Therefore let those who deny that the Son is from the Father by nature and proper to His Essence, deny also that He took true human flesh of Mary Ever-Virgin; for in neither case had it been of profit to us men, whether the Word were not true and naturally Son of God, or the flesh not true which He assumed.East: Bishop St. Epiphanius of Salamis says in 374 [The Well-Anchored Man 120], "The Son of God...was born perfectly of the holy ever-virgin Mary by the Holy Spirit."
East: Bishop St. Basil the Great of Caesarea (Doctor & Holy Hierarch) says before 379 [Homily On the Holy Birth of Christ 5], "The friends of Christ do not tolerate hearing that the Mother of God [Θεοτόκος] ever ceased to be a Virgin [παρθένος]."
West: Pope St. Siricius of Rome says in 393 [Letter to Bishop Anysius],
You had good reason to be horrified at the thought that another birth might issue from the same virginal womb from which Christ was born according to the flesh. For the Lord Jesus would never have chosen to be born of a virgin if He had ever judged that she would be so incontinent as to contaminate with the seed of human intercourse the birthplace of the Lord's body, that court of the eternal King.West: Bishop St. Ambrose the Great of Milan (Doctor) says in 396 [Letter 63:111 to the Christians at Vercellæ in PL 16:1218D], "Imitate her, holy mothers, who in her only dearly beloved Son set forth so great an example of maternal virtue; for neither have you sweeter children, nor did the Virgin seek the consolation of being able to bear another son."
West: Bishop St. Augustine the Great of Hippo (Doctor of Grace) says in 401 [On Holy Virginity 4:4],
Thus Christ by being born of a virgin, who, before she knew Who was to be born of her, had determined to continue a virgin, chose rather to approve, than to command, holy virginity. And thus, even in the female herself, in whom He took the form of a servant, He willed that virginity should be free.East: Patriarch St. Cyril of Alexandria (Doctor of the Incarnation) says after 431 [Against Those Who Do Not Wish to Confess That the Holy Virgin is the Mother of God 4], "The Word Himself, coming into the Blessed Virgin herself, assumed for Himself His own temple from the substance of the Virgin and came forth from her a man in all that could be externally discerned, while interiorly He was true God. Therefore He kept His Mother a virgin even after her childbearing."
West: Pope St. Leo I the Great of Rome (Doctor) says before 461 [Sermon 22:2 On the Feast of the Nativity in PL 54:195AB],
And by a new nativity He was begotten, conceived by a Virgin, born of a Virgin, without paternal desire, without injury to the mother's chastity: because such a birth as knew no taint of human flesh, became One who was to be the Savior of men, while it possessed in itself the nature of human substance. For when God was born in the flesh, God Himself was the Father, as the archangel witnessed to the Blessed Virgin Mary: "because the Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee: and therefore, that which shall be born of thee shall be called holy, the Son of God" [Lk 1:35]. The origin is different but the nature like: not by intercourse with man but by the power of God was it brought about: for a Virgin conceived, a Virgin bare, and a Virgin she remained.WRH: The great pontiff not only teaches the perpetual virginity of the Theotókos, but the fact of two nativities in Christ. There is one eternal nativity whereby Christ is ever-begotten of the Father, while there is one temporal nativity from the Blessed Virgin by the power of the Holy Spirit.
East: Hieromonk St. John of Damascus (Doctor) says in 743 [An Exact Exposition of the Orthodox Faith 4:14 in PG 94:1161BC],
The ever-virgin One thus remains even after the birth still virgin, having never at any time up till death consorted with a man. For although it is written, "And knew her not till she had brought forth her first-born Son," yet note that He who is first-begotten is first-born even if He is only-begotten. For the word "first-born" means that He was born first but does not at all suggest the birth of others. And the word "till" signifies the limit of the appointed time but does not exclude the time thereafter. For the Lord says, "And lo, I am with you always, even unto the end of the world," not meaning thereby that He will be separated from us after the completion of the age. The divine apostle, indeed, says, "And so shall we ever be with the Lord," meaning after the general resurrection.WRH: Here the great Doctor corrects the fallacy of previous heretics and future Protestants that the diction of St. Matthew means that St. Mary had conjugal relations with St. Joseph after the birth of her Divine Son.