"Rubum, quem víderat Móyses incombústum, conservátam agnóvimus tuam laudábilem virginitátem. Dei Génetrix, intercéde pro nobis."
"As the bush, which Moses saw unconsumed, even thus preserved we know is thy glorious virginity:O mother of God, intercede for us."
Third antiphon, Vespers.
"For by the singular gift of Him who is our Lord and God, and withal, her own son, she is to be confessed most truly and most blessedly--The mother of God 'Theotocos,' but not in the sense in which it is imagined by a certain impious heresy which maintains, that she is to be called the Mother of God for no other reason than because she gave birth to that man who afterwards became God, just as we speak of a woman as the mother of a priest, or the mother of a bishop, meaning that she was such, not by giving birth to one already a priest or a bishop, but by giving birth to one who afterwards became a priest or a bishop. Not thus, I say, was the holy Mary 'Theotocos,' the mother of God, but rather, as was said before, because in her sacred womb was wrought that most sacred mystery whereby, on account of the singular and unique unity of Person, as the Word in flesh is flesh, so Man in God is God."
-Vincent of Lerins, Commonitory for the Antiquity and Universality of the Catholic Faith, 15
"And so you say, O heretic, whoever you may be, who deny that God was born of the Virgin, that Mary the Mother of our Lord Jesus Christ ought not to be called Theotocos, i.e., Mother of God, but Christotocos, i.e., only the Mother of Christ, not of God. For no one, you say, brings forth what is anterior in time. And of this utterly foolish argument whereby you think that the birth of God can be understood by carnal minds, and fancy that the mystery of His Majesty can be accounted for by human reasoning, we will, if God permits, say something later on. In the meanwhile we will now prove by Divine testimonies that Christ is God, and that Mary is the Mother of God."
-John Cassian, The Incarnation of Christ, II:2
Here's a medieval hymn, the mirror image of the popular "Stabat Mater Dolorosa":
Stabat Mater Speciosa.
And it's really hard to pick one section of today's office to quote, the whole thing is so beautiful So, If you're good for it, Here's the whole office according to the typical version.