21 With a view to deprive of substantive divinity the Only-begotten God, Who was God the Word with God in the beginning, they make Him merely the utterance of the voice of God. The Son is related to God His Father, they say, as the words to the speaker. They are trying to creep into the position, that it was not God the eternal Word, abiding in the form of God, Who was born as Christ the Man, Whose life therefore springs from a human origin, not from the mystery of a spiritual conception; that He was not God the Word, making Himself man by birth from the Virgin, but the Word of God dwelling in Jesus as the spirit of prophecy dwelt in the prophets. They accuse us of saying that Christ was born man with body and soul different from ours. But we preach the Word made flesh, Christ emptying Himself of the form of God and taking the form of a servant, perfect according to the fashion of human form, born a man after the likeness of ourselves: that being true Son of God, He is indeed true Son of Man, neither the less Man because born of God, nor the less God because Man born of God.
Source: On the Trinity (New Advent)