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("Communication of Idioms").
A technical expression in the theology of the Incarnation. It means that the properties of the Divine Word can be ascribed to the man Christ, and that the properties of the man Christ can be predicated of the Word. The language of Scripture and of the Fathers shows that such a mutual interexchange of predicates is legitimate; in this article its source and the rules determining its use will be briefly considered.
The source of the communicatio idiomatum is not to be found in the close moral union between Christ and God as maintained by the Nestorians, nor in Christ's fullness of grace and supernatural gifts, nor, again, in the fact that the Word owns the human nature of Christ by right of creation. God the Father and the Holy Ghost have the same right and interest as the Son in all created things except in the human nature of Jesus Christ. This the Son by Assumption has made His own in a way that is not theirs, i.e., by the incommunicable property of personal union. In Christ there is one person with two natures, the human and the Divine. In ordinary language all the properties of a subject are predicated of its person; consequently the properties of Christ's two natures must be predicated of his one person, since they have only one subject of predication. He Who is the Word of God on account of His eternal generation is also the subject of human properties; and He Who is the man Christ on account of having assumed human nature is the subject of Divine attributes. Christ is God; God is man.
The communicatio idiomatum is based on the oneness of person subsisting in the two natures of Jesus Christ. Hence it can be used as long as both the subject and the predicate of a sentence stand for the person of Jesus Christ, or present a common subject of predication. For in this case we simply affirm that He Who subsists in the Divine nature and possesses certain Divine properties is the same as He Who subsists in the human nature and possesses certain human properties. The following considerations will show the application of this principle more in detail:
(1) In general, concrete terms stand for the person: hence, statements interchanging the Divine and human properties of Christ are, generally speaking, correct if both their subjects and predicates be concrete terms. We may safely say, "God is man", though we must observer certain cautions:
(2) Abstract terms generally stand for their respective nature. Now in Christ there are two natures. Hence statements interchanging the Divine and human properties of Christ are, generally speaking, incorrect if their subject and predicate, either one or both, be abstract terms. We cannot say "the Divinity is mortal", or, "the humanity is increated". The following cautions, however, must be added:
(3) In statements which interchange the Divine and the human properties of Christ, care must be taken not to deny or destroy one of Christ's natures or its properties. This is apt to be done:
(For the use of the communicatio idiomatum in a wider sense, i.e. as applied to the Body of Christ and the Sacramental Species, see EUCHARIST. See also INCARNATION; JESUS CHRIST.)