Concerning the Holy Trinity
The terms, 'Word' and 'effulgence,' then, are used because He is begotten of the Father without the union of two, or passion, or time, or flux, or separation: and the terms 'Son' and 'impress of the Father's subsistence,' because He is perfect and has subsistence and is in all respects similar to the Father, save that the Father is not begotten: and the term 'Only-begotten' because He alone was begotten alone of the Father alone. For no other generation is like to the generation of the Son of God, since no other is Son of God. For though the Holy Spirit proceeds from the Father, yet this is not generative in character but processional. This is a different mode of existence, alike incomprehensible and unknown, just as is the generation of the Son. Wherefore all the qualities the Father has are the Son's, save that the Father is unbegotten, and this exception involves no difference in essence nor dignity, but only a different mode of coming into existence. We have an analogy in Adam, who was not begotten (for God Himself moulded him), and Seth, who was begotten (for he is Adam's son), and Eve, who proceeded out of Adam's rib (for she was not begotten). These do not differ from each other in nature, for they are human beings: but they differ in the mode of coming into existence.
For one must recognise that the word ἀγένητον with only one 'ν ' signifies “uncreate” or “not having been made,” while ἀγέννητον written with double 'ν ' means “unbegotten.” According to the first significance essence differs from essence: for one essence is uncreate, or ἀγένητον with one 'ν,' and another is create or γενητή. But in the second significance there is no difference between essence and essence. For the first subsistence of all kinds of living creatures is ἀγέννητος but not ἀγένητος. For they were created by the Creator, being brought into being by His Word, but they were not begotten, for there was no pre-existing form like themselves from which they might have been born.
So then in the first sense of the word the three absolutely divine subsistences of the Holy Godhead agree: for they exist as one in essence and uncreate. But with the second signification it is quite otherwise. For the Father alone is ingenerate, no other subsistence having given Him being. And the Son alone is generate, for He was begotten of the Father's essence without beginning and without time. And only the Holy Spirit proceeds from the Father's essence, not having been generated but simply proceeding. For this is the doctrine of Holy Scripture. But the nature of the generation and the procession is quite beyond comprehension.
And this also it behooves us to know, that the names Fatherhood, Sonship and Procession, were not applied to the Holy Godhead by us: on the contrary, they were communicated to us by the Godhead, as the divine apostle says, Wherefore I bow the knee to the Father, from Whom is every family in heaven and on earth. But if we say that the Father is the origin of the Son and greater than the Son, we do not suggest any precedence in time or superiority in nature of the Father over the Son (for through His agency He made the ages), or superiority in any other respect save causation. And we mean by this, that the Son is begotten of the Father and not the Father of the Son, and that the Father naturally is the cause of the Son: just as we say in the same way not that fire proceeds from light, but rather light from fire. So then, whenever we hear it said that the Father is the origin of the Son and greater than the Son, let us understand it to mean in respect of causation. And just as we do not say that fire is of one essence and light of another, so we cannot say that the Father is of one essence and the Son of another: but both are of one and the same essence. And just as we say that fire has brightness through the light proceeding from it, and do not consider the light of the fire as an instrument ministering to the fire, but rather as its natural force: so we say that the Father creates all that He creates through His Only-begotten Son, not as though the Son were a mere instrument serving the Father's ends, but as His natural and subsistential force. And just as we say both that the fire shines and again that the light of the fire shines, So all things whatsoever the Father does, these also does the Son likewise. But whereas light possesses no proper subsistence of its own, distinct from that of the fire, the Son is a perfect subsistence, inseparable from the Father's subsistence, as we have shown above. For it is quite impossible to find in creation an image that will illustrate in itself exactly in all details the nature of the Holy Trinity. For how could that which is create and compound, subject to flux and change, circumscribed, formed and corruptible, clearly show forth the super-essential divine essence, unaffected as it is in any of these ways? Now it is evident that all creation is liable to most of these affections, and all from its very nature is subject to corruption.
Likewise we believe also in one Holy Spirit, the Lord and Giver of Life: Who proceeds from the Father and rests in the Son: the object of equal adoration and glorification with the Father and Son, since He is co-essential and co-eternal: the Spirit of God, direct, authoritative, the fountain of wisdom, and life, and holiness: God existing and addressed along with Father and Son: uncreate, full, creative, all-ruling, all-effecting, all-powerful, of infinite power, Lord of all creation and not under any lord: deifying, not deified: filling, not filled: shared in, not sharing in: sanctifying, not sanctified: the intercessor, receiving the supplications of all: in all things like to the Father and Son: proceeding from the Father and communicated through the Son, and participated in by all creation, through Himself creating, and investing with essence and sanctifying, and maintaining the universe: having subsistence, existing in its own proper and peculiar subsistence, inseparable and indivisible from Father and Son, and possessing all the qualities that the Father and Son possess, save that of not being begotten or born. For the Father is without cause and unborn: for He is derived from nothing, but derives from Himself His being, nor does He derive a single quality from another. Rather He is Himself the beginning and cause of the existence of all things in a definite and natural manner. But the Son is derived from the Father after the manner of generation, and the Holy Spirit likewise is derived from the Father, yet not after the manner of generation, but after that of procession. And we have learned that there is a difference between generation and procession, but the nature of that difference we in no wise understand. Further, the generation of the Son from the Father and the procession of the Holy Spirit are simultaneous.
All then that the Son and the Spirit have is from the Father, even their very being: and unless the Father is, neither the Son nor the Spirit is. And unless the Father possesses a certain attribute, neither the Son nor the Spirit possesses it: and through the Father, that is, because of the Father's existence, the Son and the Spirit exist, and through the Father, that is, because of the Father having the qualities, the Son and the Spirit have all their qualities, those of being unbegotten, and of birth and of procession being excepted. For in these hypo static or personal properties alone do the three holy subsistences differ from each other, being indivisibly divided not by essence but by the distinguishing mark of their proper and peculiar subsistence.
Further we say that each of the three has a perfect subsistence, that we may understand not one compound perfect nature made up of three imperfect elements, but one simple essence, surpassing and preceding perfection, existing in three perfect subsistences. For all that is composed of imperfect elements must necessarily be compound. But from perfect subsistences no compound can arise. Wherefore we do not speak of the form as from subsistences, but as in subsistences. But we speak of those things as imperfect which do not preserve the form of that which is completed out of them. For stone and wood and iron are each perfect in its own nature, but with reference to the building that is completed out of them each is imperfect: for none of them is in itself a house.
Source: Exposition of the Faith (New Advent)