The Blessed Trinity

Examines Old Testament intimations of the Trinity: the plural names of God, the theophanies and Angel of Jehovah (Burning Bush, Mambre), the Messianic Psalms (Ps. II, CIX), the Trisagion of Isaias, and the Sapiential Books (Prov. VIII; Wisd. VII; Ecclus. XXIV) — showing the gradual foreshadowing of three distinct Persons and the coming Messias as true God.

Marshals New Testament texts in which all three Divine Persons appear together: the Baptism of Christ, the Great Commission (Matt. 28:19), the Pauline benediction (2 Cor. 13:13), the Johannine comma, and the Farewell Discourse — establishing the de fide doctrine of the Trinity from explicit Scriptural testimony.

Distinguishes figurative from proper divine fatherhood in Scripture, proves that 'Father' as applied to the First Person is a proper name expressing real Paternity, establishes His real distinction from Son and Holy Ghost, and demonstrates His full Divinity — showing the First Person as unoriginate source of the other two Persons.

Three subdivisions prove Christ's Divine Sonship in the strict sense (against Socinians); His Divinity from His divine attributes, His claim to divine honours, and texts expressly calling Him 'God' (John 1:1, Rom. 9:5, Tit. 2:13, 1 John 5:20); and the Logos doctrine of St. John's prologue — His personality, hypostatic distinction, and consubstantiality with the Father.

Three subdivisions establish the personality of the Holy Ghost (not a mere force or gift but a real Person); His hypostatic difference from Father and Son (He is a distinct Hypostasis); and His full Divinity — shown from Scripture texts calling Him 'God', from divine attributes ascribed to Him, and from the ordo subsistendi placing Him as the Third Person coequal with Father and Son.

Three articles treat the principal heresies condemned by the Church: Article 1 — Crass Monarchianism (Unitarianism): denies all real distinction of Persons; Article 2 — Modalism of Sabellius: admits a Trinity of manifestations but not of real Persons; Article 3 — Subordinationism of Arius and Macedonius: denies the full Divinity of the Son and/or Holy Ghost — each with the Church's formal condemnation.

Three articles present the positive Trinitarian tradition before and after Nicaea: Article 1 — the official liturgy and baptismal formula of the early Church; Article 2 — the Ante-Nicene Fathers (Justin, Tertullian, Origen, Hippolytus) and their Trinitarian teaching; Article 3 — the Nicene and Post-Nicene Fathers (Athanasius, the Cappadocians, Augustine) and their definitive exposition of the dogma.

Chapter III treats the two immanent Processions that constitute the mystery's principle. §1 proves the Procession of the Son from the Father by eternal Generation (de fide — Nicaea), from Scripture and Tradition. §2 treats the Procession of the Holy Ghost from the Father and the Son: Article 1 refutes the Greek Schism's denial of the Filioque and presents the Church's condemnation; Article 2 proves the Catholic doctrine from Scripture, the Greek Fathers, and the Councils of Lyons and Florence.

Shows that the Trinity is an absolute mystery inaccessible to unaided reason — reason cannot discover, demonstrate, or even prove its possibility. Yet reason, guided by faith, can show that the dogma involves no contradiction, repel philosophical objections, and attain some analogical understanding by means of Patristic and Scholastic analogies from human self-consciousness.

Proves the unanimous theological conclusion that the Father generates the Son by mode of understanding (per modum intellectus) — supported by the Scriptural terms Verbum, Logos, Image, and Wisdom — and that the Holy Ghost proceeds by mode of will (per modum voluntatis) as the mutual Love of Father and Son. Treats the Augustinian psychological analogy (mens, notitia, amor) and the formal distinction between the two Processions.

Treats the precise theological concepts of hypostasis and person (following Fourth Lateran), the four real Relations in God (Paternity, Filiation, active and passive Spiration), the doctrine of subsistent Relations as the formal constitutive of Divine Persons, and the classic Scholastic questions about the real vs. virtual distinction between the Persons and their common Nature.

Distinguishes personal properties (proprietates personales — subsistent Relations: Paternity, Filiation, passive Spiration) from properties of persons (including innascibility of the Father and active Spiration shared by Father and Son), and expounds the five Notions (innascibility, Paternity, Filiation, active Spiration, passive Spiration) by which the divine Persons are known and distinguished.

Defines Appropriation (attributing to one Person what is common to all Three, to reveal the Hypostatic character of that Person) and lists the principal Appropriations: Power to the Father, Wisdom to the Son, Goodness to the Holy Ghost; treats the Missions (temporal sendings) of Son and Holy Ghost — visible and invisible — as temporal extensions of the eternal Processions.

Part II opens with the heresy of Tritheism — the error of positing three separate divine natures or substances. Treats its history (John Philoponus; Roscellinus; Gilbert de la Porrée; Joachim of Flora; Günther), its condemnation by the Church (Fourth Lateran Council's definition against Joachim), and its logical opposition to the consubstantiality defined at Nicaea.

Proves the strict numerical identity (not merely generic unity) of the one Divine Nature in three Persons from Scripture — the Shema, the Nicene formula, John 10:30, and the Baptismal formula — and from the unanimous Patristic and Conciliar teaching culminating in the homoousios of Nicaea and the Athanasian Creed.

Proves as a corollary of consubstantiality that all external operations of the Trinity (creation, sanctification, glorification) are common to all three Persons — Opera Trinitatis ad extra sunt indivisa — from Scripture, the Lateran Council of 649, the Fourth Lateran, and the theological argument from unity of nature. Distinguishes the common operation from the appropriata assigned to individual Persons.

Defines Perichoresis (Greek: περιχώρησις; Latin: circuminsessio) — the mutual Interpenetration and Inexistence of the Three Divine Persons by virtue of their consubstantiality and immanent Processions. Proves it from John 10:38 and 14:10–11 (the Father in the Son and the Son in the Father), from the Fathers (John Damascene), and from the theological argument that distinct Persons sharing one infinite Essence must mutually inexist.