4 He, therefore, being the perfect Father's perfect Son, the Only-begotten Offspring of the unbegotten God, who has received all from Him Who possesses all, being God from God, Spirit from Spirit, Light from Light, says boldly, The Father in Me, and I in the Father. For as the Father is Spirit, so is the Son Spirit; as the Father is God, so is the Son God; as the Father is Light, so is the Son Light. Thus those properties which are in the Father are the source of those wherewith the Son is endowed; that is, He is wholly Son of Him Who is wholly Father; not imported from without, for before the Son nothing was; not made from nothing, for the Son is from God; not a son partially, for the fullness of the Godhead is in the Son; not a Son in some respects, but in all; a Son according to the will of Him who had the power, after a manner which He only knows.
What is in the Father is in the Son also; what is in the Unbegotten is in the Only-begotten also. The One is from the Other, and they Two are a Unity; not Two made One, yet One in the Other, for that which is in Both is the same. The Father is in the Son, for the Son is from Him; the Son is in the Father, because the Father is His sole Origin; the Only-begotten is in the Unbegotten, because He is the Only-begotten from the Unbegotten. Thus mutually Each is in the Other, for as all is perfect in the Unbegotten Father, so all is perfect in the Only-begotten Son.
This is the Unity which is in Son and Father, this the power, this the love; our hope, and faith, and truth, and way, and life is not to dispute the Father's powers or to depreciate the Son, but to reverence the mystery and majesty of His birth; to set the unbegotten Father above all rivalry, and count the Only-begotten Son as His equal in eternity and might, confessing concerning God the Son that He is from God.
Source: On the Trinity (New Advent)