31 We see how the living Son of the living Father, He Who is God from God, reveals the unity of the Divine nature, indissolubly One and the same, and the mystery of His birth in these words, I and the Father are One. Because the seeming arrogance of them engendered a prejudice against Him, He made it more clear that He had spoken in the conscious possession of Divinity by saying, You say that I have blasphemed because I said, I am the Son of God; thus showing that the oneness of His nature with that of God was due to birth from God.
And then, to clench their faith in His birth by a positive assertion, and to guard them, at the same time, from imagining that the birth involves a difference of nature, He crowns His argument with the words, Believe the works, that the Father is in Me, and I in the Father. Does His birth, as here revealed, display His Divinity as not His by nature, as not His own by right? Each is in the Other; the birth of the Son is from the Father only; no alien or unlike nature has been raised to Godhead and subsists as God.
God from God, eternally abiding, owes His Godhead to none other than God. Import, if you see your opportunity, two gods into the Church's faith; separate Son from Father as far as you can, consistently with the birth which you admit; yet still the Father is in the Son, and the Son is in the Father, and this by no interchange of emanations but by the perfect birth of the living nature. Thus you cannot add together God the Father and God the Son, and count Them as two Gods, for They Two are One God.
You cannot confuse Them together, for They Two are not One Person. And so the Apostolic faith rejects two gods; for it knows nothing of two Fathers or two Sons. In confessing the Father it confesses the Son; it believes in the Son in believing in the Father. For the name of Father involves that of Son, since without having a son none can be a father. Evidence of the existence of a son is proof that there has been a father, for a son cannot exist except from a father. When we confess that God is One we deny that He is single; for the Son is the complement of the Father, and to the Father the Son's existence is due.
But birth works no change in the Divine nature; both in Father and in Son that nature is true to its kind. And the right expression for us of this unity of nature is the confession that They, being Two by birth and generation, are One God, not one Person.
Source: On the Trinity (New Advent)