Question: Why do Christians think that Isaiah 7:14 was a prophecy about the future and not something that only involved Ahaz during his lifetime? Doesn't Isaiah 7:14 only concern Isaiah and Ahaz? In v.10-11 it says plainly that the sign is to Ahaz, v.14 says the Lord himself shall give you (Ahaz) a sign. It's all addressed from Isaiah to Ahaz not one thing is addressed to the future, such as the time will come when such and such happens.
Response: The key issue here is whether the prophecy in Isaiah 7:14 was directed at Ahaz and had to be fulfilled during his lifetime or whether it was directed at the house of David and could be fulfilled long after the time of Ahaz. A plain reading of Isaiah 7 supports the view that the latter is true, that the Christian view of this prophecy is correct.
In Isaiah 7:13, the prophet Isaiah turns his attention from King Ahaz, who is a member of the house of David, and addresses the house of David in general:
Then Isaiah said, "Hear now, you house of David! Is it not enough to try the patience of men? Will you try the patience of my God also? (Isaiah 7:13, NIV)
Then, in verse 14, Isaiah speaks of a sign:
Therefore the Lord himself will give you a sign: The virgin will be with child and will give birth to a son, and will call him Immanuel. (Isaiah 7:14, NIV)
As pointed out by an article at jesusplusnothing.com/questions/Virginbirth.htm, the word you in Isaiah 7:14 is plural, adding more support to the view that the prophecy is being given to the house of David and not to just one person, such as Ahaz.
From these two verses, it is clear that Isaiah is addressing the house of David, meaning the descendants of King David, and that the sign is directed at the house of David.
As other commentators have pointed out, the house of David is being told that despite the fact that many people in the land of Israel during Isaiah's time, 2700 years ago, were concerned that the Assyrians would invade their land and destroy them, God would preserve the house of David. About 700 years after the time when Isaiah gave this prophecy, the Messiah, Jesus, was born into the house of David, in fulfillment of the prophecy in Isaiah 7:14.
More details about Isaiah 7:14 can be found here:
Did Matthew make a mistake when he translated almah as virgin?
Isaiah foreshadowed the virgin birth of Jesus
Isaiah 7:14 is About the Virgin Birth