This article is written to provide fundamental concepts in preparation for my next article, Is God He or She, which explains why God is Father, not mother. It draws from St. Thomas Aquinas’ perennial truths about the One and Triune God. In his Compendium of Theology, Thomas begins with God’s unity of Substance and then moves to His triuninty of Persons.
God’s Unity of Substance
God is one Being just as you and I are one being. However, whereas we are body-soul composites, God is one Being who is one spiritual Substance. He is not a composition of substances. God cannot be broken down into parts, separated, or the like. His spiritual Substance is His complete, infinite, eternal, immutable, spiritual essence; in one word, He is Divinity.
Accordingly, God’s intellect and will are the same as his essence. He is not part intellect and part will, and His essence cannot be divided. Unlike humans, God is intellect and will, whereas we have intellect and will. Humans understand by moving from potential to actual understanding, but God is pure act. Since God knows and acts perfectly and eternally, He is essentially (of the essence) intellect and will.
The key for understanding what follows is that God is substantially and essentially one. Only by understanding his unity can we begin to understand His triunity.
Trinity of Persons in God
As the Intelligent Being, God is intellect and will from whom all other intelligent beings (i.e. angels and humans) receive their intellects and wills. The intellect beholds objects that enter it, and the will moves us toward objects we perceive to be good.
Our wills are attracted to the good and repulsed by the evil our intellects behold. Even though we sometimes desire what is evil, we desire it because it brings about some good (e.g. pleasure, relaxation, satiation, etc.). For example, pleasure is good, but if we obtain it by evil means, or abuse the faculties in which pleasure is found, we commit sin.
In God, however, sin and evil do not exist. His knowledge of evil is good in the way that it is good for a judge to know evil for justice to be rightly executed. Therefore, God’s intellect eternally beholds Himself as the greatest good, without admixture of evil, which is pure and total perfection, and His will eternally loves the greatest good (Himself).
The Son, Word, Image of God
When the intellect beholds and understands an object, a conception is formed. When you and I know something, we know a particular thing. Then we abstract its essences (i.e. its “whatness”) and apply our knowledge of that thing to other similar things. But God knows Himself perfectly.
By knowing Himself perfectly, God generates a perfect Conception of Himself. This Conception is distinct, not separate or different, from the Knower and is properly called Image. Accordingly, the Knower of the Conception is Father and the Conception, who is the perfect Image of the Father, is Son. A word communicates a conception in some way. Therefore, the Son is also the Word.
Given what we know so far, it would seem that the Father knows the Son, but not conversely. However, unlike humans who generate concepts, even about themselves, their conceptions do not have intellects.
With God, however, He is His intellect; he does not simply have an intellect. Therefore, the Son who is the perfect Image of the Father, has everything the Father has, including His intellect and will. Thus, the Father and the Son have the same intellect and will. Since the Father knows Himself perfectly, and the Son has all that belongs to the Father, the Son knows the Father perfectly.
The Holy Spirit, The Love of God
Understanding how the Holy Spirit is the third Person of the Trinitarian God is a little more difficult to grasp because we cannot imagine love in the same way we can imagine an image. Nevertheless, by understanding that “as the object known is in the knower, as known, so the beloved must be in the lover, as loved.” Thomas says,
[U]nderstanding takes place by a certain assimilation of the knower to the object known; and so, the object known must be in the knower in the sense that a likeness of it is present in him. But the act of loving takes place through a sort of impulse engendered in the lover by the beloved; the beloved draws the lover to himself. Accordingly, the act of loving reaches its perfection, not in a likeness of the beloved, in the way that the act of understanding reaches perfection in a likeness of the object understood, but in a drawing of the lover to the beloved in person.
Since God is not simply an intellect that is an infinite storehouse of knowledge, but also a will that wills the greatest good, He beholds the Son as Him who is loved, and the Son beholds the Father as one who is loved. Therefore, the Love who proceeds from the Father and the Son is a third Person of the Trinity, the Holy Spirit. Just as the Son is the Word (Truth) personified, the Holy Spirit is Love personified.
Additionally, the Father and the Son breath the Holy Spirit (Latin: spiritus, breath) with one breath because the Son has the same intellect and will as the Father. Therefore, when we say in the Nicene Creed that the Holy Spirit proceeds from the Father and the Son, this procession is by way of one eternal breath of love.
Finally, from what has been stated above, relation is the only distinction among the three Persons. The Father, Son, and Holy Spirit do not differ in any way other than by their relations. The Father is not the Son or the Holy Spirit, the Son is not the Father or the Holy Spirit, and the Holy Spirit is not the Father or the Son. Other than this relational distinction, the Father, Son, and Holy Spirit have all things in common.