In the fifth century, the heretic Nestorius was spreading the false doctrine that Jesus Christ was not true God and true Man.
Therefore, Our Lady was not the Mother of God. St. Cyril, Patriarch of Alexandria, attempted to convince Nestorius of his errors by a series of letters, which presented clearly and with meekness the irrefutable truths of the Faith.
When Nestorius was obdurate in his falsehoods and had no argument that could refute the Patriarch, he began to create personal lies against the saint. Several of St. Cyril's compatriots urged him to soften his position thereby allowing Nestorius to spread his malicious heresy unopposed.
St. Cyril answered: "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 his anathema, by means of his partisans, at us, at the other Bishops of the Catholic Church, and at the ancient Fathers, who in all ages and places in 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'. If our fear of some disturbance is stronger that our zeal for God's glory and thus prevents us from speaking the truth, how shall we dare in the presence of the Catholic people to celebrate the holy martyrs, whose glory lies in the very fact that by their lives they made example of the words: 'To fight for justice even unto death?'"
St. Cyril knew that the best way to spread error is to protect it and retreat from proclaiming the Truth.
Error flourishes when men do nothing to expose it. Let us ask the Holy Patriarch through the intercession of Our Lady for the fortitude to love the Truth and to proclaim it with zeal , not weakening in its defense.