We understand God to be God, but He’s not so easy to grasp.
1 From what has been said, then, one must hold that in the divine nature three Persons subsist: the Father, the Son, and the Holy Spirit; and that these three are one God, distinguished from one another by relations only. For the Father is distinguished from the Son by the relations of paternity and innascibility; the Son from the Father by the relation of sonship; the Father and the Son from the Holy Spirit by spiration, so to say; and the Holy Spirit from the Father and the Son by the procession of love, by this He proceeds from each of Them.
Notes This helps in discussing the nature of God with Muslims: distinguished from one another by relations only“. One God still!
2 Beside these three Persons, no fourth in the divine nature can be asserted. For the divine Persons, since they agree in essence, cannot be distinguished except by relation of origin, as is clear. These relations of origin one must understand not as a procession which inclines to what is without—for what proceeds thus is not co-essential with its principle—one must understand them as proceeding within.
Of course, a thing which proceeds and remains its own principle is found only in the operation of the intellect and will, as was made clear.
Hence, the divine Persons cannot be multiplied save by the requirements of the procession of the intellect and will in God. It is, of course, not possible that there be in God more than one proceeding within His understanding, because His act of understanding is one, simple, and perfect, for in, understanding Himself He understands all things else.
And thus, there can be in God but one proceeding of the Word. In like manner, too, must the proceeding of Love be one only, for the divine will act is one and simple—by loving Himself He loves all things else.
Therefore, it is not possible that in God there be more than two Persons proceeding: one by way of intellect, as Word—namely the Son; the other by way of Love, as the Holy Spirit. There is also one Person who does not proceed—namely, the Father. Therefore, in the Trinity there can be only three Persons.
3 Again, let the divine Persons be distinguished by proceeding. But the mode of a person in proceeding can be but threefold: namely, to be altogether not proceeding, which is the Father’s mode; to be proceeding from one who does not proceed, which is the Son’s; to be proceeding from one who Proceeds, which is the Holy Spirit’s. Therefore, it is impossible to assert more than three Persons.
4 We grant, of course, that in other living things relations of origin can be multiplied—for example, in human nature there can be many fathers and many sons—but in the divine nature this is altogether impossible. For sonship, since in one nature it is of one species, cannot be multiplied except by matter or by subject; this is also the case with other forms. Hence, since in God there is neither matter nor subject, and since the relations are themselves subsistent (which is clear from what was said above) it is impossible that there be a plurality of sonships of God. The same reasoning holds for the other Persons. Thus, in God there are only three Persons.
5 Of course, an objector may say that in the Son who is perfect God there is infinite intellective power, and thus He can produce a word; in like fashion, since there is in the Holy Spirit infinite goodness which is the principle of communication, He will be able to communicate the divine nature to another person.
But such a one ought to consider that the Son is God, as begotten not as begetting; and so the intellective power is in Him as proceeding in the way of the Word, and not in Him as producing the Word. Similarly, since the Holy Spirit is God as proceeding, there is infinite goodness in Him as the Person receiving, and not in Him as communicating the infinite goodness to another. For the Persons are not distinguished from one another except by relations, as is clear from the things said above.
Therefore, all the fullness of divinity is the Son, numerically identical with that in the Father, but with the relation of birth, as it is in the Father with the relation of active generation. Hence, if the relation of the Father be attributed to the Son, all distinction is removed. And the same reasoning holds for the Holy Spirit.
Notes The next argument really helps firm things up. Consider how your consider yourself. There is something more there than a “blind” machine.
6 Now, this divine Trinity has a likeness in the human mind which we can consider. For the mind itself, because it understands itself, conceives within itself a word. And this is nothing but the intelligible intention of the mind, which is called the mind understood and exists within the mind. When this mind further loves itself, it produces its very self in the will as beloved. Of course, it does not proceed further within itself, but the cycle is concluded when by love it returns to the very substance from which the proceeding began by the intention understood.
The proceeding extends to external effects when from love of itself it proceeds to make something. Thus, three things are discovered in the mind: the mind itself, the source of the proceeding, existing in its nature; and mind conceived in the intellect; and mind beloved in the will. For all that, these three are not one nature, for the mind’s act of understanding is not its being; and its will act is neither its being, nor its act of understanding. For this reason, also, the mind understood and the mind beloved are not persons, since they are not subsisting. Even the mind itself existing in its nature is not a person, for it is not the whole which subsists, but a part of the subsistent; namely, of the man.
7 Therefore, in our mind one finds a likeness of the divine Trinity in regard to proceeding, “and this multiplies the Trinity.” For from the exposition this is clear: there is in the divine nature God unbegotten, who is the source of the whole divine proceeding, namely the Father; there is God begotten by way of a word conceived in the intellect, namely the Son; there is God by way of love proceeding, namely the Holy Spirit. Of course, no further proceeding is discovered within the divine nature, but only a proceeding to exterior effects.
In this, of course, the mind fails in representing the divine Trinity: the Father and the Son and the Holy Spirit are one in nature, and in each of these the person is perfect, simply because the act of understanding and the act of will are the divine being itself, as was shown. For this reason one considers the divine likeness in man just as one considers the likeness of Hercules in stone: with regard to the representation of form, not with regard to the agreement of nature. And so one says that in the mind of man there is the “image of God” according to the Word: “Let us make man to our image and likeness” (Gen. 1:26).
8 One also finds in other things a likeness of the divine Trinity, so far as anything in its substance is one, formed in a kind of species, ordered in some fashion. just as is clear from the things said, the conception of the intellect in intelligible being is like the species formation in natural being, love, of course, is like the inclination or order in a thing of nature. And so the species of things in nature from afar represent the Son; their order, of course, the Holy Spirit. Accordingly, by reason of the remote and obscure representation in irrational things, one speaks of the “vestige” of the Trinity in them, not of the “image”; so we read in Job (11:7): “Would you comprehend the steps of God?” and so forth.
9 And this is enough to say about the divine Trinity for the present.