The communicatio idiomatum finds it source in the incarnation where the Divine Word became flesh in the person of Christ (John 1:1,14). This means that in the one person of Jesus are two distinct natures: divine and human. We call this the Hypostatic Union. Yet, we see in the Bible that the attributes of both natures are ascribed to the one person of Christ. In other words, the attributes of both divinity and humanity are both ascribed to the one person of Jesus. Therefore, the communicatio idiomatum means "that the properties of both, the human and the divine natures, are now the properties of the person, and are therefore ascribed to the person."1Again, this means that the one person of Jesus can exhibit attributes of divinity (omnipresence, all knowing, etc.) and at the same time exhibit attributes of humanity (eating, walking, learning, growing, etc.). The communicatio idiomatum does not mean, however, that anything particular to the divine nature was communicated to the human nature. Likewise, it does not mean that anything particular to the human nature was communicated to the divine nature.
To make this more clear, let's look at some verses that illustrate this principle.
John 17:5, "And now, glorify Thou Me together with Thyself, Father, with the glory which I had with Thee before the world was."
Notice here that Jesus, the person, is laying claim to the glory He had with the Father before the world was. Jesus is laying claim to the attribute of pre-existence before the world was created. How can Jesus, the man, lay claim to this since Jesus, the man, didn't exist until He was born on earth? The answer is that Jesus, the person, has two natures: divine and human and the attributes of the divine nature were ascribed to the single person of Christ. Let me repeat this:John 17:5demonstrates that the attributes of the divine nature (the divine pre-existence before the world was), was attributed to the person of Jesus. Therefore, Jesus could lay claim to the glory of the divine nature that existed before the world was because the attributes of the divine nature are ascribed to the single person of Jesus.
Another verse that demonstrates this quality is...
John 3:13, "And no one has ascended into heaven, but He who descended from heaven, even the Son of Man."
Here, Jesus claims to have descended from heaven. How can this be unless Jesus is pre-existent in some way? There is no teaching in the Bible that people pre-exist. We have our formation and beginning in the womb. Yet, Jesus is stating he has a pre-existence. Again, this is an example of the attributes of Jesus divine nature in pre-existence, being attributed to the single person of Jesus.
Remember, we see thatJohn 1:1-2,14says, "In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God... 14 And the Word became flesh, and dwelt among us, and we beheld His glory, glory as of the only begotten from the Father, full of grace and truth." This means that the Word which was God and was with God became flesh. We then say that in the incarnation there is the indwelling in the person of Christ Jesus of two distinct natures: divine and human. In other words, Jesus is both God and man, divine and human and the qualities of both the divine and the human are reckoned/attributed to the single person of Jesus. It is simple and profound.
Following are other verses that illustrate this point with comments after each one:
John 6:62, "What then if you should see the Son of Man ascend where He was before?"
Here Jesus is claiming to have a prior existence in an ascended position. How can this be if Jesus was just a man with a beginning on earth? The answer is that He is both God and man and that the quality of the divine pre-existent nature is ascribed to the person of Jesus, so that Jesus could rightfully claim to have descended out of heaven where He was before.
Acts 20:28, "Be on guard for yourselves and for all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood."
It says that God purchased the church with His own blood. Here we see the attributes of humanity being ascribed to God in and through the atoning work of Christ. Since Jesus is God in flesh, and as a man He had blood, it can be said that God purchased the church with His own blood.
As you can see, the communicatio idiomatum is taught in scripture. But, it isn't just a theological curiousity. It has a vitally important part in the nature of the atonement.
The atonement and the communicatio idiomatum
When we sin, we offend an infinitely holy God because we break God's law. How are we able to appease an infinitely holy God when we are finite and unholy? We can't. Therefore, the only one left who can appease God, who can offer a sacrifice sufficient to please an infinitely holy God, is God himself. Therefore, God the Word became flesh and dwelt among us (John 1:1,14). In Jesus are two natures: divine and human. Now, let's look at the cross.
In the sacrifice of Jesus, we have Jesus dying. But, did the divine nature of Christ also die? No it did not, since God cannot die. If the divine "side" of Jesus didn't die, then how is the sacrifice of Christ of infinite value? The answer is found in the communicatio idiomatum because in this teaching (as we have seen in the scriptures above), the quality and attributes of the divine nature were ascribed to the person of Christ. So, even though the divine side of Jesus didn't die, the person of Christ did die and the person of Christ was able to claim the divine attributes as His own. Therefore, the death of Christ was infinitely valuable and able to save us from our sins.
But, in the cults, in those who deny that Jesus is God in flesh, they have a sacrifice on the cross that is not infinite in value. They have the sacrifice of a created thing, a Jesus that is not divine in nature. Their false Christ, who is not God in flesh (John 1:1,14), is not able to offer a sacrifice of infinite value which is sufficient to cleanse us of all our sins. Therefore, they are still in their sins. This is why Jesus said inJohn 8:24, "I said therefore to you, that you shall die in your sins; for unless you believe that I am He, you shall die in your sins.” Remember, when Moses asked God what His name was, God said inExodus 3:14, "I AM WHO I AM”; and He said, “Thus you shall say to the sons of Israel, ‘I AM has sent me to you.’” Jesus also said inJohn 8:55-59, "Jesus said to them, “Truly, truly, I say to you, before Abraham was born, I am.” 59 Therefore they picked up stones to throw at Him; but Jesus hid Himself, and went out of the temple." The Jews wanted to kill Jesus. Why? They told us later inJohn 10:33when they said to Jesus, "For a good work we do not stone You, but for blasphemy; and because You, being a man, make Yourself out to be God.” But, the cults deny that Jesus is God -- just like the Jews did, and so Jesus' words become all the more important: "I said therefore to you, that you shall die in your sins; for unless you believe that I am He, you shall die in your sins," (John 8:24).