Communication of Idioms and Christological Predication is the Catholic rule that what belongs truly to either nature of Christ may be predicated of the one divine person who possesses both natures, without transferring a divine property into humanity or a human limitation into divinity.
One subject of divine and human predicates
Jesus Christ is not a partnership between a divine subject and a human subject. The eternal Son assumed a complete human nature, so the same person who is eternally begotten of the Father was born of Mary, suffered, died, rose, and reigns. Scripture can therefore say that the Lord of glory was crucified and that the Son of Man came down from heaven. The grammatical subject is the person; the ground of each predicate is the nature according to which it is true.
This rule explains the title Mother of God. Mary did not originate the divine nature or the eternal generation of the Son, yet the one whom she conceived and bore according to his humanity is truly God the Son. The Council of Ephesus defended Theotokos precisely to protect the unity of Christ's person. Refusing the title because birth is a human event divides the one Christ; using it as though Mary generated the Godhead confuses person, nature, and origin.
Concrete and abstract predication
Concrete names ordinarily designate the subsisting person: God, man, Son, Lord, and Jesus Christ. Abstract terms designate natures or properties: divinity, humanity, passibility, eternity, and bodily extension. Thus one may say that God suffered in the flesh, because God the Son is the personal subject of suffering according to his humanity. One may not say that the divine nature suffered or that humanity is eternal. The distinction preserves the force of the confession while excluding a mixture of attributes.
Chalcedonian boundaries
The communication of idioms depends upon the Hypostatic Union: two complete natures remain without confusion, change, division, or separation in one person and hypostasis. It is not a verbal trick that manufactures unity after the fact. Because the union is real, personal predication is real. Because the natures remain distinct, their natural properties remain distinct. Christ's human body is local and passible; his divinity is immense and impassible. Both statements are true of the one Lord under the proper qualification.[1][2]
Catholic usage consequently rejects both Nestorian and monophysite distortions. A merely moral or functional conjunction cannot justify saying that God was born and died for us. Conversely, attributing created limitation to divinity as divinity destroys divine immutability, while absorbing the humanity into divinity destroys the reality of obedience, suffering, and redemption. The rule serves the dogma received at Ephesus and Chalcedon; it does not dissolve the dogma into an unrestricted exchange of properties.
Christological propositions in worship
The Church's prayers, hymns, and sacramental formulas frequently address the one Christ through predicates rooted in one nature. The Crucified is adored because the crucified person is the divine Son, not because pain becomes a divine attribute. The Sacred Heart is worshiped as the heart of the Word made flesh, not as a detached organ or symbol. Sound devotion follows Christological predication toward the person and avoids treating a nature, faculty, image, or bodily member as an independent subject.
Use in theology and catechesis
Precise qualifications such as according to his humanity and according to his divinity are sometimes necessary, especially where a sentence might imply contradiction. Christ knows all things in the divine knowledge he possesses with the Father and Spirit; his created human intellect truly learned and operated in a human mode. He is equal to the Father according to divinity and less than the Father according to humanity. These are not two Christs but distinct predicates of one person grounded in two natures.
Catechesis should neither avoid strong biblical language nor repeat it without the Church's grammar. The communication of idioms safeguards the realism of the Incarnation: God the Son truly entered human history, and the man Jesus is truly the eternal Son. It also sets limits on speculative phrases, alleged private revelations, and devotional exaggerations. A proposition about Christ must identify its personal subject, natural ground, and conformity with the conciliar confession before rhetorical intensity is mistaken for orthodoxy.
References
- 1.Catechism of the Catholic Church, 464–483 — The one divine person of the Word in two complete natures, the communication of idioms, Christ's true human knowledge and will, and the dogmatic boundaries fixed by the early ecumenical councils
- 2.Third Council of Constantinople — The sixth ecumenical council's dogmatic confession of two natural wills and two natural operations in the one person of Christ, without division, change, separation, or confusion