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Since we have reached this point in our discourse, and find ourselves compelled to address the third opinion, come, let us speak concerning the Spirit. We shall not be stingy with our words, so that the Word original: "Λόγος" (Logos) — referring here to Christ as the divine Word from above may pour out his streams upon us and provide a spiritual defense for the Spirit.
What, then, is the nature of the Spirit? Some have supposed him to be an "activity" original: "ἐνέργεια" (energeia) — a force or operation rather than a distinct being, others a creature, others God. Still others did not know which of these he was, out of reverence for the Scripture (as they claim), because the Bible does not clearly reveal the matter. For this reason, they neither worship nor dishonor him, maintaining a middle ground—which is actually a miserable state to be in. Even among those who have understood him to be God, some are pious only in their private thoughts, while others dare to speak it aloud. Among the wise men who discuss the Godhead, I hear some who diminish the power of the Spirit, some who exaggerate it, and others who measure it only against their own limited assumptions.
To put it briefly, is the Spirit a creature or God? If he is a creature, how do we believe in him? How are we perfected in him? For it is not the same thing to believe in something as it is to believe about it. The first belongs to divinity, whereas the second can apply to any common thing.
But if he is God, he is neither a creature, nor a product, nor a "fellow-servant" original: "σύνδουλον" — a term used by heretics to suggest the Spirit was a subordinate being. For if he were a creature, he would be a fellow-servant; and if a fellow-servant, how does he set us free? How does he sanctify? How does he deify original: "θεοῖ" — refers to theosis, the Eastern Christian concept of humans participating in the divine life? How does he enlighten? How does he give life? How can he be the source of light and life? Or how can it be said of him, "from whom are all things, and through whom are all things, and in whom are all things," as it is said of the Father and the Son?
If he is God, will he not be God alongside the Father and the Son? Certainly he will. If he is God, why is he not "unbegotten" original: "ἀγέννητος" — the unique property of the Father? Or why is he not a "son"? For he must be one of these if he comes from God. If he is unbegotten, then there are two things without beginning. If he is begotten, he must come either from the Father or from the Son. If he comes from the Father, then there are two sons, and they are brothers—either as twins, or as one being elder and the other younger.
But just as among physical bodies there are some things that are unlike and others that are of the same kind, so also in God: there is the Unbegotten [the Father], the Begotten [the Son], and the Proceeding original: "ἐκπορευτόν" — the unique property of the Spirit [the Holy Spirit]. We must use these names to preserve the distinct properties of their natures. Just because we speak of "unbegottenness," "birth," and "procession," we do not therefore divide the essence term: Ousia (essence) — the underlying "stuff" or nature of God, which is one. If we did, we would fall into the error of Polytheism, which we flee.
Even in physical bodies, a difference in individual properties does not destroy the identity of the nature. For example: man, and man, and man are all one "man" by nature, even if they differ in their personal traits. So it is with the divinity. "Unbegotten," "Begotten," and "Proceeding" are not names of the nature itself, but of a relationship original: "σχέσεως" (scheseos) and the distinction between them. The name of the nature is "God." Just as among humans there is "man," "Peter," and "Paul," so in the divinity there is "God," "Father," "Son," and "Holy Spirit."
The Father, the Son, and the Holy Spirit are one God in three, or three in one. Consider these examples: the sun's disk, its ray, and its light; or a spring, a river, and the water that comes from it. One contains the cause, another is from the cause, and the third is from both. In this way, the divinity remains undivided, and the property of the persons term: Prosopa (persons) — the distinct identities of the Trinity remains unconfused.
Do not ask the "how" or "in what manner" this works. For the nature of God is unsearchable and beyond all human thought. Only believe that one God is worshipped in three individual realities term: Hypostases (persons/substances) — the concrete existence of the Father, Son, and Spirit: Father, Son, and Holy Spirit; to whom be glory and power forever and ever. Amen.
Glossary of technical terms from the footer:
Gregory of Nazianzus; Theology; Holy Spirit; Trinity; Unbegotten; Begotten; Proceeding; Essence (Ousia); Person (Hypostasis); God.