The Threeness of God

I. The Indispensability of the Doctrine of the Trinity
Israel confesses God as one (Deut 6:4).

Christians agree, but go on to describe God as Trinity ('Tri-unity')
to speak about God in light of Jesus Christ (for whom Father and Spirit are both other and one),
to guard the apostolic traditions and insights, and
to guide further exploration of God and God's works.
II. Is 'Trinity' Biblical?
The Old Testament only hints (Gen 18:1-16, Isa 6:3, Ps 33:6, Prov 8, Gen 1:1-3, 1:26).
Jews worship the risen Jesus, the Father, and the Spirit as the one God of Israel.
Jesus' and the Church's life bring God's threeness into plain view.
The Father:
Jesus calls God "the Father" (abba/ho pater; Matt 11:27).
Jesus reveals the Father (John 14:9) and does his will.
The Son:
Jesus calls himself "the (only) Son", the Father's heir (John 3:16, Rom 8:32, Col 1:13).
Jesus shows the Father as "only begotten God" (John 1:18).
Jesus is Creator, Judge, and Savior.
Believers confess that "Jesus is Lord".
The Holy Spirit:
The Spirit proceeds from the Father (John 15:26).
The Spirit conceives Jesus (Luke 1:35).
Jesus receives the Spirit from the Father (Luke 3:22, Acts 2:33).
Jesus gives the Spirit to humanity (Acts 2:33, John 14:16, 26, John 20:22).
The three together:
Explicit triads (Matt 28:19, 2 Cor 13:13).
Subtle triads (Luke 1:35, 24:49, Rom 14:17-18, 15:16, 15:30, 1 Cor 12:4-6, 2 Cor 1:21-22, 2 Cor 3:3, Gal 3:11-14, 4:6, Eph 2:18, 2:20-22, 3:14-16, Phil 3:3, Col 1:6-8, 2 Thess 2:13, Titus 3:4-6, Heb 6:4-6, 1 Pet 1:2, 4:14, 1 John 4:2, 4:13-14).

III. Further Development in Christian Tradition
Explicit Trinitarian language develops to serve
worship practice (liturgy),
initiation (baptism) and confession (creeds),
intellectual reflection ("theology"), and
arguments against adversaries (polemics, apologetics).
The results are two vocabularies and two models of God, crafted in response to several Trinitarian heresies:
IV. Basic Trinitarian Vocabulary
in ... one "what" three "who's":
Greek ousia hypostasis
Latin esse, substantia persona, subsistentia
English being, essence, substance person, subsistence, "way of being"
but not person, form ... parts, beings, members, people, entities, forms, substances ...
V. Eastern Orthodoxy ...
follows the Cappadocian Fathers (Basil the Great, Gregory of Nyssa, and Gregory Nazianzus):
The Father is the source of divinity and unity.
The persons' distinctions are relational: interrelationships constitute the Father as Father, Son as Son, Spirit as Spirit.
African proverb: "A person becomes a person through other persons."
Example: It's a Wonderful Life.
God's unity is social and dynamic: coinherence (three turned-in-mirrors) or perichoresis.
Iconic illustration: Rublev's Trinity.
... against Modalistic Monarchianism
Modalists have God as one source with three roles (Creator, Redeemer, Sanctifier).
Personhood would be the many masks of an ancient actor (persona, prosopon).
Example: Mike Myers in Austin Powers.
Why only three?
Can you have a personal relationship with a role?
Does a role reveal an actor?
To whom does Jesus pray? (Sabellianism: persons as sequential).
Who was on the cross? (Patripassianism: "the Father suffered").
Is the cross just an act?
Wouldn't God 'need' creation to have these relationships?
Illustrations: ice-water-steam; three-leaf clover.
Oneness Pentecostalism is modalistic.
VI. Latin Orthodoxy ...
Catholics and Protestants follow Tertullian and Augustine:
God's unity is substantial: The persons share the divine essence.
The persons' distinction is relational.
Illustration: Lakota Trinity.
The economy of salvation shows the Spirit's double-procession.
So t
he West alters the Nicene Creed, adding the filioque (the Spirit proceeds "also from the Son").
Humanity in God's image has "vestiges of the Trinity":
Lover, beloved, love;
Memory, intellect, will.

... against Dynamic Monarchianism and Arianism ...
Adoptionism has Jesus inheriting 'divinity'.
Personhood means legal status.
The Spirit is the impersonal inheritance, sign, and seal of Jesus' adoption.
Example: Marcus Aurelius adopting Maximus in Gladiator.
What does an adopted (not begotten) Jesus show of his adoptive parent?
How is God involved on the cross?

... as well as Tritheism

God's "unity" would merely be the moral unity of three individuals.
Personhood means individuality, independence, consciousness (as in modern psychology).
Example: The Three Musketeers.

Illustration: egg yolk-white-shell.
How does one such person reveal another?
Are Son and Spirit equal (cf. John 5:18)?
Or are they unequal (subordinationism)? (Cf. Col 1:15, John 14:28, 1 Cor 15:28, Phil 2:5-11.)
Can we divide God's glory among specific persons (cf. Deut 6:12-15)?
On the cross, is God a dysfunctional family?