I understand that the phrase "Mother of God" appears in the prayer commonly referred to as the "Hail Mary" prayer. I'm wondering, though, what the origin is of the phrase "Mother of God". Did it first appear in the Hail Mary prayer or does it predate that?
Wikipedia's entry on theotokos ("mother of God") already answers this pretty well:
The Hail Mary Prayer, though based in Luke, by contrast, dates to the 1050s:-
Although Mary is the Mother of God, she is not his mother in the sense that she is older than God or the source of her Son’s divinity, for she is neither. Rather, she is the Mother of God in the sense that she carried in her womb a divine person—Jesus Christ, God "in the flesh".
If this is not so then it is like saying that Mary did not carry God in her womb, but only carried Christ’s human nature. This assertion will be same as heresy of Nestorianism. On the contrary Mary carried and gave birth to the person of Jesus Christ, and the person she gave birth to was God.
Here are some of the summarized forms of Mother of God references from very early age of Christianity:
As the length of this answer is increasing, I will restrict here in mentioning only the names and dates when these references were made:
The other fellas addressed this pretty well, but I figured, what the heck, might as well muddy the waters with some extra stuff from the "Short Catechism of Mary"
However, in "The Secret of the Rosary" St. Louis De Montfort says
So, there seems to be some confusion as to the dates. But no confusion as to the meaning of Mother of God in the context of the Rosary. And whatever words St. Dominic uttered, I highly doubt they failed to contain a plea to the Holy Mother of God.
St. Louis continues in his explanation of the Hail Mary:
And those are all the things that the title Mother of God entails. All of which had been gleaned through Tradition and Scripture since the beginning of the Church.
She's the channel of graces (Mediatrix), because it is through her the Christ comes into the world, all the Gospels attest to this.
John, who was intimately acquainted with her story, knows that Jesus gave her to us all from the cross
He also recounts a story of Mary's intercession
which makes her the advocate that St. Louis talks about.
Now, you might think, "I know this guy, he's just a crazy Catholic Mariolotrist who would say anything to arrive at the point that any thing good you can say about Mary makes her Divine Son happy."
But the point is, you're asking for a precise date when a particular dogma was infallibly proclaimed and I'd say it was 451 in the Council of Chalcedon which produced the following creed:
but it's really, the unchanging Tradition of the Catholic Church that Mary is the Mother of God and it was known by her little people long before it was written in stone by her big people.