St. John of Damascus: Theotokos

Moreover we proclaim the holy Virgin to be in strict truth the Mother of God. For inasmuch as He who was born of her was true God, she who bare the true God incarnate is the true mother of God.

For we hold that God was born of her, not implying that the divinity of the Word received from her the beginning of its being, but meaning that God the Word Himself, Who was begotten of the Father timelessly before the ages, and was with the Father and the Spirit without beginning and through eternity, took up His abode in these last days for the sake of our salvation in the Virgin’s womb, and was without change made flesh and born of her.

For the holy Virgin did not bare mere man but true God: and not mere God but God incarnate, Who did not bring down His body from Heaven, nor simply passed through the Virgin as channel, but received from her flesh of like essence to our own and subsisting in Himself. For if the body had come down from heaven and had not partaken of our nature, what would have been the use of His becoming man? For the purpose of God the Word becoming man was that the very same nature, which had sinned and fallen and become corrupted, should triumph over the deceiving tyrant and so be freed from corruption, just as the divine apostle puts it, For since by man came death, by man came also the resurrection of the dead (1 Cor. 15:21). If the first is true the second must also be true.

Hence it is with justice and truth that we call the holy Mary the Mother of God. For this name embraces the whole mystery of the dispensation. For if she who bore Him is the Mother of God, assuredly He Who was born of her is God and likewise also man.

~St. John of Damascus (645-749) whose feast is celebrated today

(from "An Exact Exposition of the Orthodox Faith" 3:12)