I have been rereading the Orations of Gregory Nazianzen (Nazianzus). I started with the 19 orations translated into English by Martha Vinson, published by the Catholic University of America Press (volume 107 in the Fathers of the Church series - Google preview). None of these orations are included in the collection of Gregory's works published in volume VII of The Nicene and Post-Nicene Fathers (second series - PDF copy).
From Oration 20 we read:
We must neither be so partial to the Father that we actually strip him of his fatherhood, for whose father would he in fact be if his son were different in nature and estranged from him along with the rest of creation? Nor by the same token, should we be so partial to Christ that we fail to preserve this very distinction, his Sonhood, for whose son would he in fact be if there were no causal relationship between his Father and himself? Nor again should we diminish the Father's status as source, proper to him as Father and generator, since he would be the source of small and worthless things were he not the cause of deity contemplated in the Son and Spirit. It is our duty then both to maintain the oneness of God and to confess three individual entities, or Persons, each with his distinctive property.
The oneness of God [i.e. the Father] would, in my view, be maintained if both the Son and Spirit are causally related to him alone without being merged or fused into him and if they all share one and the same divine movement and purpose. And all three individually existing entities will be maintained if we do not think of them as fusing or dissolving or mingling, lest those with an excessive devotion to unity end up destroying the whole. And the individual properties will be maintained if, in the case of the Father, we think and speak of him as being both source and without source (I use the term in the sense of causal agent, fount, and eternal light)...The Father, then, is without source: his existence is derived neither outside nor from within himself. In turn, the Son is not without source if you understand "Father" to mean causal agent, since the Father is the source of the Son as causal agent... (Pages 111, 112.)
The teaching of Gregory that the Father is the "causal agent", "source", "fount" of the Son (and Spirit) is a foundational aspect of his doctrine of the Trinity, and is found in a number of his works. Directly related to this teaching is his understanding of the Father as "greater" (John 14:28) than the Son. Note the following from Oration 30:
As your third point you count the Word Greater ; and as your fourth. To My God and your God. And indeed, if He had been called greater, and the word equal had not occurred, this might perhaps have been a point in their favour. But if we find both words clearly used what will these gentlemen have to say? How will it strengthen their argument ? How will they reconcile the irreconcilable? For that the same thing should be at once greater than and equal to the same thing is an impossibility; and the evident solution is that the Greater refers to origination, while the Equal belongs to the Nature ; and this we acknowledge with much good will. But perhaps some one else will back up our attack on your argument, and assert, that That which is from such a Cause is not inferior to that which has no Cause ; for it would share the glory of the Unoriginate, because it is from the Unoriginate. And there is, besides, the Generation, which is to all men a matter so marvellous and of such Majesty. For to say that he is greater than the Son considered as man, is true indeed, but is no great thing. For what marvel is it if God is greater than man ? Surely that is enough to say in answer to their talk about Greater. (NPNF 7.312.)
Gregory follows the majority of pre-Nicene Church Fathers that the Biblical description of the Father being "greater" than the Son (John 14:28) should not be limited to the Son's incarnation; and that, the Father is in a very concrete sense "greater" than the Son because he is the "causal agent" of the Son.
My rereading of Gregory's orations has impressed upon me some important aspects of his reflections on the doctrine of the Trinity: first, the monarchy of God the Father, which includes His function as the "causal agent", "source", "fount" of the Son and Holy Spirit; second, his emphasis on the individuality of the Father, Son and Holy Spirit, without diminishing their equality of nature; and third, his ability to refute both Sabellianism and Arianism at the same time while engaged in the process of expounding his own Trinitarian thought.
Grace and peace,