III. From the union of the two natures flows the grace of baptism. He makes a direct appeal to Pulcheria for her help.
But from Christ's marvellous sharing of the two natures, the mystery of regeneration shone upon us that through the self-same spirit, through whom Christ was conceived and born, we too, who were born through the desire of the flesh, might be born again from a spiritual source: and consequently, the Evangelist speaks of believers as those “who were born not of bloods, nor of the will of the flesh, nor of the will of man, but of God.” And of this unutterable grace no one is a partaker, nor can be reckoned among the adopted sons of God, who excludes from his faith that which is the chief means of our salvation.
Wherefore, I am much vexed and saddened that this man, who seemed before so laudably disposed towards humility, dares to make these empty and stupid attacks on the one Faith of ourselves and of our fathers. When he saw that his ignorant notion offended the ears of Catholics, he ought to have withdrawn from his opinion, and not to have so disturbed the Church's rulers, as to deserve a sentence of condemnation: which, of course, no one will be able to remit, if he is determined to abide by his notion.
For the moderation of the Apostolic See uses its leniency in such a way as to deal severely with the contumacious, while desiring to offer pardon to those who accept correction. Seeing then that I possess great confidence in your lofty faith and piety, I entreat your illustrious clemency, that, as the preaching of the Catholic Faith has always been aided by your holy zeal, so now, also, you will maintain its free action. Perchance the Lord allowed it to be thus assailed for this reason that we might discover what sort of persons lurked within the Church. And clearly, we must not neglect to look after such, lest we be afflicted with their actual loss.
Source: Letters (New Advent)