The Common English Bible renders Isaiah 7:10ff thusly:
10 Again the LORD spoke to Ahaz: 11 “ Ask a sign from the LORD your God. Make it as deep as the graveq or as high as heaven. ” 12 But Ahaz said, “ I won’t ask; I won’t test the LORD . ” 13 Then Isaiah said, “ Listen, house of David! Isn’t it enough for you to be tiresome for people that you are also tiresome before my God? 14 Therefore, the Lord will give you a sign. The young woman is pregnant and is about to give birth to a son, and she will name him Immanuel. 15 He will eat butter and honey, and learn to reject evil and choose good. 16 Before the boy learns to reject evil and choose good, the land of the two kings you dread will be abandoned. 17 The LORD will bring upon you, upon your people, and upon your families days unlike any that have come since the day Ephraim broke away from Judah—the king of Assyria.”
Translations which have ‘virgin’ in verse 14 mistakenly render the Hebrew word ‘Almah’ which simply means ‘young girl’. If the author had wished to say ‘virgin’ he would have used ‘Bethulah’ – the word for virgin in Hebrew. It is at precisely this point that we need to recall the historical context of Isaiah’s theological declaration: An invading army is approaching and God promises to deliver the city if the people will turn to him in faith. Ahaz isn’t sure it will happen so God tells him that the sign of his promise’s truth will be that this pregnant young woman will give birth to her son and before that son is weaned, deliverance will have dawned.
Christian readers who claim to take Scripture seriously and who yet ignore the very vocabulary of the passage and import into it a prediction of the birth of Christ violate it violently.
But does that mean Matthew has it wrong? Indeed not. Why? Because he has taken from this passage its ‘theology of deliverance from the enemy as promised by God’ and applied it to the birth of Jesus, whom he thus portrays as the ‘promise of deliverance’ par excellance.
Here again then we see Matthew the theologian doing the work of a theologian and reading his source theologically. Matthew is preaching and his text is Isaiah. Matthew isn’t doing history and neither is Isaiah (not primarily anyway). And the proof? Jesus wasn’t named Immanuel. He was named Yeshua.
And anyway, if Isaiah had wanted to say ‘virgin’ he could have. He just didn’t. An ‘almah’ is not necessarily a ‘bethulah’.
Are you saying that Matthew knew that ‘almah didn’t mean virgin and was consciously changing the meaning to make his theological point?
he didn’t change the meaning. he adopted the LXX’s reading because it fit his purpose in both respects. i.e., to show jesus as deliverance and to assert the uniqueness of his existence.