39 Again, the unity of Begetter and Begotten, manifested in sameness of nature and true oneness of kind, proves that the Father was seen in His true nature. And this is shown by the Lord's next words, Believe ye not that I am in the Father, and the Father in Me? In no other words than these, which the Son has used, can the fact be stated that Father and Son, being alike in nature, are inseparable. The Son, Who is the Way and the Truth and the Life, is not deceiving us by some theatrical transformation of names and aspects, when He, while wearing Manhood, styles Himself the Son of God.
He is not falsely concealing the fact that He is God the Father; He is not a single Person Who hides His features under a mask, that we may imagine that Two are present. He is not a solitary Being, now posing as His own Son, and again calling Himself the Father; tricking out one unchanging nature with varying names. Far removed from this is the plain honesty of the words. The Father is the Father, and the Son is the Son. But these names, and the realities which they represent, contain no innovation upon the Divine nature, nothing inconsistent, nothing alien.
For the Divine nature, being true to itself, persists in being itself; that which is from God is God. The Divine birth imports neither diminution nor difference into the Godhead, for the Son is born into, and subsists with, a nature that is within the Divine nature and is like to it, and the Father sought out no alien element to be mingled in the nature of His Only-begotten Son, but endowed Him with all things that are His own, and this without loss to the Giver. And thus the Son is not destitute of the Divine nature, for, being God, He is from God and from none other; and He is not different from God, but is indeed nothing else than God, for that which is begotten from God is the Son, and the Son only, and the Divine nature, in receiving birth as a Son, has not forfeited its Divinity.
Thus the Father is in the Son, the Son is in the Father, God is in God. And this is not by the combination of two harmonious, though different, kinds of being, nor by the incorporating power of an ampler substance exercised upon a lesser; for the properties of matter make it impossible that things which enclose others should also be enclosed by them. It is by the birth of living nature from living nature. The substance remains the same, birth causes no deterioration in the Divine nature; God is not born from God to be ought else than God. Herein is no innovation, no estrangement, no division. It is sin to believe that Father and Son are two Gods, sacrilege to assert that Father and Son are one solitary God, blasphemy to deny the unity, consisting in sameness of kind, of God from God.
Source: On the Trinity (New Advent)