“If she had not been a virgin, there would have been no sign, since a sign has to be something out of the ordinary and beyond the laws of nature, something new and unexpected, something that makes an impression on those who see it and hear of it.
That is why it is called a sign, because it stands out. This would not be the case if it could be confused with other common events; so that, if the discourse refers to a woman who gives birth according to the law of nature, why would this be called a sign, since it is something that happens every day?
Therefore, in beginning his speech, he did not say simply: Behold, a virgin, but: Behold, the Virgin. By adding the article, he indicates a unique virgin, distinct from all the others.”
John Chrysostom, Commentary on Isaiah 7, s; PG 56, 84.
If you are interested in more quotes on Virgin Mary, you can find them in Mary and the Fathers of the Church on Amazon in both in Kindle and paperback formats.
References:
Gambero, L., 1999. Mary and the Fathers of the Church 1st ed. Translated by Buffer, T., San Francisco: Ignatius Press.