Many of the words Christians use to talk about God can seem unfamiliar at first; terms like essence, person, eternal generation, and divine simplicity. They may sound technical, but they belong to the whole church, not just theologians.
These words aren't additions to Scripture. They are careful ways the church has learned to summarize and protect what Scripture teaches about God. They help us speak truthfully about the Father, Son, and Holy Spirit and guard the gospel from error.
Theology matters because words matter, and words matter because God has spoken. This brief glossary is designed to help you better understand these key Trinitarian terms and speak more faithfully about the God who has revealed Himself.
Trinity
God is one divine being in three persons: Father, Son, and Holy Spirit. The Father is truly God, the Son is truly God, and the Holy Spirit is truly God, yet there are not three gods but one God. The three persons eternally exist in perfect unity, sharing the one undivided divine essence.
The word Trinity does not appear in Scripture, but the reality it names does, and the church has rightly used this word to summarize what Scripture teaches.
"Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit." , Matthew 28:19
Divine Person (or Subsistence)
A divine person is a real, eternal distinction within the one God. The Father is not the Son, the Son is not the Spirit, and the Spirit is not the Father. These are not three roles God plays in sequence, nor three names for the same undifferentiated deity, but three genuinely distinct persons who eternally exist as the one God.
The persons are distinguished not by essence but by eternal relations of origin: the Father is unbegotten, the Son is begotten of the Father, and the Spirit proceeds from the Father and the Son. Person answers the who question. The older tradition also uses the word subsistence for this reality, acknowledging the limits of any human term applied to the being of God.
"But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things." , John 14:26
Divine Essence
Essence refers to the one divine being or nature shared fully and indivisibly by all three persons. Each person of the Trinity is not a part of God, nor does each possess only a portion of deity. Each person is truly and wholly the one God.
Essence answers the what question. The Father, Son, and Holy Spirit each truly and entirely are the one infinite, eternal divine being.
"Hear, O Israel: The LORD our God, the LORD is one." , Deuteronomy 6:4
Eternity of God
God is eternal. He has no beginning, no end, and no development. He does not grow, change, or come into being. He simply is.
The Father, Son, and Holy Spirit have always existed together in the fullness of the divine life. There was never a time when the Son did not exist. His eternity is not merely unending duration but the utter absence of succession, limitation, or change.
"In the beginning was the Word, and the Word was with God, and the Word was God." , John 1:1
Equality of the Divine Persons
The Father, Son, and Holy Spirit are equally and fully God. None is greater or lesser in deity, power, glory, or majesty. The one divine essence, with all its infinite perfections, belongs wholly and equally to each person.
This must be distinguished from the voluntary submission the Son takes on in the economy of salvation. When the incarnate Son says "the Father is greater than I" (John 14:28), He speaks as the God-man in the work of redemption, not as a being lesser in nature. Eternal equality and economic humility are both true and must not be confused.
"I and the Father are one." , John 10:30
Inseparable Operations
All of God's works toward creation are the unified work of the one triune God. Father, Son, and Holy Spirit never act independently of one another.
Scripture may emphasize one person in a particular act: the Father sending, the Son accomplishing redemption, the Spirit applying it to the church. But every divine act is the act of the one God, undivided in power and will, with each person acting in a manner fitting to who He is.
"...who through the eternal Spirit offered himself without blemish to God..." , Hebrews 9:14
Eternal Generation of the Son
The Son is eternally from the Father: not created, not beginning in time, but eternally begotten. The Nicene Creed confesses Him as "God from God, Light from Light, true God from true God, begotten not made, of one substance with the Father."
The Father has never existed without the Son. This eternal relation of generation distinguishes them without dividing their deity.
"For as the Father has life in himself, so he has granted the Son also to have life in himself." , John 5:26
Procession of the Holy Spirit
The Holy Spirit eternally proceeds from the Father and the Son. This is not an event in time but an eternal relation within the life of God, the reality that distinguishes the Spirit from the Father and the Son while affirming His full and equal deity.
The church has consistently confessed that the Spirit proceeds from both the Father and the Son (filioque, meaning "and the Son"), in accord with the historic Western confession and the teaching of Scripture taken as a whole.
"But when the Helper comes, whom I will send to you from the Father, the Spirit of truth, who proceeds from the Father, he will bear witness about me." , John 15:26
Divine Simplicity
God is not composed of parts, layers, or separable attributes. He is entirely and indivisibly Himself. The theological term for this is simplicity: God is what He has. His attributes are not additions to His essence but identical with it.
God's love is not something alongside His justice. God is His love, His justice, His wisdom, in one undivided act of being.
"God said to Moses, 'I AM WHO I AM.'" , Exodus 3:14
Two Errors the Church Has Rejected
Understanding what the church confesses requires knowing what it has rejected.
Modalism
Modalism teaches that God is one person who appears in different roles or modes at different times: the Father at creation, the Son at the incarnation, the Spirit at Pentecost. This denies the real, eternal relations of origin among the three persons that Scripture clearly reveals. It is not a Trinity at all, but a masked unitarianism.
Subordinationism
Subordinationism teaches that the Son or Spirit is less than the Father in nature, essence, or deity. This was the error of Arianism, the heresy that the Son is a created being exalted above all other creatures but not truly God. The church rejected this at Nicaea and has rejected every revival of it since. The Son and Spirit are truly, eternally, and equally God.
Ad Intra and Ad Extra
Theologians distinguish between God as He is in Himself and God as He acts toward creation.
Ad intra
Ad intra ("within") refers to God's own eternal life: the Father eternally begets the Son, the Spirit eternally proceeds from the Father and Son, and the three persons dwell together in the fullness of the divine life apart from any reference to creation.
Ad extra
Ad extra ("outside") refers to God's works in history: creation, providence, and redemption. The Father sends the Son; the Son accomplishes redemption; the Spirit applies it to the church. These works in history reflect, without exhausting, the eternal relations within God's own life. The Son is sent because He is the Son. The Spirit is given because He proceeds.
Perichoresis
The Father, Son, and Holy Spirit eternally dwell in one another in perfect unity and love. They are genuinely distinct yet never separated, divided, or confused. Each person exists in and with and through the others, without mixture and without separation.
The church calls this mutual indwelling perichoresis (Greek) or circumincession (Latin).
"Believe me that I am in the Father and the Father is in me." , John 14:11
Communion with God
The gospel brings believers into communion with the triune God. Through union with Christ, the believer is brought to the Father by the Holy Spirit. This is not a deification of nature. It is communion by grace: real participation in the life of God through the mediating work of the Son and the indwelling of the Spirit.
Every act of Christian devotion is a movement toward the Father, through the Son, by the Holy Spirit. This is not a pious sentiment. It is the structure of the Christian life as the triune God has ordained it.
"...that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ." , 1 John 1:3