And we proclaim the holy Virgin to be properly and truly Mother of God (Theotokos).

Russian Floral Icon of Panaghia For, as He who was born of her is true God, so is she truly Mother of God who gave
birth to the true God who took His flesh from her. Now, we do not say that God was born of her in the sense that the divinity of the Word has its beginning of being from her, but in the sense that God the Word Himself, who was
timelessly begotten of the Father before the ages and exists without beginning and eternally with the Father and the Holy Ghost, did in the last days come for our salvation to dwell in her womb and of her was, without undergoing change, made flesh and born. For the holy Virgin did not give birth to a mere man but to true God, and not to God simply, but to God made flesh. And He did not bring His body down from heaven and come through her as through a channel, but assumed from her a body consubstantial with us and subsisting in Himself. Now, had the body been brought down from heaven and not been taken from our nature, was there any need for His becoming man? God the Word was made man for this reason : that that very nature which had sinned, fallen, and become corrupt should conquer the tyrant who had deceived it. Thus should it be freed from corruption, as the divine Apostle says: Tor by a man came death: and by a man the resurrection of the dead. If the first was true, then so is the second…

…Hence, it is rightly and truly that we call holy Mary the Mother of God, for this name expresses the entire mystery of the Incarnation.

Exposition of the Orthodox Faint, by Saint John of Damascus