41 And so, lest we should believe and say that the Father works in the Son through His own omnipotent energy, and not through the Son's possession, as His birthright, of the Divine nature, Christ says, Believe Me, that I am in the Father, and the Father in Me. What means this, Believe Me? Clearly it refers back to the previous, Show us the Father. Their faith— that faith which had demanded that the Father should be shown— is confirmed by this command to believe.
He was not satisfied with saying, He that has seen Me has seen the Father also. He goes further, and expands our knowledge, so that we can contemplate the Father in the Son, remembering meanwhile that the Son is in the Father. Thus He would save us from the error of imagining a reciprocal emanation of the One into the Other, by teaching Their unity in the One nature through birth given and received. The Lord would have us take Him at His word, lest our hold upon the faith be shaken by His condescension in assuming Humanity.
If His flesh, His body, His passion seem to make His Godhead doubtful, let us at least believe, on the evidence of the works, that God is in God and God is from God, and that They are One. For by the power of Their nature Each is in the Other. The Father loses nothing that is His because it is in the Son, and the Son receives His whole Sonship from the Father. Bodily natures are not created after such a fashion that they mutually contain each other, or possess the perfect unity of one abiding nature.
In their case it would be impossible that an Only-begotten Son could exist eternally, inseparable from the true Divine nature of His Father. Yet this is the peculiar property of God the Only-begotten, this the faith revealed in the mystery of His true birth, this the work of the Spirit's power, that to be, and to be in God, is for Christ the same thing; and that this being in God is not the presence of one thing within another, as a body inside another body, but that the life and subsistence of Christ is such that He is within the subsisting God, and within Him, yet having a subsistence of His own.
For Each subsists in such wise as not to exist apart from the Other, since They are Two through birth given and received, and therefore only one Divine nature exists. This is the meaning of the words, I and the Father are One, and He that has seen Me has seen the Father also, and I in the Father and the Father in Me. They tell us that the Son Who is born is not different or inferior to the Father; that His possession, by right of birth, of the Divine nature as Son of God, and therefore nothing else than God, is the supreme truth conveyed in the mysterious revelation of the One Godhead in Father and Son.
And therefore the doctrine of the generation of the Only-begotten is guiltless of ditheism, for the Son of God, in being born into the Godhead, manifested in Himself the nature of God His Begetter.
Source: On the Trinity (New Advent)