In the book of Isaiah it is written: Behold, the virgin will be with child, and will give birth to a son, and they will name him Emmanuel.

Yet when blessed Gabriel revealed this mystery to the holy Virgin who was to be the Mother of God, he said:

Do not he afraid, Mary, for you have found favour in God’s sight. You will conceive in your womb and bear a son and you will name him Jesus, for it is he who will save his people from their sins.

How is this? Does the message of the holy angel contradict that of the prophet? By no means.

Speaking mysteries in the Spirit, the inspired Prophet foretold that God would be with us, naming him “Emmanuel” in consideration of his divine nature and of the plan whereby he became incarnate.

The blessed angel on the other hand called him by a name that signified his function: he has in fact saved his people, and on this account he is called “Saviour”.

This is how hosts of angels announced the Good News to the shepherds at the time when he humbled himself to be born in the flesh for our sake:

Be not afraid, they said; today we bring you good tidings of a great joy for the whole people: a saviour has been born this day in the city of David, and he is Christ the Lord.

Rightly then is he named Emmanuel, because being God by nature he became God-with-us when he was made man.

And yet he is also named Jesus, because being God, and being made man, he had the task of saving the world.

So when he came forth from his mother’s womb (for according to the flesh he was truly born of her) his name was conferred on him.

It was inappropriate for God the Word to be named “Christ” before his birth in the flesh: how should he be called Christ, the Anointed One, when he had not yet been anointed?

But, when he was born as man, then there was given to him the name that belonged to him in virtue of his human condition.

Cyril of Alexandria (c. 376-444): Commentary on Isaiah Lib. 4, Oratio 4 (PG 70, 1035-1038), from the Monastic Office of Vigils, December 20th in Advent Year 1.