My Scripture students always know what the first question will be on every exam I give: What's the difference between exegesis and eisegesis? I figure if they can't correctly make the distinction between the two, there's no way they can pass a course in the Bible.
The answer isn't complicated. Exegesis takes place when we take out of the text what the original author intended us to take out of it. We work at understanding the background against which the author writes and the questions he or she is trying to answer. Eisegesis, on the other hand, happens when, ignoring the intentions of the sacred author, we put our own preconceived ideas and thoughts into a biblical text and then excitedly proclaim, "Look what I found!"
No biblical text is more eisegeted by Christians than today's Isaiah passage.
As we hear in our gospel pericope, followers of Jesus eventually took Isaiah's words to Ahaz and gave them a meaning neither the prophet nor the king would have originally understood. Matthew explains Jesus' unique conception by simply stating, "All this took place to fulfill what the Lord had said through the prophet: 'Behold, the virgin shall conceive and bear a son, and they shall name him Emmanuel, when means God is with us.'" A classic case of eisegesis.
First, the Hebrew word "almah" - which we Christians like to translate as "virgin" - describes not only a woman who has yet to experience sexual intimacy, it more frequently is employed to characterize a woman of childbearing age who has not yet given birth to a child. Scripture scholars agree the latter is meant here.
Second, in context, Ahaz is being forced to make a decision which will affect not only him but his immediate family. They probably will be massacred if he chooses the wrong option. It's a decision he should have made yesterday. Why would anyone think he has over 700 years to wait for a sign?
Third, the almah here could only be Mrs. Ahaz. Isaiah interprets her pregnancy as a sign that Yahweh's not going to permit the king's family to be wiped out.
Fourth, the son to be born, Hezekiah, will eventually turn out to be a far better king than his father ever was. Having him on the throne was like having El (God) with us.
Putting Jesus and Mary into Isaiah 7 would be continuing the eisegesis which Matthew began.
Our faith in Jesus as God and our belief in the circumstances of his virginal conception developed only after his death and resurrection. If we overlook the angelic annunciations to Mary in Luke and Joseph in Matthew, no one could have imagined Jesus' divine prerogatives until, as Paul tells the Romans, "(God) established Jesus as Son of God in power according to the Spirit of holiness through the resurrection from the dead." Once we buy into what the late Raymond Brown presumed, that biblical annunciations are almost always literary devices employed by the authors to let their readers in on the deeper meaning of the events they're narrating, then we're forced to admit that those closest to the historical Jesus probably had a far more difficult time recognizing who he actually was than we post-resurrection disciples have today.
Perhaps we should give ourselves not only over to correctly exegeting Scripture, but also to correctly exegeting the world around us. If we don't surface the risen Jesus in all we do and experience, we're not taking out of this world what God originally put into it. That's a sin of eisegesis for which we'll have to answer at the exam we'll have to take at the pearly gates.