John Cassian, On the Incarnation of the Lord, Against Nestorius VII.29: In the next place he puts the authority of S. Athanasius

Athanasius also, priest of the city of Alexandria, a splendid instance of constancy and virtue, whom the storm of heretical persecution tested without crushing him: whose life was always like a clear glass, and who had almost obtained the reward of martyrdom before attaining the dignity of confessorship: Let us see what was his view of the Lord Jesus Christ and the mother of the Lord. “This then,” he says, “is the mind and stamp of Holy Scripture, as we have often said; viz., that in one and the same Saviour two things have to be understood: (1) that He was ever God, and is Son, Word, and Light, and Wisdom of the Father, and (2) that afterwards for our sakes He took flesh of the Virgin Mary the Theotocos, and was made man.” Again after some other matter: “Many then were saints and clean from sin: Jeremiah also was sanctified from the womb, and John, while still in the womb leapt for joy at the voice of Mary the Theotocos.” He certainly says that God, the Son of God, who (to declare the faith of all in his words) is “the Word, and Light and Wisdom of the Father,” took flesh for our sakes; and therefore he calls the Virgin Mary Theotocos, because she was the Mother of God.