I don't believe you can prove that.
Actually, nothing much in the bible can be proven either way if we look at it from the outside, because it doesn't provide much terms of historical verifiability.
So I don't you can prove that the sign was about the messiah, just because of the author (Matthew?) say so in the gospel, because the gospel totally ignore all other signs in Isaiah 7 and 8 (not just one verse Matthew had quoted and misrepresented), which indicated the sign is related to the war in Isaiah's time.
The only thing that confirm we can confirm about Isaiah 7 (and 8, as well as 2 Kings 15 & 16) is that there are Assyrian sources do show that Ahaz and Judah was at war with Israel and Aram, and that the fortune of the war favored Judah only because the King of Assyria did attack Aram and Israel. This Assyrian king is named Tiglath-Pileser III (reign 745–727 BCE), so that much of biblical history we can confirm to be true.
The verses that followed Isaiah 7:14 - verses 15, 16 & 18 (as well as Isaiah 8:1-4) indicated that while the sign is about how the war will end in Ahaz's favour, it does not say anything about the child being "the Messiah". The sign clear indicate that these event about the war will happen when the child reach a certain age, therefore not Jesus. And nothing in Isaiah 7:14-17 say the child will be king, messiah or saviour, let alone being the son of God.
The child in 7:14-17 and 8:3-4 referred to the same war against Israel and Aram, and the same king of Assyria being involved, so Immanuel of Isaiah 7 must be the same child being born to Isaiah and the prophetess: Maher-shalal-hash-baz
Isaiah 8:18 clearly also state that Isaiah and his children (sons) were the potent of Judah's survival:
Isaiah 8:18 said:
18 See, I and the children whom the Lord has given me are signs and portents in Israel from the Lord of hosts, who dwells on Mount Zion.
I have already stated it all of these before, in my previous reply, and nothing you have stated have refute anything that I have already given.
All you have just Matthew's say-so (interpretation) to JUST ONE VERSE, all of which, based on the Greek translation to the word - parthenos "virgin", and not the original Hebrew word
almah "young woman" (or more precisely
ha'almah, "the young woman".
No where in that one verse indicated that the child will be king, priest, prophet, messiah, son of god, etc. No where in the verse (Isaiah 7:14) stated that the child will be "anointed"?
Is that what the messiah mean? The "Anointed One"?
Lastly, 7:14 stated in Hebrew transliteration:
Isaiah7:14 said:
hinneh ha‘almah harah veyoledet ben; veqara’t shemo ‘immanu ’el
The word harah or
הָרָה֙ means "pregnant" or "with a child".
Isaiah 7:14 is similar to when the angel or God informed Hagar was pregnant with Ishmael, in Genesis 16:11:
Genesis16:11 said:
hinach harah veyoladet ben vekarat shemo Yishma'el
Genesis16:11 said:
Behold, thou art with child and shalt bear a son, and shalt call his name Ishmael...
Genesis16:11 said:
“Now you have conceived and shall bear a son; you shall call him Ishmael...
Although, there are no almah in Genesis 16:11, it does translate
harah to
"with child" in KJV or
"have conceived" in NRSV, like that of Isaiah 7:14, as in the
"pregnant young woman" or
"young woman with child".
And if we look at the recent translation of Isaiah 7:14, from the Dead Sea Scrolls, we get:
Isaiah 7:14 said:
14 Therefore the Lord 149 himself 150 will give y[ ou a sign. Loo] k, the young woman has conceived and is bearing a son, and his name will be 151 Immanuel.
Furthermore, if we look at the transliteration to Isaiah 8:3 again, we seen the word
harah repeated again, and KJV used the words "harah" as being translated to "conceived", which mean the prophetess was already pregnant:
Isaiah 8:3 said:
3 And I went unto the prophetess; and she conceived, and bare a son. Then said the Lord to me, Call his name Mahershalalhashbaz.
Why did the KJV translated the word harah to
"shall conceive" in Isaiah 7:14, but translated as
"conceived" in Isaiah 8:14 or
"have conceived" in Genesis 16:11?
This showed the KJV's inconsistencies in translating the word "harah". There 2 other KJV translations to harah, and they have been - "with child":
Exodus21:22 said:
...hurt a woman with child...
Jeremiah 31:8 said:
...the woman with child...
Those are (textual) evidences that I have managed to find.
Even Jews who read the same book as Christians do, don't see Isaiah 7:14 relating to anything about the messiah, let alone Jesus.
You have nothing more than Matthew's say-so, and that's not really worth much, when anyone who read Isaiah 7 & 8 in their entirety can see it doesn't relate to the messiah, let alone b