The following was written in partial fulfillment of the requirements of Dr. Steve McKinion's Christian Theology II class at Southeastern Baptist Theological Seminary.
The “communication of attributes” (Latin communicatio idiomatum) is a summary of one of the essential Christian affirmations about the Incarnation of the Son of God: that in the Incarnation, the person Jesus shared both divine and human attributes. The properties of divinity and humanity both fully existed in Jesus the Messiah; they did not cross into each other, but were united in the Son. The Son then could choose to use whichever attributes were appropriate to him at a given time: he could both ride in a boat (consistent with being a human being) or forgive sins (consistenly only with being God). This carries through to all aspects of the Son’s work upon his incarnation: he has a human will and a divine will, neither of which overrides the other but both of which are exercised together as appropriate. The divine will is already in line with the will of the Father; the human will he chooses to subject to the Father. By analogy: one may know both English and French, and if so one speaks each language as appropriate—there is no implied division in a person simply because he knows more than one language.
This communication of attributes was accomplished so that no part of either the Son’s pre-existing humanity divinity or newly-embraced humanity was either changed or diminished in the Incarnation. The two were united, but they were not altered or subsumed. This important reality was articulated in the four negations of the Chalcedonian definition:
In Jesus, the two natures were united without confusion. The Incarnation was not like mixing wine and water to produce a tertium quid, a third thing like both of the others but not actually either of them, even while no longer distinguishable from each other. Jesus was both God and man, not a new thing that was neither wholly God nor wholly man.
In Jesus, the two natures were united without diminishing. The Incarnation was not like putting a drop of wine in the ocean so that the wine simply dissipates and is meaningless. Thus Apollinarian view that the Son’s divine nature subsumed and overwhelmed the human nature was rejected. In Jesus both human and divine natures were fully present, and neither overwhelmed the other.
In Jesus, the two natures were united without mixture. The Incarnation was not the result of combining two things that were unlike to produce a new thing which could still be separated into the old things. In other words, the Son’s becoming man was not like sodium joining with chloride to produce salt, which is entirely different from either sodium or chloride.
In Jesus, the two natures were united without separating. The Incarnation was not like bundling together a pair of sticks which could just as easily be pulled apart. The Son did not shed his humanity upon his resurrection or his ascension; he remains now and forever a human being as well as divine. Thus all the experiences, actions, and sayings of Jesus are attributable to one person (rather than suggesting that the divine Son did some things and the human being did another—one stick and another stick acting in distinct ways). Rather, one person, Jesus, walked (and walks) with both natures, lived (and lives) both kinds of lives, existed (and exists) in both ways.
Thus, the attributes of God and of humanity were both communicated—they had communion—in Christ. In Jesus, the human and divine did not mingle into a distinctionless soup, did not overwhelm each other, cannot be separated, and did not create some new thing unlike human or God. Jesus is fully God and fully man, forever.