That this should be the sign of His nativity, that He should be born of a virgin— man and God— a son of man and a Son of God
In Isaiah: “And the Lord went on to speak to Ahaz, saying, Ask you a sign from the Lord your God, in the height above and in the depth below. And Ahaz said, I will not ask, and I will not tempt the Lord my God. And He said, Hear, therefore, O house of David: it is no trifling contest unto you with men, since God supplies the struggle. On this account God Himself will give you a sign. Behold, a virgin shall conceive, and shall bear a son, and you shall call His name Emmanuel. Butter and honey shall He eat; before that He knows to prefer the evil, He shall exchange the good.” This seed God had foretold would proceed from the woman that should trample on the head of the devil. In Genesis: “Then God said unto the serpent, Because you have done this, cursed are you from every kind of the beasts of the earth. Upon your breast and your belly shall you crawl, and earth shall be your food all the days of your life. And I will place enmity between you and the woman and her seed. He shall regard your head, and you shall watch his heel.”
Source: The Treatises of Cyprian (New Advent)