27 And so the Son, Who does the Father's works and demands of us that, if we believe not Him, at least we believe His works, is bound to tell us what the point is as to which we are to believe the works. And He does tell us in the words which follow:— But if I do, and you will not believe Me, believe the works, that you may know and be sure that the Father is in Me, and I in Him. It is the same truth as is contained in I am the Son of God, and I and the Father are One.
This is the nature which is His by birth; this the mystery of the saving faith, that we must not divide the unity, nor separate the nature from the birth, but must confess that the living God was in truth born from the living God. God, Who is Life, is not a Being built up of various and lifeless portions; He is Power, and not compact of feeble elements, Light, intermingled with no shades of darkness, Spirit, that can harmonise with no incongruities. All that is within Him is One; what is Spirit is Light and Power and Life, and what is Life is Light and Power and Spirit.
He Who says, I am, and I change not, can suffer neither change in detail nor transformation in kind. For these attributes, which I have named, are not attached to different portions of Him, but meet and unite, entirely and perfectly, in the whole being of the living God. He is the living God, the eternal Power of the living Divine nature; and that which is born from Him, according to the mysterious truth which He reveals, could not be other than living. For when He said, As the living Father has sent Me, and I live through the Father, He taught that it is through the living Father that He has life in Himself.
And, moreover, when He said, For as the Father has life in Himself, so has He given to the Son also to have life in Himself, He bore witness that life, to the fullest extent, is His gift from the living God. Now if the living Son was born from the living Father, that birth took place without a new nature coming into existence. Nothing new comes into existence when the Living is begotten by the Living; for life was not sought out from the non-existent to receive birth; and Life, which receives its birth from Life, must needs, because of that unity of nature and because of the mysterious event of that perfect and ineffable birth, live always in Him that lives and have the life of the Living in Himself.
Source: On the Trinity (New Advent)