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St. Cyril on the Perpetual Virginity

But in answer to those who argue that, if He were brought forth in the flesh, the Virgin was corrupted: and if she were not corrupted, that He was brought forth only in appearance, we say; the prophet declares, "the Lord, the God of Israel, hath entered in and gone out, and the gate remaineth closed." If, moreover, the Word was made flesh without sexual intercourse, being conceived altogether without seed, then was He born without injury to her virginity.

 
In what sense then her firstborn? By firstborn she here means, not the first among several brethren, but one who was both her first and only son: for some such sense as this exists among the significations of "firstborn." For sometimes also the Scripture calls that the first which is the only one; as "I am God, the First, and with Me there is no other." To shew then that the Virgin did not bring forth a mere man, there is added the word firstborn; for as she continued to be a virgin, she had no other son but Him Who is of the Father.
 
COMMENTARY OF ST. CYRIL, PATRIARCH OF ALEXANDRIA, UPON THE GOSPEL OF ST. LUKE.(1859) BOOK I, PAGE 9