The Trinity

Beliefs about the nature of God differ significantly across Christian denominations (see Christian Denominations for how this splits by tradition). The doctrine held by the majority of the world’s churches is that God has a triune nature — the Trinity — which defines God as one Being who exists eternally as three distinct persons: the Father, the Son, and the Holy Spirit. Put simply, it is “one God in three persons.”¹ The three persons are not to be confused with one another — the Father is not the Son, the Son is not the Holy Spirit, and the Holy Spirit is not the Father — yet all three are said to be co-equal and co-eternal, “each God, whole and entire.” Each person is said to play a different role in how God relates to the world: in the plan of salvation, the Father is said to have sent the Son, Jesus, who died on the cross for the sins of mankind, while the Holy Spirit sanctifies believers and inspires them in daily life.

A key element of the doctrine is the incarnation: the second person of the Trinity, the Son, is said to have taken on human flesh in the person of Jesus. Jesus is thus held to be the God-man, with two natures — one divine, one human — fully God and fully man at once. As a result of the incarnation, Trinitarian doctrine holds that humanity has been permanently incorporated into the Godhead: the Son retains an inseparable divine and human nature forever, even after the crucifixion, resurrection, and ascension — existing in heaven as a glorified man who is, at the same time, God.

A diagram of the doctrine

Christians have long used a simple diagram, the Shield of the Trinity, to summarize this relationship at a glance. It dates to at least the 12th century and became especially popular in 13th-century English manuscripts, though the English name “Shield of the Trinity” itself only came into common use in the 20th century — earlier writers called it the Scutum Fidei, Latin for “shield of faith.”²

How the doctrine developed

The word “Trinity” does not appear anywhere in the Bible. Trinitarian Christians hold that the concept is nonetheless implicit in how the New Testament speaks of the Father, Son, and Holy Spirit together — for instance in the Great Commission (Matthew 28:19) — but the fully worked-out doctrine took shape gradually, through several centuries of controversy and debate.³ The First Council of Nicaea (325 AD) affirmed the Son’s full divinity against Arianism; the First Council of Constantinople (381 AD) extended this to the Holy Spirit; and the doctrine’s classic formulation — “one substance, three persons” — is usually credited to the Athanasian Creed, compiled around 500 AD, well over four centuries after Jesus.⁴ See Major Events in Christian History for these councils in their fuller historical context.

How Trinitarians and Unitarians each read the evidence

Trinitarians justify the doctrine through verses such as Genesis 1:26 (“Let us make mankind in our image”) and Matthew 28:19 (the baptismal formula “in the name of the Father, and of the Son, and of the Holy Spirit”). They further point to the miracles attributed to Jesus in the New Testament, and to his statement “I am” in John 8:58, read as a deliberate echo of God’s self-naming in Exodus 3:14.

Unitarian Christians, by contrast, argue for the absolute oneness of God from verses like Deuteronomy 6:4 (“Hear, O Israel: the Lord our God, the Lord is one”) and Isaiah 45:5-6 (“I am the Lord, and there is no other”), together with New Testament verses such as John 17:3 and Mark 12:28-34, where Jesus himself affirms that same oneness.⁵ Unitarian Christian churches remain a small minority — see Christian Denominations for where groups like Jehovah’s Witnesses, Christadelphians, and Iglesia ni Cristo fall on this question relative to the Trinitarian majority.

For a detailed question-and-answer treatment of the doctrine and the arguments on both sides, see The Trinity: Questions & Answers.

References

1. "Trinity" — Encyclopedia Britannica, for the doctrine's standard definition and its final formulation in the Athanasian Creed.
2. "Shield of the Trinity" — Wikipedia, for the diagram's medieval origin and naming history.
3. "Christianity: Attempts to define the Trinity" — Encyclopedia Britannica, for the doctrine's gradual development through the early church's controversies.
4. "Homoousios" — Encyclopedia Britannica and "Nicene Creed" — Encyclopedia Britannica, for the Council of Nicaea's formulation and the Nicene Creed's text and history.
5. "Unitarianism and Universalism" — Encyclopedia Britannica, for the Unitarian Christian position and its history.