The Trinity is perhaps the most distinctive Christian doctrine and demonstrates a unique understanding of what is meant by the word God. In the mystery of the Trinity, we confess there is one God in three persons (CCC 253). These persons are not distinguished in substance, nature, or essence. Although this is a mysterious mode of being which we cannot fully understand, it is certainly possible to further our understanding so as to know God better. In order to clarify what is meant by this it is necessary to clearly define what is meant by these terms.
Essence, nature, and substance are closely related to one another. The essence of a thing is whatever is necessary to be that thing and also to distinguish it from other things. Nature connotes the same reality as essence, but with a logical distinction of acting or operation rather than being. Substance denotes individual existence and the subject for any accidental properties, for example a board cut in half is still of the same substance with an accidental change in size, but a board burnt to ashes is subject to a substantially different mode of existence (Week 2). When understood in human terms we can subtly differentiate these ideas although the differences may be virtual rather than real. A man can be said to be a rational animal that is composed of body and soul for a determined end, subsisting in an individual body and soul whose existence is contingent and not necessary. God, on the other hand, exists necessarily and is not composed of parts nor subject to change or accidental properties (ST Q 3 A 7 co). His existence is not caused and does not differ from His essence and so there cannot be a real distinction in His essence, nature, or substance (ST Q 3 A 4 co).
Boethius defined a person as a substance with a rational nature (Garrigou-Lagrange III.I). Each person is individual, complete, endowed with intellect and exists independently (Week 2). In the east the Greek term hypostasis came to convey the idea of personhood, although originally hypostasis often connoted the substance of a thing. St Basil distinguished hypostasis (individual and real) from ousia which signified the substance (Garrigou-Lagrange III.II). Thus, person is really distinct from substance and essence. This distinction is critically important to understanding not only the Trinity, but related concepts such as the Incarnation and the hypostatic union.
All of these definitions and distinctions are important to understand the Trinity, however, reason by itself is not sufficient to arrive at knowledge of the Trinity (Garrigou-Lagrange VI .I). Our reason must operate within the bounds of sense information and so we can reason toward God from his effects: those things that He created but we cannot perceive His essence (ST Q 12 Art 12 co). This does provide us with the ability to know a great deal about Him analogically such as His existence and certain properties of His existence. However, since God’s “creative power is one and the same in the three persons” it pertains to His nature which is one and does not reveal the plurality of persons in God (Garrigou-Lagrange Ch VI.I.4).
Fortunately, God reveals the mysteries of His intimate life for our salvation (ST Q1 A1 co). These mysteries are handed down in faith, but the natural order confirms their truth (Garrigou-Lagrange VI.I). God was not compelled in His creation nor in our redemption. The revelation of the Trinity enables the correct understanding of the Incarnation and the gifts of the Holy Spirit which are only comprehensible through the Trinity (ibid). Although reason cannot prove the Trinity, even after it was revealed, it can show that the Trinity and related teachings are not in opposition to natural reason.
As regards divine personhood we must then examine what distinguishes the person from the essence among the Divine Persons to have a better understanding of the Trinity. Each person is an intelligent and free substance, or in other words, a subject with intellect and will (Garrigou-Lagrange III.I). A person signifies the who: that which thinks, chooses, and experiences. Whereas essence signifies the kind of thing something is. The person unites the essence with existence whereby it exists separately from other like essences, one individual man is different then another although they share the same nature. These are closely related since each person must have a nature; indeed, human persons are agents with a human nature or essence. Our experience as human can help us understand person and essence, and in so doing, help us understand God better by predicating these categories to God analogically.
The divine essence is necessarily one because there can be only one infinitely perfect being. Among other reasons, if there were more than one, they would need to be distinguished by some feature. This means that one of them either possesses a perfection the other lacks or has a privation and could not be perfect (ST Q 11 A 3 co). Further, since every perfection is attributed to God and person signifies the most perfect kind of subsistence, namely that with a rational nature, person can be rightly predicated to God analogically (Garrigou-Lagrange III.III). Thus, there is one God who is a personal being. God exists independently with intellect and will, having every perfection.
A person signifies a distinct substance with an intellectual nature (Garrigou-Lagrange III.I). Yet, in the Trinity it is claimed that God is three persons and above that there can only be a single being with the divine essence. The persons of the Trinity are not distinguished by substance and being but rather by real relations of origin (CCC 254). These relations are based on action internal to God, processing from intellect and will (ST Q 28 A 4 co). Each person subsists as a free and intelligent subject but understands with the same essential intellection and loves with the same essential love (Garrigou-Lagrange III.IV).
When a man generates children, a child is produced as an external action whereas the child is an individual, independent substance from the father. His nature is communicated externally. The generation of the Word by the Father is not so. This generation is an internal action of intellect through which the Father begets the Son (Garrigou-Lagrange I.I). Begetting implies an origin of a like nature, however, since God is in no way potential this action must be eternally actualized (Garrigou-Lagrange I.II). The Son shares in the divine essence which is indistinguishable from the essence of the Father which is wholly identical because the infinite essence of God is not divisible and is not multiplied in this action (Garrigou-Lagrange “Post-Nicene Testimonies”). In God essence and existence are the same and so the Father and the Son are the same being. Yet, the Son is distinguishable from the Father as a subject. The one begotten is distinguished from the one who begets. The Father and the Son are two persons who share one divine nature.
In a like manner, a third person proceeds from the Father and Son by internal action of will (ST Q27 A3 co). Again, the one divine essence is communicated to the person of the Holy Spirit who is a separate subject while preserving oneness in being. Thus, the Spirit proceeds from the Father and Son and all three are united with the same essential intellection and love. As such, the Trinity of persons is reconciled with the unity of God by way of consubstantiality.
The Trinity is a critical concept in Christianity. Although we could not learn of the Trinity by reason alone, we can use reason explore this mysterious mode of being and are able to demonstrate that it is not unreasonable. This does not mean that it is easily or fully understood. God’s revelation about His nature reveals His love for us in our creation and redemption. In turn, we can better love God as we know Him more deeply.
Aquinas, Thomas. Summa Theologiae, Kevin Knight, 2017, www.newadvent.org/summa/. Accessed 18 March 2022.
Catholic Church. Compendium OF THE CATECHISM OF THE CATHOLIC CHURCH. 2005, www.vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html. Accessed 18 March 2022.
Garrigou-Lagrange, Reginald. The Trinity and God the Creator. “EWTN Global Catholic Television Network”, www.ewtn.com/catholicism/library/trinity-and-god-the-creator-10197. Accessed 18 March 2022.