How Christians explain that Isaiah 7:14 refers to a virgin?

Ver. 23. Behold a virgin,[5] &c. The Jews sometimes objected, as we see in St. Justin's dialogue with Tryphon, that the Hebrew word alma, in the prophet Isaias, signified no more than a young woman. But St. Jerome tells us that alma signifies a virgin kept close up. Let the Jews, says he, shew me any place in which the Hebrew word alma, is applied to any one that is not a virgin, and I will own my ignorance. Besides the very circumstances in the text of the prophet, are more than a sufficient confutation of this Jewish exposition; for there a sign, or miracle, is promised to Achaz; and what miracle would it be for a young woman to have a child, when she had ceased to be a virgin? (Witham) --- How happens it that nowhere in the gospels, or in any other part, do we find Christ called Emmanuel? I answer, that in the Greek expression the name is given for the thing signified; and the meaning is: He shall be a true Emmanuel, i.e., a God with us, true God and true man. (Estius) --- The text says, they shall call, i.e., all men shall look upon Him as an Emmanuel. Again, his name shall be called Wonderful, Counsellor, the mighty, the Prince of peace, &c., i.e., He shall be all these, not so much nominally, as really and in effect. (Haydock)

Haydock Bible commentary, Matthew 1 Verse 23 http://haydock1859.tripod.com/id1730.html

"Therefore the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel." Isaiah 7:14

As Azygos notes, it is not much a sign if a young women gives birth to a son. What would be the significance or the reason for the middle phrase.

However, the more decisive reason is that most Christians believe the gospels to be divinely inspired. In Matthew it is written:

So all this was done so that it might be fulfilled which was spoken by the Lord through the prophet, saying: "Behold, the virgin shall be with children and bear a son, and they shall call his name Immanuel," which is translated "God with us". (Matthew 1:22-23).

Regardless of later usage, exegesis should look at the original language and the likely original meaning of a text. In the original Hebrew, Isaiah 7:14 uses the word 'almah, which means 'young woman' and is used only in this sense in nine other references in the Old Testament. The Hebrew word for 'virgin' is betulah and is used exclusively in that sense more than fifty times in the Hebrew Bible. Ian Wilson says in Jesus, page 46, that while 'almah carries a general connotation of eligibility for marriage, this does not necessarily mean virginity. Nevertheless, nearly all English-language Christian Bibles say 'virgin' in Isaiah 7:14.

In chapter 7, in the reign of Ahaz, Jerusalem has survived a threat from Assyria and now the Aramaeans and Israelites (Ephraim) are the next threat (*). However, in 7:7-9, the Lord tells Isaiah that they will not succeed, and that Israel will be destroyed within just sixty five years (... within threescore and five years shall Ephraim be broken, that it be not a people.). He offers to provide Ahaz with a sign from God, to dissuade him from forming an alliance with Assyria, but Ahaz refuses. Isaiah presses the point, saying that a son shall be born, called Immanuel, and that he will be holy and there will be peace and prosperity. In other words, I propose that Isaiah believed that this good news would help him persuade the king that God favoured Isaiah’s advice (**).

In spite of Christian tradition, this oracle could probably be linked to the birth of a royal baby in Isaiah 9:5-6:

For a child is born to us, a son is given us; upon his shoulder dominion [kingship] rests. They name him Wonder-Counselor, God-Hero, Father-Forever, Prince of Peace. His dominion is vast and forever peaceful, From David's throne, and over his kingdom, which he confirms and sustains By judgment and justice, both now and forever. The zeal of the LORD of hosts will do this!

(*) At this stage, we must distinguish Israel (Ephraim) from Judah. They were separate nations and occasionally at war, as in this case. In later centuries, Judah glossed over these differences and adopted the name ‘Israelite’ for themselves, perhaps with a view to justifying a claim on the rich northern lands of what was later known as Samaria, in line with its chief city and capital, Samaria.

(**) I used to think that 7:14 refers to the prophet's wife and her baby (Isaiah 8:3), simply because of the proximity to 7:14, but I admit I was being lazy - a son born to a sometimes irritating prophet is not a sign that would make a king feel blessed by God . As I understood Isaiah better, I realised that the reference was to the royal baby mentioned in 9:5-6. In particular, only the expectation of a son would make the king feel blessed by God and willing to heed the words of his prophet. Isaiah 9:5-6 describes the king's son in raptuous terms, followed in the remainder of chapter 9 by news of the defeat of Israel, which had so recently threatened Judah.