5 Therefore let us with steadfast piety believe in one God, the Father, and the Son, and the Holy Spirit; let us at the same time believe that the Son is not [the person] who is the Father, and the Father is not [the person] who is the Son, and neither the Father nor the Son is [the person] who is the Spirit of both the Father and the Son. Let it not be supposed that in this Trinity there is any separation in respect of time or place, but that these Three are equal and co-eternal, and absolutely of one nature: and that the creatures have been made, not some by the Father, and some by the Son, and some by the Holy Spirit, but that each and all that have been or are now being created subsist in the Trinity as their Creator; and that no one is saved by the Father without the Son and the Holy Spirit, or by the Son without the Father and the Holy Spirit, or by the Holy Spirit without the Father and the Son, but by the Father, the Son, and the Holy Spirit, the only one, true, and truly immortal (that is, absolutely unchangeable) God. At the same time, we believe that many things are stated in Scripture separately concerning each of the Three, in order to teach us that, though they are an inseparable Trinity, yet they are a Trinity. For, just as when their names are pronounced in human language they cannot be named simultaneously, although their existence in inseparable union is at every moment simultaneous, even so in some places of Scripture also, they are by certain created things presented to us distinctively and in mutual relation to each other: for example, [at the baptism of Christ] the Father is heard in the voice which said, “You are my Son;” the Son is seen in the human nature which, in being born of the Virgin, He assumed; the Holy Spirit is seen in the bodily form of a dove, — these things presenting the Three to our apprehension separately, indeed, but in no wise separated.
6. To present this in a form which the intellect may apprehend, we borrow an illustration from the Memory, the Understanding, and the Will. For although we can speak of each of these faculties severally in its own order, and at a separate time, we neither exercise nor even mention any one of them without the other two. It must not, however, be supposed, from our using this comparison between these three faculties and the Trinity, that the things compared agree in every particular, for where, in any process of reasoning, can we find an illustration in which the correspondence between the things compared is so exact that it admits of application in every point to that which it is intended to illustrate? In the first place, therefore, the similarity is found to be imperfect in this respect, that whereas memory, understanding, and will are not the soul, but only exist in the soul, the Trinity does not exist in God, but is God. In the Trinity, therefore, there is manifested a singleness [simplicitas] commanding our astonishment, because in this Trinity it is not one thing to exist, and another thing to understand, or do anything else which is attributed to the nature of God; but in the soul it is one thing that it exists, and another thing that it understands, for even when it is not using the understanding it still exists. In the second place, who would dare to say that the Father does not understand by Himself but by the Son, as memory does not understand by itself but by the understanding, or, to speak more correctly, the soul in which these faculties are understands by no other faculty than by the understanding, as it remembers only by memory, and exercises volition only by the will? The point, therefore, to which the illustration is intended to apply is this—that, whatever be the manner in which we understand, in regard to these three faculties in the soul, that when the several names by which they are severally represented are uttered, the utterance of each separate name is nevertheless accomplished only in the combined operation of all the three, since it is by an act of memory and of understanding and of will that it is spoken—it is in the same manner that we understand, in regard to the Father, the Son, and the Holy Spirit, that no created thing which may at any time be employed to present only one of the Three to our minds is produced otherwise than by the simultaneous, because essentially inseparable, operation of the Trinity; and that, consequently, neither the voice of the Father, nor the body and soul of the Son, nor the dove of the Holy Spirit, was produced in any other way than by the combined operation of the Trinity.
7. Moreover, that sound of a voice was certainly not made indissolubly one with the person of the Father, for so soon as it was uttered it ceased to be. Neither was that form of a dove made indissolubly one with the person of Holy Spirit, for it also, like the bright cloud which covered the Saviour and His three disciples on the mount, or rather like the tongues of flame which once represented the same Holy Spirit, ceased to exist as soon as it had served its purpose as a symbol. But it was otherwise with the body and soul in which the Son of God was manifested: seeing that the deliverance of men was the object for which all these things were done, the human nature in which He appeared was, in a way marvellous and unique, assumed into real union with the person of the Word of God, that is, of the only Son of God—the Word remaining unchangeably in His own nature, wherein it is not conceivable that there should be composite elements in union with which any mere semblance of a human soul could subsist. We read, indeed, that “the Spirit of wisdom is manifold;” but it is as properly termed simple. Manifold it is, indeed, because there are many things which it possesses; but simple, because it is not a different thing from what it possesses, as the Son is said to have life in Himself, and yet He is Himself that life. The human nature came to the Word; the Word did not come, with susceptibility of change, into the human nature; and therefore, in His union to the human nature which He has assumed, He is still properly called the Son of God; for which reason the same person is the Son of God immutable and co-eternal with the Father, and the Son of God who was laid in the grave—the former being true of Him only as the Word, the latter true of Him only as a man.
8. Wherefore it behooves us, in reading any statements made concerning the Son of God, to observe in reference to which of these two natures they are spoken. For by His assumption of the soul and body of a man, no increase was made in the number of Persons: the Trinity remained as before. For just as in every man, with the exception of that one whom alone He assumed into personal union, the soul and body constitute one person, so in Christ the Word and His human soul and body constitute one person. And as the name philosopher, for example, is given to a man certainly with reference only to his soul, and yet it is nothing absurd, but only a most suitable and ordinary use of language, for us to say the philosopher was killed, the philosopher died, the philosopher was buried, although all these events befell him in his body, not in that part of him in which he was a philosopher; in like manner the name of God, or Son of God, or Lord of Glory, or any other such name, is given to Christ as the Word, and it is, nevertheless, correct to say that God was crucified, seeing that there is no question that He suffered this death in his human nature, not in that in which He is the Lord of Glory.
Source: Letters (New Advent)