24. Since the Word is from God, He must be Son. Since the Son is from everlasting, He must be the Word; else either He is superior to the Word, or the Word is the Father. Texts of the New Testamentwhich state the unity of the Son with the Father; therefore the Son is the Word. Three hypotheses refuted— 1. That the Man is the Son; 2. That the Word and Man together are the Son; 3. That the Word became Son on His incarnation. Texts of the Old Testamentwhich speak of the Son. If they are merely prophetical, then those concerning the Word may be such also
24 But it is not so; for in truth much is said in the Old also about the Son, as in the second Psalm, 'You are My Son, this day have I begotten You;' and in the ninth the title, Unto the 'end concerning the hidden things of the Son, a Psalm of David;' and in the forty-fourth, 'Unto the end, concerning the things that shall be changed to the Sons of Korah for understanding, a song about the Well-beloved;' and in Isaiah, 'I will sing to my Well-beloved a song of my Well-beloved touching my vineyard.
My Well-beloved has a vineyard;' Who is this 'Well-beloved' but the Only-begotten Son? As also in the hundred and ninth, 'From the womb I begot You before the morning star,' concerning which I shall speak afterwards; and in the Proverbs, 'Before the hills He begot me;' and in Daniel, 'And the form of the Fourth is like the Son of God;' and many others. If then from the Old be ancientness, ancient must be the Son, who is clearly described in the Old Testament in many places. 'Yes,' they say, 'so it is, but it must be taken prophetically.'
Therefore also the Word must be said to be spoken of prophetically; for this is not to be taken one way, that another. For if 'You are My Son?' refer to the future, so does 'By the Word of the Lord were the heavens established;' for it is not said 'were brought to be,' nor 'He made.' But that 'established' refers to the future, it states elsewhere: 'The Lord reigned,' followed by 'He so established the earth that it can never be moved.' And if the words in the forty-fourth Psalm 'for My Well-beloved' refer to the future, so does what follows upon them, 'My heart uttered a good Word.'
And if 'From the womb' relates to a man, therefore also 'From the heart.' For if the womb is human, so is the heart corporeal. But if what is from the heart is eternal, then what is 'From the womb' is eternal. And if the 'Only-begotten' is 'in the bosom,' therefore the 'Well-beloved' is 'in the bosom.' For 'Only-begotten' and 'Well-beloved' are the same, as in the words 'This is My Well-beloved Son. ' For not as wishing to signify His love towards Him did He say 'Well-beloved,' as if it might appear that He hated others, but He made plain thereby His being Only-begotten, that He might show that He alone was from Him.
And hence the Word, with a view of conveying to Abraham the idea of 'Only-begotten,' says, 'Offer your son your well-beloved;' but it is plain to any one that Isaac was the only son from Sara. The Word then is Son, not lately come to be, or named Son, but always Son. For if not Son, neither is He Word; and if not Word, neither is He Son. For that which is from the father is a son; and what is from the Father, but that Word that went forth from the heart, and was born from the womb? For the Father is not Word, nor the Word Father, but the one is Father, and the other Son; and one begets, and the other is begotten.
Source: Four Discourses Against the Arians (New Advent)