“Thus was fulfilled the mystery ordained before all ages and announced of old by the prophets: “Behold, the virgin shall conceive in her womb and bear a son and shall call his name Emmanuel, which translated means: God-with-us” (Is 7:14).
This ancient name also contains the revelation of the whole mystery, namely, that God will be with men. For it says: Emmanuel, which means God-with-us.
No one, then, ought to let himself be led astray by the calumnies of the Jews, who assert that the prophet was speaking of a young woman and not of a virgin. Then the text would say: “Behold the young woman shall conceive in her womb.” In any case, it is the most absurd thing thing imaginable to think that what was given to us by the Lord as a sign should be something so common and familiar to nature.
What does the prophot really say? “And the Lord went on to speak with Ahaz thus: ‘Ask for yourself a sign from the Lord, in the depths or in the heights: And Ahaz said. ” I will not ask and I will not tempt the Lord’ (Is 7:10-12) And a little later it continues: “Therefore the Lord himself will give you a sign. Behold the virgin shall conceive in her womb.” (Is 7:14).
Then, since Ahaz does not ask for a sign, neither in the depths nor from the heights, so that you might understand that he who descended into the lowest regions of the earth is the same who rose above all the heavens (Eph 4:9-10), the Lord himself gave a sign: a sign truly extraordinary and portentous, far superior to the common law of nature: the same woman is both virgin and mother; and though remaining in the holy condition of virginity, she also obtains the blessing of childbearing.”Basil of Caesarea , On the Generation of Christ 4; PG 31, 1465 B-1468 A.
Gambero, L., 1999. Mary and the Fathers of the Church 1st ed. Translated by Buffer, T., San Francisco: Ignatius Press. p. 143-144.