52 Thus God out of regard for human weakness has not set forth the faith in bare and uncertain statements. For although the authority of our Lord's mere words of itself compelled their acceptance, He nevertheless has informed our reason by a revelation which explains their meaning, that we might learn to know His words, I and the Father are one, by means of that which was itself the cause of the unity in question. For in saying that the Father speaks in His words, and works through His working, and judges through His judgment, and is seen in His manifestation, and reconciles through His reconciliation, and abides in Him, while He in turn abides in the Father—what more fitting words, I ask, could He have employed in His teaching to suit the faculties of our reason, that we might believe in Their unity, than those by which, through the truth of the birth and the unity of the nature, it is declared that whatever the Son did and said, the Father said and did in the Son?
This says nothing of a nature foreign to Himself, or added by creation to God, or born into Godhead by a partition of God, but it betokens the divinity of One Who by a perfect birth is begotten perfect God, Who has so confident an assurance of His nature that He says, I in the Father and the Father in Me, and again, All things whatsoever the Father has are Mine. For nought of the Godhead is lacking in Him, in Whose working and speaking and manifestation God works and speaks and is beheld.
They are not two Gods, Who in their working and words and manifestation put on a semblance of unity. Neither is He a solitary God. Who in the works and words and sight of God, Himself worked and spoke and was seen as God. The Church understands this. The Synagogue does not believe, philosophy does not know, that being One of One, Whole of Whole, God and Son, He has neither by His birth deprived the Father of His completeness, nor failed to possess the same completeness in Himself by right of His birth. And whosoever is caught in this folly of unbelief is a disciple either of the Jews or of the heathen.
Source: On the Trinity (New Advent)