The Trinity is perhaps the most profound and important teaching of Christianity. In the mystery of the Trinity, we confess that there is one God in three persons. In other words, the persons of the Trinity as not distinct from one another in substance, nature, or essence. Although it is difficult for us to understand this fully it is possible to know God better by reason.
Reason on its own is not sufficient for us to arrive at knowledge of the Trinity. All human knowledge operates within the bounds of sense information. Hence, everything we know about God naturally is necessarily connected with creation. We understand God through his effects and natural principles but are not able to perceive Him essentially (ST I, Q. 12, A.12, co). In the Trinity, the effects proceed from God’s omnipotence or creative power. Yet, these are common to each of the persons of the Trinity and we cannot distinguish between the persons of the Trinity from this alone (Garrigou-Lagrange VI.I.4). Hence, natural reason is not up to the task of knowing the Trinity.
Fortunately, we do not need to rely on reason alone. God reveals the mysteries of His intimate life for our salvation (ST I QI AI co). We find ample evidence of the Trinity in scripture. The Old Testament demonstrates the Trinity, but not clearly without the aid of the New Testament (Pohle PI CH I). In the Old Testament, we find many references to the plurality of God which are not opposed to His unity. For example, in the creation we find references to God making man “in our own image, after our likeness” and is described as becoming like “us” (Genesis 1:26, 3:22). At the oaks of Mamre the Lord is described as appearing like three men (Genesis 18:1-2). In the book of Isaiah, we find the Trisagion (Isaiah 6:3). This triple call of Holy points towards God’s plurality (Pohle PI Ch I SI.I). While these hint at God’s plurality, we see clearer evidence in the New Testament.
In passages that describe the Baptism of the Lord, we see explicit evidence of the clear distinction of designation and operations in the interactions between the Father, Son, and the Holy Spirit. When the Son is baptized the Father speaks from heaven. He declares that Jesus is His Son (Matthew 3:17). In the Gospel of Luke, we see the heavenly origin of the Holy Spirit who descends from heaven (Luke 3:22). In fact, He is described as the Spirit of God which is God himself (Garrigou-Lagrange Introduction). Each person addresses each other by distinct names. We can see that the Father is not the Son because he calls the Son by name (Garrigou-Lagrange Introduction). The Holy Spirit descends from heaven while the Father addresses the Son.
Further, at the Last Supper, Christ tells his disciples that he would go to the Father and send them the paraclete (John 14:16-26). We see a clear indication that Christ is distinct from both Father and Holy Spirit. In addition, the Father must be distinct from the Holy Spirit because the Father is sending Him (Pohle PI Ch I SII). We also find Jesus describing himself as one with the Father. Taken together, we find both distinction and unity.
However, it is not clear how this distinction and unity are to be understood together. While the mysteries of the Trinity are handed down by faith, we can still use reason to confirm their truths (Garrigou-Lagrange VI.I). 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 I, 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, personhood 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.
When discussing created things we can see that they are composed. For example, man is a rational animal composed of body and soul for a determined end. He subsists in an individual body and soul whose existence is contingent and not necessary. On the other hand, God is composed in no way and exists necessarily nor is He subject to change or accidental properties (ST I, Q. 3, A. 7, co). The question remains, if there is only one God and that God is perfectly simple, then what does it mean for God to be three persons?
Corporeal beings act outside of themselves (Garrigou-Lagrance I.I). However, like a man’s intellect can produce a thought internally, God’s processions are ad intra. There are two immanent actions which are to know and to will. In God, intellection is the same as being because God’s Word produces substantially and communicates His nature (Garrigou Lagrange I.I). This does not imply any change but generates likeness without any movement of non-being to being. In human generation, children receive the same nature as their parents although they are distinct substances. In the case of God, Father and Son also have the same nature. However, in God nature and being are the same, and hence they are consubstantial.
This also holds true regarding the action of volition (Garrigou-Lagrange I.III). The Holy Spirit is the terminus of the procession of love just as the Word is the terminus of the intellectual procession. God is simple and therefore the divine nature is also communicated by the procession of love (Garrigou-Lagrange I.III). Yet, these two processions are distinct from one another because “intellect assimilates a thing to itself… but the will by its nature is not an assimilative faculty or power” (Garrigrou-Lagrange I.IV). Thus, from these two processions, there arise four relations which are: paternity, filiation, active spiration, and passive spiration. However, a father cannot be his own son. In this way, we see the distinction between the persons of the Trinity: opposition of relation. Father is distinct from Son because of the opposition of paternity and filiation (Garrigou-Lagrange II.III). Father and Son are distinct from the Holy Spirit because of the opposition of active and passive spiration. However, there is no opposition in paternity/spiration or filiation/spiration and so no other distinction is possible (Garrigou-Lagrange II.IV). Hence, there are three persons who are distinct from one another, but share the same essence and being. However, there is no composition because in God relation really is essence (ST I, Q. 28, A. 3, AD. 1).
Great care must be taken when speaking about the Holy Trinity because errors can be made both in denying the unity or the plurality of God. It is the case that the divine nature is one and so names that signify the divine essence substantively are predicated to the three persons singly (Garrigous-Lagrange XIII.III). On the other hand, words that describe the divine essence adjectivally are predicated plurally. Great care must be taken to not express a distinction of nature or any inequality between the persons of the Trinity. At the same time, we must refrain from statements that exclude a plurality of persons such as singular or solitary.
Although we cannot know the Trinity from reason it can help us to know there is no contradiction in the dogma of the Trinity (“Elucidation of the Mystery of the Holy Trinity”).In addition, although speaking precisely about the Trinity is difficult we can employ language in such a way as to clarify what is meant by terms and not use language which confuses the issue. Finally, God reveals Himself to us and through that revelation, we come to know Him better