Cyrill of Alexandria
On the Nature of the Son's Unity with the Father. This argument is connected to a particular difficulty that leads us to understand that the Son exists from the very essence of the Father rather than emerging from outside that essence, demonstrating His co-eternality with the Father. If, as you propose, O opponents of the truth, the Son emanates from non-existence and only comes into being at His birth, then it follows that He would be akin to other rational beings, who are titled God, Son, and Wisdom due to their relationship with God.
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This is a characteristic of rational creatures, whose beauty reflects the Divine not through their inherent nature but through the grace bestowed by the Giver, as expressed in Scripture: 'I say, "You are God, and are sons of the Highest."' Since we, through our adoption as children of God, receive His honor, what then do we attribute to the communion of the Word that allows Him to be recognized as both Son and God? We commune with the Holy Spirit, and to think otherwise of the Word is a misunderstanding. He declares about the Spirit, 'He shall receive of Mine' (John 16:14). What is it that He receives? The answer must be 'from the Father'.
What embodies this communion? Just as warmth originates from fire, or fragrance from a flower, so the Spirit touches our souls, as Scripture indicates that He proceeds from the Father. What emanates from the Father and manifests in the Word? Whether it arises from the essence of the Father or from another source, where does this received Word come from? If it is external, then the Son does not partake of the Father; rather, he is sanctified by something else, which is a notion too unholy to entertain. Conversely, if you consider that what is given to the Word in communion originates from the essence of the Father, you face a dilemma: either you acknowledge a division within God's nature, or you assert that such processes in God occur without division or suffering, unintentionally undermining the integrity of the Son's begetting by introducing such notions.
If God begets without division and does so impassibly from His own essence, there is no obstacle to affirming that which is begotten is indeed the living Word of the Father. He who proceeds from the eternal Father must equally be eternal, for He possesses and preserves the Father's divine nature within Himself, asserting the truth of His statement: 'He who has seen me has seen the Father.' The Most High reveals Himself not within the created but through His eternal nature.