Why do Christians believe Isaiah 7:14 was a prophecy of Jesus's birth in Matthew 1:23?
Isa 7:1 And it came to pass in the days of Ahaz the son of Jotham, the son of Uzziah, king of Judah, that Rezin the king of Syria, and Pekah the son of Remaliah, king of Israel, went up toward Jerusalem to war against it, but could not prevail against it.
Isa 7:2 And it was told the house of David, saying, Syria is confederate with Ephraim. And his heart was moved, and the heart of his people, as the trees of the wood are moved with the wind.
Isa 7:3 Then said the LORD unto Isaiah, Go forth now to meet Ahaz, thou, and Shearjashub thy son, at the end of the conduit of the upper pool in the highway of the fuller's field;
Isa 7:4 And say unto him, Take heed, and be quiet; fear not, neither be fainthearted for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah.
Isa 7:5 Because Syria, Ephraim, and the son of Remaliah, have taken evil counsel against thee, saying,
Isa 7:6 Let us go up against Judah, and vex it, and let us make a breach therein for us, and set a king in the midst of it, even the son of Tabeal:
Isa 7:7 Thus saith the Lord GOD, It shall not stand, neither shall it come to pass.
Isa 7:8 For the head of Syria is Damascus, and the head of Damascus is Rezin; and within threescore and five years shall Ephraim be broken, that it be not a people.
Isa 7:9 And the head of Ephraim is Samaria, and the head of Samaria is Remaliah's son. If ye will not believe, surely ye shall not be established.
Isa 7:10 Moreover the LORD spake again unto Ahaz, saying,
Isa 7:11 Ask thee a sign of the LORD thy God; ask it either in the depth, or in the height above.
Isa 7:12 But Ahaz said, I will not ask, neither will I tempt the LORD.
Isa 7:13 And he said, Hear ye now, O house of David; Is it a small thing for you to weary men, but will ye weary my God also?
Isa 7:14 Therefore the Lord himself shall give you a sign; Behold a virgin shall be with child and she shall conceive, and bear a son, and shall call his name Immanuel