Gregory ( Against Eunomius , 3.3) insists that only a Trinitarian theology can truly affirm the goodness of God.  He assumes the Scriptural titles for the Son - light, truth, life, glory - and asks whether the Father could ever have been without these goods.  If He was once without the Son, then He was once without these goods.  God is good because God is Triune.

What is God? he asks.  And he answers: “Well, God is a Father. It follows that He is what He is from eternity: for He did not become, but  is a Father: for in God that which was, both is and will be. On the other hand, if He once was not anything, then He neither is nor will be that thing: for He is not believed to be the Father of a Being such that it may be piously asserted that God once existed by Himself without that Being. For the Father is the Father of Life, and Truth, and Wisdom, and Light, and Sanctification, and Power, and all else of a like kind that the Only-begotten is or is called. Thus when the adversaries allege that the Light ‘once was not,’ I know not to which the greater injury is done, whether to the Light, in that the Light is not, or to Him that has the Light, in that He has not the Light. So also with Life and Truth and Power, and all the other characters in which the Only-begotten fills the Father’s bosom, being all things in His own fullness. For the absurdity will be equal either way, and the impiety against the Father will equal the blasphemy against the Son: for in saying that the Lord ‘once was not,’ you will not merely assert the non-existence of Power, but you will be saying that the Power of God, Who is the Father of the Power, ‘was not.’ Thus the assertion made by your doctrine that the Son ‘once was not’ establishes nothing else than a destitution of all good in the case of the Father. See to what an end these wise men’s acuteness leads, how by them the word of the Lord is made good, which says, ‘He that despises Me despises Him that sent me’: for by the very arguments by which they despise the existence at any time of the Only-begotten, they also dishonour the Father, stripping off by their doctrine from the Father’s glory every good name and conception.”