The doctrine of the Holy Trinity is the central mystery of the Christian faith — that the one God exists eternally as three persons: Father, Son, and Holy Spirit, of one substance, power, and eternity. The Westminster Confession of Faith, Chapter 2, Section 3, states this with remarkable brevity:
In the unity of the Godhead there be three persons, of one substance, power, and eternity: God the Father, God the Son, and God the Ghost: the Father is of none, neither begotten, nor proceeding; the Son is eternally begotten of the Father; the Holy Ghost eternally proceeding from the Father and the Son.
The word "Trinity" does not appear in Scripture, but the pattern of revelation demands it. Three biblical affirmations together necessitate the doctrine: (1) There is one God (Deut 6:4, Isa 44:6); (2) The Father is God, the Son is God (John 1:1, 20:28), and the Holy Spirit is God (Acts 5:3-4); (3) These three are personally distinct — the Father is not the Son, the Son is not the Spirit, the Spirit is not the Father. ^[raw/en/wcf-ch02-s03.md]
Key passages include: - The baptismal formula (Matt 28:19) — one name, three persons - The baptism of Jesus (Matt 3:16-17) — Son in the water, Spirit descending, Father speaking - The Prologue of John (John 1:1-2, 14) — the Word was with God and was God - The Farewell Discourse (John 15:26) — the Spirit proceeds from the Father and Son - The Apostolic Benediction (2 Cor 13:14) — grace, love, and communion
WCF 2.3 specifies the personal properties that distinguish the three persons:
These relations of origin are the sole distinctions among the persons. The divine essence is undivided — all three persons share the identical attributes of deity.
The Reformed tradition distinguishes (without separating) the immanent Trinity (God as He is in Himself) from the economic Trinity (God as He acts in creation and redemption). What God is toward us in time — Father sending, Son redeeming, Spirit applying — He is in Himself from eternity. As John Calvin wrote, the Father is "the beginning of activity, the fountain and source of all things; the Son, wisdom, counsel, and the ordered disposition of all things; and the Spirit, the power and efficacy of that activity."
The divine decree itself is Trinitarian in shape. In the covenant of redemption (pactum salutis), made before the foundation of the world:
Herman Witsius called this the "economy of the covenants," showing that the eternal decree is not a cold fate but the personal agreement of the triune God concerning the salvation of the elect. ^[raw/en/wcf-ch02-s03.md]
The Trinity is not a speculative abstraction but the shape of the Christian life:
The Confession was drafted at a time when Socinianism was reviving ancient anti-Trinitarian heresies. The Socinians denied the deity of Christ, the personality of the Spirit, and the satisfaction of Christ. Against them, the Divines confessed the homoousios of Nicaea and the filioque of the Western tradition, using the language of "eternal generation" and "procession" that had defined orthodoxy since the fourth century.