20 Since, then, all things were made through Him, come to our help and tell us what it was that was made not without Him. That which was made in Him is life. That which was made in Him was certainly not made without Him; for that which was made in Him was also made through Him. All things were created in Him and through Him. They were created in Him, for He was born as God the Creator. Again, nothing that was made in Him was made without Him, for the reason that God the Begotten was life, and was born as Life, not made life after His birth; for there are not two elements in Him, one inborn and one afterwards conferred.
There is no interval in His case between birth and maturity. None of the things that were created in Him was made without Him, for He is the Life which made their creation possible. Moreover God, the Son of God, became God by virtue of His birth, not after He was born. Being born the Living from the Living, the True from the True, the Perfect from the Perfect, He was born in full possession of His powers. He needed not to learn in after time what His birth was, but was conscious of His Godhead by the very fact that He was born as God of God. I and the Father are One, are the words of the Only-begotten Son of the Unbegotten.
It is the voice of the One God proclaiming Himself to be Father and Son; Father speaking in the Son and Son in the Father. Hence also He that has seen Me has seen the Father also; hence All that the Father has, He has given to the Son; hence As the Father has life in Himself so has He given to the Son to have life in Himself; hence No one knows the Father save the Son, nor the Son save the Father; hence In Him dwells all the fullness of the Godhead bodily.
Source: On the Trinity (New Advent)