40 Lest they, whose faith conforms to the Gospel, should regard this mystery as something vague and obscure, the Lord has expounded it in this order— Do you not believe Me, that I am in the Father, and the Father is in Me? The words that I speak unto you I speak not of Myself, but the Father that dwells in Me, He does His works. In what other words than these could, or can, the possession of the Divine nature by Father and Son be declared, consistently with prominence for the Son's birth?
When He says, The words that I speak unto you I speak not of Myself, He neither suppresses His personality, nor denies His Sonship, nor conceals the presence in Himself of His Father's Divine nature. While speaking of Himself— and that He does so speak is proved by the pronoun I— He speaks as abiding in the Divine substance; while speaking not of Himself, He bears witness to the birth which took place in Him of God from God His Father. And He is inseparable and indistinguishable in unity of nature from the Father; for He speaks, though He speaks not of Himself.
He Who speaks, though He speak not of Himself, necessarily exists, inasmuch as He speaks; and, inasmuch as He speaks not of Himself, He makes it manifest that His words are not His own. For He has added, But the Father that dwells in Me, He does His works. That the Father dwells in the Son proves that the Father is not isolated and alone; that the Father works through the Son proves that the Son is not an alien or a stranger. There cannot be one Person only, for He speaks not of Himself; and, conversely, They cannot be separate and divided when the One speaks through the voice of the Other.
These words are the revelation of the mystery of Their unity. And again, They Two are not different One from the Other, seeing that by Their inherent nature Each is in the Other; and They are One, seeing that He, Who speaks, speaks not of Himself, and He, Who speaks not of Himself, yet does speak. And then, having taught that the Father both spoke and wrought in Him, the Son establishes this perfect unity as the rule of our faith— But the Father that dwells in Me, He does His works. Believe Me, that I am in the Father, and the Father in Me; or else believe for the very works' sake. The Father works in the Son; but the Son also works the works of His Father.
Source: On the Trinity (New Advent)