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First, we need to define what we mean by "person." The traditional formulation of the Trinity is "one God in three persons." However, in modern language, the concept of "person" is practically identical with "being," and it has connotations such as "personality." Within the Trinity, these are "persons" only in a relative sense, in which sense the Father and the Son are not "persons" either, in the same way that humans are persons. It is more correct to speak of three subjects (supposita) or three subsistencies.
The Holy Spirit is truly God—coequal, consubstantial, and coeternal with the Father and the Son. He is not a mere force, energy, or divine influence, nor is He a separate deity alongside the Father and the Son. He is the third Person of the Blessed Trinity, fully God, yet distinct in personhood. To speak clearly and faithfully, we must begin by carefully defining what it means to be a “person” in the Trinity, and how this applies to the Holy Spirit.
The term “person” in Trinitarian theology does not carry the modern psychological or behavioral meaning it often does in casual speech today. It does not refer to personality traits or psychological self-awareness. Rather, in classical Catholic theology, especially as defined by Boethius and developed by Aquinas, a “person” is an individual substance of a rational nature. In God, however, we must refine this further, since God's nature is not divided among the Persons. In the Trinity, there is one divine essence, one intellect, one will—shared entirely and equally among the Father, the Son, and the Holy Spirit. The distinction among the Persons lies not in what they are, but in how they are related. The Father is unbegotten, the Son is begotten of the Father, and the Holy Spirit proceeds from the Father and the Son as from one principle.
Therefore, the Holy Spirit is not “another god,” but the same God. He is the same divine being who is eternally breathed forth as the mutual Love between the Father and the Son. He is not a separate mind, not a separate nature, but He is a distinct person—a unique I within the Godhead. This can be difficult to grasp because our human concept of “personality” is often tied to distinct centers of consciousness, and this creates confusion. Some ask: does the Trinity contain three egos, three “I”s, three consciousnesses? Are we not then veering toward tritheism? But this is not how the Church speaks. In fact, we must distinguish carefully: while there are indeed three Persons—and hence three I’s—there is only one divine essence, and therefore only one divine intellect and one divine will. So while the Father can say “I,” and the Son can say “I,” and the Holy Spirit can say “I,” these three Persons do not constitute three separate minds, nor do they each possess a different divine essence. They each fully are the one and only God.
To understand this, the analogy often employed by the Church Fathers and developed by Aquinas is that of relations. In God, the Persons are distinct only by their relations of origin. The Father is the origin without origin. The Son is eternally begotten, meaning He receives the fullness of the divine nature from the Father in an eternal act of generation. The Holy Spirit proceeds not by generation but by spiration—He is spirated by the mutual love of the Father and the Son. He is not their creation, not their product in time, but their eternal gift to each other, their shared breath of divine Love. That is why the Church speaks of Him as the “Gift,” the bond of charity, the living flame of divine communion. And yet, as the Nicene Creed proclaims, He is “the Lord and Giver of Life, who proceeds from the Father and the Son, who with the Father and the Son is adored and glorified.” He is not a mere expression of God’s power or presence—He is God.
This understanding guards us from two major errors: on one side, modalism, which says that God is only one person appearing in three different “modes” or roles; and on the other side, tritheism, which would treat the Father, Son, and Holy Spirit as three distinct gods. The Catholic faith avoids both extremes by carefully affirming one essence in three Persons. The Holy Spirit is not simply the “power” of God, as some heretical sects suggest. He is not a faceless “force” like in pantheistic systems. A force does not speak, does not love, does not intercede, does not grieve. But the Holy Spirit does all of these things in Scripture. He teaches (John 14:26), He intercedes (Romans 8:26), He distributes gifts (1 Corinthians 12:11), He is lied to (Acts 5:3–4), He sends (Acts 13:2), and He is named equally with the Father and the Son in baptism (Matthew 28:19). These actions are not those of an impersonal energy but of a divine person.
The Holy Spirit, then, is not a separate deity, nor is He a mere function or action of God. He is the third Person of the Most Holy Trinity. He is the Love breathed eternally between the Father and the Son. And because love is not something God has but something God is (1 John 4:8), the Holy Spirit is not simply what unites the Trinity but who unites the Trinity. He is not the passive effect of the Father and the Son, but a Person in His own right—possessing the fullness of divinity, acting with divine authority, and worthy of divine worship.
The Thomistic tradition preserves this mystery by maintaining the essential unity of God’s being while also upholding the real distinction of persons. God is one substance, one divine being, but He exists as three subsistent relations. This language may seem technical, but it protects the mystery: we do not invent explanations out of human categories. Rather, we confess what has been revealed—what Christ has spoken, what the Spirit has inspired, and what the Church has handed on with clarity and reverence. In this confession, we adore not three gods, but one God in three Persons. And in this mystery, we discover that God is not solitary, but eternally relational: an infinite communion of love, into which we are drawn by grace.
So when we ask, “Who is the Holy Spirit?”, the answer is not a what but a who. He is the Lord, the Giver of Life, the Sanctifier, the Teacher, the Advocate. He is the divine Person who comes to dwell in the souls of the baptized, making them temples of God. He is not an impersonal presence but the personal Love of the Father and the Son, poured into our hearts. And by Him, we cry out “Abba, Father” (Romans 8:15), because He draws us into the inner life of the Trinity, where the Love that is God becomes our own life and joy forever.