St Gregory the Theologian
‘What then, say they, is there lacking to the Spirit which prevents His being a Son? for if there were not something lacking He would be a Son. We assert that there is nothing lacking—for God has no deficiency. But the difference of manifestation, if I may so express myself, or rather of their mutual relations one to another, has caused the difference of their Names. For indeed it is not some deficiency in the Son which prevents His being Father (for Sonship is not a deficiency), and yet He is not Father … For the Father is not Son, and yet this is not due to either deficiency or subjection of Essence; but the very fact of being Unbegotten or Begotten, or Proceeding has given the name of Father to the First, of the Son to the Second, and of the Third, Him of Whom we are speaking, of the Holy Ghost that the distinction of the Three Persons may be preserved in the one nature and dignity of the Godhead … The Three are One in Godhead, and the One Three in properties; so that neither is the Unity a Sabellian one [with no distinction of persons], nor does the Trinity countenance the present evil distinction [of the Arians, in which the Son and the Holy Spirit are not of the same essence as the Father].’