in him a second St. John Chrysostom. The joy of the good was of short duration. In the very year of his exaltation, on Christmas Day 428, Nestorius, taking advantage of the immense concourse which had assembled in honor of the Virgin Mother and Her Child, pronounced from the episcopal pulpit the blasphemous words: “Mary did not bring forth God; Her Son was only a man, the instrument of the Divinity.” The multitude shuddered with horror. Eusebius, a simple layman, rose to give expression to the general indignation, and protested against this impiety. Soon a more explicit protest was drawn up and disseminated in the name of the members of this grief-stricken Church, launching an anathema against anyone who should dare to say: “The Only-begotten Son of the Father and the Son of Mary are different Persons.” This generous attitude was the safeguard of Byzantium, and won the praise of Popes and Councils.
When the shepherd becomes a wolf, the first duty of the flock is to defend itself. It is usual and regular, no doubt, for doctrine to descend from the Bishops to the faithful, and those who are subject in the Faith are not to judge their superiors. But in the treasure of Revelation there are essential doctrines which all Christians, by the very fact of their title as such, are bound to know and defend. The principle is the same whether it be a question of belief or conduct, dogma or morals. Treachery like that of Nestorius is rare in the Church, but it may happen that some pastors keep silence for one reason or another in circumstances when religion itself is at stake. The true children of Holy Church at such times are those who walk by the light of their Baptism, not the cowardly souls who, under the specious pretext of submission to the powers that be, delay their opposition to the enemy in the hope of receiving instructions which are neither necessary nor desirable. …
As it always happens, there were pacifists who, though not sharing Nestorius’ errors, thought it would be best not to answer him for fear of embittering him, increasing the scandal, and wounding charity. St. Cyril thus answers that false virtue, which fears the affirmations of the Catholic Faith more than the audacity of heresy: “What! Nestorius dares to suffer men to say in public and in his presence that he who calls Mary the Mother of God is to be anathema! He hurls anathema, through his partisans, at us, at the other Bishops of the Universal Church and the ancient Fathers, who in all ages and all places with one accord have acknowledged and honored the Holy Mother of God! And have we not the right to repay him in his own coin and say, If anyone denies that Mary is the Mother of God, let him be anathema? Nevertheless, out of regard for him, I have not yet uttered these words.”
Men of this type, also represented in all ages, revealed the true motive of their hesitation when, after insisting on the advantages of peace and their ancient friendship with Nestorius, they suggested timidly that it would be dangerous to oppose so powerful an adversary. “Could I but satisfy the Bishop of Constantinople and heal the wounded spirit of my brother by suffering the loss of all my possessions!” was St. Cyril’s reply. “But the Faith is at stake. The scandal has spread through the Church, and all men are inquiring about the new doctrine. If we, who have received from God the office of teacher, fail to remedy such great evils, will there be flames enough for us at the Day of Judgment? I have already been struck by insult and calumny——let it pass. If only the Faith be safe, I will yield to none in my love of Nestorius. But if the Faith suffers through the deeds of some——let there be no doubt about it——I will not risk my soul even if instant death threaten me. If the fear of some disturbance is stronger than our zeal for God’s glory and prevents us from speaking the truth, how shall we dare in the presence of the Christian people to celebrate the Holy Martyrs, whose glory lies in the very fact that they carried out in their lives the words: Even unto death fight for justice (Eccl. 4: 33)?”
— Dom Prosper Guéranger, O.S.B., The Liturgical Year, IV (pp. 379-380).