The communicatio idiomatum is the doctrine that the attributes of both Christ's divine and human natures are properly predicated of his one undivided Person. Because Jesus is one Person with two natures, what is true of either nature is true of him — the whole Person. So we can say: "God died" (because the divine Person genuinely died in his human nature); "the Son of Man is in heaven" (John 3:13, because the divine Person who took on human nature remains omnipresent); "the Lord of glory was crucified" (1 Cor 2:8). This is not a confusion of natures — it is the grammar of incarnation. It protects against Nestorius, who so separated the natures that Christ was effectively two persons, and who notoriously refused to call Mary Theotokos (God-bearer) — because in his framework, only the human Jesus was born of Mary, not the divine Son. Cyril of Alexandria rightly insisted: one Person means the properties of both natures are his — fully, really, personally.
COMMUNICATIO IDIOMATUM. [Latin: the communication of properties.] A theological term expressing the doctrine that the properties, attributes, and actions belonging to either the divine or the human nature of Christ may be predicated of his whole Person. Thus what the Scriptures declare of Christ as God — his eternity, omnipotence, and glory — and what they declare of him as man — his birth, suffering, and death — are all equally true of the one Person who is both God and man. The doctrine preserves the unity of Christ's Person while respecting the integrity of each nature.
• Acts 20:28 — "Care for the church of God, which he obtained with his own blood." (God has blood — the human attribute is predicated of the divine Person.)
• 1 Corinthians 2:8 — "They crucified the Lord of glory." (The Lord of glory — divine title — was crucified — human suffering.)
• John 3:13 — "No one has ascended into heaven except he who descended from heaven, the Son of Man who is in heaven." (The Son of Man — human title — is simultaneously in heaven.)
• Romans 1:3–4 — "Descended from David according to the flesh…declared to be the Son of God in power." (Human descent and divine Sonship — one Person.)
• Galatians 4:4 — "God sent forth his Son, born of woman." (The eternal Son of God was born of woman — divine attribute communicated to human event.)
Latin: communicatio — sharing, participation, imparting. From communicare (to share, to make common) → communis (common, shared). Related: "communion," "community," "communicate."
Latin: idioma (from Greek ἰδίωμα) — a distinctive characteristic, a peculiarity. From idios (ἴδιος) — one's own, peculiar, private. Related: "idiom," "idiosyncrasy," "idiot" (private citizen).
Greek: κοινωνία τῶν ἰδιωμάτων (koinōnia tōn idiōmatōn) — the patristic Greek equivalent; koinōnia (G2842) = fellowship, sharing, participation.
The doctrine's grammar is rooted in the unity of the hypostasis (G5287): because there is one hypostasis (Person), predications of either nature belong to that one Person.
Modern Christology often ignores communicatio idiomatum because modern Christianity has largely lost the grammar of two-natures-one-Person theology. The result: popular piety treats Jesus as alternately "turning on" his divinity for miracles and "turning it off" for hunger or fatigue — as if the natures were modes or phases rather than permanent, simultaneous realities. This is functional Nestorianism: a Jesus who compartmentalizes his natures. The Lutheran tradition pushed communicatio idiomatum very far — claiming Christ's human body receives divine ubiquity (omnipresence), which Reformed theologians disputed. Reformed theology maintains that the natures remain distinct even as their attributes are truly predicated of the one Person. The practical point: when the NT says "God bled for you" or "the Creator was born in a manger" — it is not sloppy language. It is the precise language of the Word made flesh, whose single Person holds both the infinite and the finite in unbreakable union.
Latin: communicatio → communicare → communis
→ com- (together) + munis (bound by obligation, gift)
→ PIE root *mei- (to change, to go, to exchange)
Related: common, commune, community, communion, excommunicate
Latin: idioma → Greek idiōma → idios (ἴδιος)
→ PIE root *s(w)e- (one's own, reflexive)
Related: idiom, idiosyncrasy, idiopathic, idiot (archaic: private person)
Historical development:
Origen, Tertullian: early formulations
Cyril of Alexandria: defender against Nestorius (430s)
Council of Ephesus (431): Theotokos affirmed — Mary bore God
Council of Chalcedon (451): two natures, one person codified
Lutheran vs. Reformed: genus maiestaticum (Lutheran claim that
Christ's humanity receives divine majesty including ubiquity —
rejected by Reformed as confusing the natures)