First Sunday of Advent, 2013
Therefore the Lord himself will give you a sign. Behold, a virgin shall conceive and bear a son, and shall call his name Immanuel. (Isaiah 7:14)

“Behold, a virgin shall conceive and bear a son,and his name shall be called Emmanuel.”(which means, God with us). (Matthew 1:23)

But who shall say that Christ and Christians have no connection with Israel, seeing that Israel was the grandson of Abraham, to whom first, as afterwards to his son Isaac, and then to his grandson Israel himself, that promise was given, which I have already mentioned, namely: “In thy seed shall all nations be blessed”? That prediction we see now in its fulfillment in Christ. For it was of this line that the Virgin was born, concerning whom a prophet of the people of Israel and of the God of Israel sang in these terms: “Behold, a virgin shall conceive, and bear a son; and they shall call His name Emmanuel.” For by interpretation, Emmanuel means, “God with us.”    — St. Augustine, Homilies on the Gospels

My comments:
First, the prophecy of Isaiah takes place about 750 years before the birth of Jesus. Second, the important point that St. Augustine makes is that the birth of Jesus is the fulfillment of the Covenant The Lord made with Abraham, a Covenant that took 1,900 years to be fulfilled: “and by your descendants shall all the nations of the earth bless themselves, because you have obeyed my voice.” (Genesis 22:18)