Understanding the Blessed Trinity
[A] On the level of nature, we can show that God is a rational being, possessing intellect, power, and will in unlimited measure.
[B] On the level of virtue, we deduce that God is a trinity. God is both object and subject, and God's knowledge of Himself is so complete and unified that it is a kind of a copy of Himself. This copy is the second person of the Trinity, the Image that the Father beholds in His mind, God the Son, who is the fullness of the Father's self-knowledge. Now God's essence is prior to His knowledge of that essence. We say that the Father "begot" the Son. And indeed, there is no reason why their relationship cannot be called somewhat metaphorically "paternity" and "filiation."
Paternity, because although there "was" no actual begetting -- God is one in His eternal present -- the logical priorness is still distinguishable. The begetting of the Son is not by will but by nature. God's self-knowledge and self-love are part of an eternal act of His self-actualization. God is surprised and delighted to realize that He is God. In grasping who He is, God begets the Son; in seeing Himself to be a spectacular and matchless being, He spirates adoration for Himself or the Holy Spirit.
Filiation, because God's knowledge is both true and "true to His essence," as in: conforms to it, never falters from reality, never errs, never sins.
The Holy Spirit is a person for a different reason. It proceeds as love of the will. Now one is what one values or loves. One's preferences, value judgements reveal one's character or personality. Now the deistic God has the following properties. (1) The Father's love of friendship for the Son is identical in meaning to His love of self, as well as to His love of concupiscence, as the Father-subject delights in the Son-object. (2) The Father loves Himself and only Himself, being in fact unaware that anything other than Him exists. (3) The Father finds no flaws in Himself and loves what He sees in the Son with full abandon. As a result, His love is a revelation to whom it may concern of the Father's entire essence and in proceeding from Him, it becomes a third person of the Trinity called the Holy Spirit.
In God, there is no distinction between the ideal and real, or between objectivity and subjectivity. Regarding the Father-Son relation, the Father is the ideal thought; the Son is the reality of the Father's self-actualization. The Son is both objective as the knowledge of the mind, and yet also subjective, as the Father creates His own reality. Regarding the Father-Holy Spirit relation, love is an ideal subjective feeling, yet the procession of love reveals the objective Father and the real Son.
As a reward for the Son's perfect fealty, the Father wills to the Son the perfect good and gives to Him everything He has, and that is Himself or the vision of His whole essence. This gift is the second name of the Holy Spirit. To summarize then, the Father is called Power and Principle; the Son, Knowledge and Image; the Holy Spirit, Love and Gift.
This unqualified integrity coupled with infinity ensure that God has the holiest personality, i.e., that He is a perfectly moral being. Note that in God, the self-regarding and other-regarding virtues are one. God's relations to His creatures are contained virtually in the Father's relation to the Son.
[C] On the level of happiness, we see that God is a unity. God's essence as a thinking being is manifested in God's having thoughts. What are the thoughts about? Himself. God comprehends Himself in a single self-image or self-conception.
But comprehending oneself which in this case is holding of one's image in one's mind is owning oneself which pertains to power. (God's power to create the world is due to His 3rd level of goodness.)
Further, since God's happiness lies not in anything outside God but rather within Himself, for God, again, love of concupiscence is the same as love of self. In keeping Himself in His mind, God ipso facto unites Himself and His understanding of Himself. But love is the only 2nd-level unitive force. Hence, God to His thought is as lover to the beloved. Insofar as God loves the thing He owns, He loves and enjoys Himself.
We see that the act of God's intellect, the act of His power, and act of His will are one and the same thing, namely, God's conceiving and contemplating Himself. Consequently, the distinction between the three faculties is illusory. God is one and supremely so.
When it comes to virtue, God is fully self-conscious; when it comes to happiness, He is fully self-forgetful.
God gives it all He has got (which is both necessary and sufficient); He does not hold back any of His powers from "achieving" happiness, in whatever it lies, such as in His self-contemplation augmented after creation with perfectly executed care for His children. Two things can be predicated of God's happiness: (1) that it is unmixed with any sorrow, except insofar as the sorrows of men are felt in the union of God and humanity through love by all so united; and (2) that it is infinite, e.g., God's capacity for joy meets with no limitations.
We can see that the human and divine natures are the same, consisting of the now familiar three faculties. Here the things attributed to God and creatures can be univocal. The human and divine personalities are completely different or different in kind, because God is a Trinity, and man is not. Predication in this case is equivocal. The human and divine happiness are different in degree, because God is perfectly happy, and man is and can only be happy imperfectly. Regarding this, names are said of God and creatures analogically.
St. Thomas argued that another way to differentiate between the Son and the Holy Spirit is to say that the Father does not proceed at all, the Son proceeds from a single origin, and the Holy Spirit proceeds from two. Knowledge follows upon essence, and love, upon both. And that is also why the Father is the "first" person; the Son, "second"; and the Holy Spirit, "third."
With this understanding of the Trinity we arrive at a surprising and non-trivial corollary. It is impossible that there could exist a being which has at least one but no more than two of the "omni" properties of God. In other words, either there exists a being who is omnipotent (p), omniscient (s), and all-loving (l) at the same time -- the deistic God -- or there are only "us" -- beings who are not omnipotent and not omniscient and not all-loving. Thus, there cannot be, for example, a being who is all-powerful, all-knowing, and yet not all-loving. The proof is as follows:
(1) Suppose ~p, s, l. Then God's generative power is defective. Therefore the Son cannot be equal to the Father, the object ≠ the subject, and ~s.
(2) Suppose p, ~s, l. Then God does not know himself completely. Since what is not known cannot be loved, ~l.
(3) Suppose p, s, ~l. If God does not love something within himself, then he must not be perfectly happy. But given p and s, he can remove the imperfection "instantly." Therefore l.
Alternatively, if God does not love what he knows to be perfect, then he is not really perfect after all. Therefore either ~p or ~s or both.
What if there is no love in God at all and the Father and the Son are indifferent to each other? Then, since love is a unitive force and the Holy Spirit unites the two into one simple being, there would have to be two gods, not one. But that is impossible (for other reasons). Same conclusion follows.
(4) ~p, ~s, l. There is, we suppose, a being whose essence is incomplete (in fact, finite) and with limited knowledge who nonetheless loves perfectly. But one can only love what one has or hopes to attain. Since here is no way for our finite God to "become" the infinite Christian God by any finite or even potentially infinite process, such love would be an overkill and an absurdity. Such a God could still be compassionate, but he would be aware of his limitations, have needs, and therefore could not love unconditionally 100%. He would be like a good angel or a human saint. Therefore, l is false. The disproof of the remaining two propositions is left as an exercise for the reader.
To end on a relaxed note, we can even connect all this with politics. There's the doctrine of the separation of powers. The branches of government correspond to the Trinity as follows: the Father is the lawgiver ("He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous." (Mt 5:45)), the Son is the judge ("Moreover, the Father judges no one, but has entrusted all judgment to the Son." (Jn 5:22)), the Holy Spirit is the enforcer of the law ("Do not cast me from your presence or take your Holy Spirit from me." (Ps 51:11)).
So separation of powers is not merely a useful tool of limiting government if we are to have government at all; the fact that political powers can be separated by the intellect (though not, of course, the question of whether they ought to be separated or how) is grounded in the nature of God Himself.