Why does Isaiah say that the son born of a virgin will be called “Emmanuel” but the angel tells Mary and Joseph to name him “Jesus”?
First, let’s take a look at the passages in question. Isaiah is one of the prophets from the Old Testament. Here are his words:
Isa 7:14 Therefore the Lord himself will give you this sign: the virgin shall be with child, and bear a son, and shall name him Immanuel.
From the very beginning, the Church has considered this to be a prophesy about Christ. Now, here is where the angel tells Mary and Joseph to name this son “Jesus”:
Lk 1:31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus.
Mt 1:21 she will bear a son, and you shall call his name Jesus, for he will save his people from their sins."
Immediately after verse 21, Matthew quotes the passage from Isaiah above and applies it to Jesus. Here is where we are confronted with the question at hand. The angel says He shall be named “Jesus”, but then right after that Matthew quotes Isaiah where he says that He shall be called “Immanuel.”
Here I think is the answer: Isa 7:14 should be read in the same way that a similar passage shortly after it is read. Look at what Isaiah says in ch. 9:
Isa 9:6 For to us a child is born, to us a son is given; and the government will be upon his shoulder, and his name will be called "Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace."
Now, no one reads this and thinks that our Savior’s name should have been “Wonderful,” or “Counselor,” or “Mighty God.” Yes, He is to be called these things, as Isaiah says, but not because any one of them is to be His name. Instead, He is called these things because that’s what He is. These titles speak to the essence of the Son of Mary.
“Immanuel” is the same type of thing. Isaiah isn’t meaning to say that His name will be Immanuel. Instead, he is saying that this son truly is “God with us,” just as He is a wonderful counselor, a mighty God, an everlasting father, and a prince of peace.
The angel says that His actual name shall be “Jesus.” Why? Because He will “save His people from their sins.” And this is fitting. The name “Jesus” means “Jehovah is salvation” or “Yahweh saves.”