Someone once mentioned that a real music connoisseur is a person who can listen to the William Tell Overture and not think of the Lone Ranger. In a parallel way, a real Scripture student is someone who can listen to today's Isaiah 7 passage and not think of Jesus of Nazareth.
During this part of the liturgical year, Isaiah's words to Ahaz play in the background of our minds when we think of Jesus' birth in the same way Rossini's music dedicated to a Swiss hero runs through our minds when we see the masked hero and his faithful Indian companion riding into the sunset. Neither Rossini nor Isaiah intended their compositions to be applied to the people who usually pop into our minds when we hear them today.
In some sense, there's less excuse for us to "eisegete" Isaiah than Rossini. We need only read the verses preceding our liturgical passage to discover that Isaiah couldn't have had Jesus in mind when he originally promised, "The virgin will conceive and bear a son, and shall name him Emmanuel." This pregnancy is the 8th century B.C.E. sign Ahaz needs to convince him not to join Israel and Syria in revolting against Assyria.
The king's overriding fear revolves around his family's safety. If he refuses to join this anti-Assyrian alliance, they'll be killed. If he does join the revolt and it fails, Assyria will also slaughter his family. Isaiah assures Ahaz that his wife's pregnancy is the sign he wants. (The Hebrew word "alma" isn't the technical word for a virgin. It simply refers to a woman who has yet to give birth. We have no parallel word in English for such a woman or girl. But we do have one for cattle: heifer.) The prophet can't help but add a slam at Ahaz in his proclamation The heir to the throne won't be a chip off the old block; he'll be a much better king than his father. His reign will so emphasize Yahweh's will that it'll be like having "God among us" - Emmanuel.
How could Matthew take these words out of their original context, latch on to just one meaning of alma, and make Jesus the Emmanuel? No problem. No one in the first century C.E. followed today's rules of exegesis. Taking sentences (and even words) out of context was not only permitted, it was expected. (Back then biblical scholars presumed that if two Scripture verses have the same number of syllables, they must be saying the same thing!).
Matthew is simply following the early Christian practice of finding Jesus tucked away in the words of the Hebrew prophets, turning these "consciences of their people" into predictors of what Christians experienced in their relationship with the risen Jesus. Christians eventually would even go so far as to change the order of the books of the Hebrew Scriptures, placing the prophetic writings immediately before the gospels to emphasize their "prediction-fulfillment" theology.
Though we today dismiss Matthew's "fulfillment" passages as Christian eisegesis, no one can dismiss the effect the risen Jesus had in the lives of his first followers. Not only did it lead them to reinterpret their Scriptures, more importantly, it led them to reinterpret their daily lives.
Paul, for instance, reminds the Romans that his experience of Jesus
has forced him to change his basic direction in life. No longer is he
content just to follow the 613 laws of Moses, he's now received "the
grace of apostleship to bring about the obedience of faith, for the sake
of his name, among all the Gentiles, among whom are you also, who are
called to belong to Jesus Christ." Quite a change in lifestyle.
Perhaps if we spent less time trying to surface predictions of Jesus in the Hebrew Scriptures and more time surfacing the risen Jesus in our everyday lives, it would be easier to understand our sacred authors' frame of mind when they composed our Christian Scriptures.