Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel.Isaiah prophesied the coming of Christ. During the times of our Old Testament friends, the Savior was promised.
Immanuel - God with us
What is the significance of this name for Christ? Not only is he the Son of God. Not only is he the Christ Child. Not only is he the Savior. He is God.
This is when they knew that God would be among them.
Only a few had known the physical presence of God - and none of their accounts were something as familiar as the human form. Isaiah told us that the virgin would have a son. And that son would be God.