The communicatio idiomatum (or the communication of properties) is one of the more important doctrines related to the incarnation of Christ, and yet it is not exactly one of the more well-known or commonly-discussed doctrines in our day.
The Westminster Confession of Faith, in its chapter entitled “Of Christ the Mediator” puts it this way:
“Christ, in the work of mediation, acts according to both natures, by each nature doing that which is proper to itself; yet, by reason of the unity of the person, that which is proper to one nature is sometimes in Scripture attributed to the person denominated by the other nature” (8.7).
That, for example, is why Acts 20:28 can speak of “the church of God, which he obtained with his own blood” (ESV, emphasis mine). Can God bleed? In the person of Christ, yes, but only according to His human nature. But because of the unity of His person, the Son of God can properly be said to have suffered, bled, and died.
The communicatio and some of its implications are helpfully summarized by Louis Berkhof:
“[The communicatio idiomatum] means that the properties of both, the human and divine natures, are now the properties of the person, and are therefore ascribed to the person. The person can be said to be almighty, omniscient, omnipresent, and so on, but can also be called a man of sorrows, of limited knowledge and power, and subject to human want and miseries. We must be careful not to understand the term to mean that anything peculiar to the divine nature was communicated to the human nature, or vice versa; or that there is an interpenetration of the two natures, as a result of which the divine is humanized, and the human is deified (Rome). The deity cannot share in human weakness; neither can man participate in any of the essential perfections of the Godhead.” (Systematic Theology, p.324)
In his book, The Glory of Christ, John Owen (1616-1683) explains how all of this relates to the earthly life, ministry, and death of Christ. He writes,
“The Lord Christ suffered and did many things both in his life and in his death as a human being. But all that he did and suffered as a human being was done and suffered by his whole person, even although what he did and suffered as a human being was not actually done and suffered by his divine nature. Because his human nature was part of his whole person, what he did as a human being could be said to have been done by God himself as God, e.g. God purchased his church ‘with his own blood; (Acts 20:28).” (p.43-44)
So we do not speak of the human nature of Christ dying for our sins, but of the death of Christ Himself (i.e. his whole person), according to His human nature. As Owen puts it, all that He did and suffered “was done and suffered by his whole person,” and yet also “not actually done and suffered by his divine nature.” Only this doctrine, properly understood, truly does justice to the incarnation of Christ, as well as to both His divine and human natures.