Although Nicaea gave us the basis of the Creed which bears its name, it failed to say much about the Holy Spirit. It was left to St. Basil and St. Gregory Nazianzus to show that the Spirit, too, was of the same Substance as the Father and the Son. Their greatest achievement, however, was to make sense of the One-ness and the Three-ness of the Trinity.

The Trinity was of the same Substance: the Father was God, the Son was God and the Holy Spirit was God – but the Father was not the Son, neither was either of those two Persons the Holy Spirit, although they are what the Father is in Substance. They came up with the word hypostasis to express the difference. The Son and the Spirit are what the Father is (God) but they are not who the Father is. They relate to each other as Persons in a communion of love which is not to be explained.

The relationship between Father and Son is that the former begets the latter. This mode of eternal filial origination is the distinct hypostatic character of the Son – His Sonship lies in that He is begotten of the Father (before all worlds, God of God, True light of True light, begotten not made).

How has he been begotten? I re-utter the question with loathing. God’s begetting ought to have the tribute of our reverent silence. The important point is for you to learn that he has been begotten. As to the way it happens, we shall not concede that even angels, much less you, know that. Shall I tell you the way? It is a way known only to the begetting Father and the begotten Son. Anything beyond this fact is hidden by a cloud and escapes your dull vision. [Oration 29.8 6]

The Spirt issues perennially from the Father, and this mode of eternal spiration is the distinct hypostatic character of the Spirit, who, however, proceeds from the Father through the Son. The Spirit’s mode of origin – spiration, is what distinguishes the Third Person of the Trinity from the Second.

What, then, is “proceeding”? You explain the ingeneracy of the Father and I will give you a biological account of the Son’s begetting and the Spirit’s proceeding – and let us go mad the pair of us for prying into God’s secrets. What competence have we here? We cannot understand what lies under our feet, cannot count the sand in the sea, “the drops of rain or the days of this world,” much less enter into the “depths of God” and render a verbal account of a nature so mysterious, so much beyond words. [Oration 31.8]

So, all Three Persons are God, but each in a distinct, hypostatic realisation. The Divine Nature is not a common property of three different entities, it is, as St. Gregory showed, a personal being (that of the Father) that is hypostatically realised by the Son and the Spirit as they each derive from and relate back to the Father. The Father is the dynamic cause of the Trinity – Three Persons, One Substance.

This I give you to share, and to defend all your life, the One Godhead and Power, found in the Three in Unity, and comprising the Three separately, not unequal, in substances or natures, neither increased nor diminished by superiorities or inferiorities; in every respect equal, in every respect the same;
Just as the beauty and the greatness of the heavens is one; the infinite conjunction of Three Infinite Ones, Each God when considered in Himself; as the Father so the Son, as the Son so the Holy Ghost; the Three One God when contemplated together; Each God because Consubstantial; One God because of the Monarchia.
No sooner do I conceive of the One than I am illumined by the Splendour of the Three; no sooner do I distinguish Them than I am carried back to the One. [Oration 40.41]

St. Gregory himself, rightly warns us that to engage too much in theological reflection led to the danger of dazzling the mind by speaking about mysteries that even the angels cannot comprehend. After that little excursion – something upon which we can all agree.