Our times do not seem like a high-water mark for doctrinal preaching, for a variety of reasons (one of which might be a foolish notion that doctrinal preaching is opposed to textual or expository preaching). And some doctrines seem harder to preach than others, they seem to be set forth primarily as a safeguard against error, but to serve no purpose other than defense of more fundamental truths.
I suspect it is easy to feel that way about the doctrine of the communicatio idiomatum, set forth in WCF VIII.7 in these words:
VII. 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.
While it can be seen that this is certainly an explanation of the workings of Scripture phrases such as “crucified the Lord of Glory” (1 Corinthians 2:8; other texts cited in supports of this doctrine include Acts 20:28 and John 3:13), and that this usage itself serves as a safeguard against Nestorianism, the use of the doctrine may not be immediately apparent. But Martin Luther provides an example, not only of preaching doctrine, but of preaching this doctrine and doing so clearly, simply, and movingly. From Volume 24 of the Works edited by Jaroslav Pelikan, speaking of John 14:16:
We have stated often enough that in the divine essence of Christ and the Father there are two distinct Persons. Therefore when speaking of Christ here one must teach clearly that He is one Person, but that there are two distinct natures, the divine and the human. Again, just as there the nature or the divine essence remains unmingled in the Father and in Christ, so also the Person of Christ remains undivided here. Therefore the attributes of each nature, the human and the divine, are ascribed to the entire Person, and we say of Christ: “The Man Christ, born of the Virgin, is omnipotent and does all that we ask � not, however, according to the human but according to the divine nature, not by reason of His birth from His mother, but because He is God’s Son.” And again, “Christ, God’s Son, prays the Father, not according to His divine nature and essence, according to which He is coequal with the Fathe,r but because He is true man and Mary’s Son.” Thus the words must be brought together and compared according to the unity of the Person. The natures must always be differentiated, but the Person must remain undivided.
And now since He is believed as one Person, God and man, it is also proper for us to speak of Him as each nature requires. Therefore we should consider what Christ says according to His human nature and what He says according to His divine nature. For where this is not observed and properly distinguished, many types of heresy must result, as happened in times gone by, when some people asserted that Christ was not true God and others that He was not true man. They were unable to follow the principle of differentiating between the two types of discourse on the basis of the two natures.
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Yes, all that Scripture says of Christ covers the whole Person, just as though both God and man were one essence. Often it uses expressions interchangeably and assigns the attributes of both to each nature. This is done for the sake of the personal union, which we call the “communication of properties.” Thus we can say: “The Man Christ is God’s eternal Son, by whom all creatures were created, Lord of heaven and earth.” And by the same token we say: “Christ, God’s Son (that is, the Person who is true God), was conceived and born of the Virgin Mary, suffered under Pontius Pilate, crucified and dead.”
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Here we must again confess with our Creed: “I believe in Jesus Christ, God the Father’s only Son, our Lord, born of the Virgin mary, suffered, was crucified, died.” It is always one and the same Son of God, our Lord. Therefore it is certain that Mary is the mother of the real and true God and that the Jews crucified not only the Son of Man but also the true Son of God. For I do not want a Christ in whom I am to believe and to whom I am to pray as my Savior who is only man. Otherwise I would go to the devil. For mere flesh and blood could not erase sin, reconcile God, remove His anger, overcome and destroy death and hell, and bestow eternal life.
Furthermore since the angels in heaven adore Him and call Him Lord as He lies in the manger, and say to the shepherds, according to Luke 2:11: “To you is born this day … a Savior, who is Christ the Lord.” He must be true God. For the angels do not worship mere flesh or human nature. Therefore it follows that both God and man must dwell in this Person. And when you speak of Christ, you speak of an undivided Person, who is both God and man; and he who sees, hears, or finds Christ with the faith of the heart surely encounters not only the man Christ but also the true God. Thus we do no let God sit idly in heaven among the angels; but we find Him here below, lying in the manger and on His mother’s lap. We summarize and say: “Wherever we encounter this Person, there we surely encounter the Divine Majesty.”
As has often been stated, all this makes it possible for us to withstand the devil and to vanquish him in the hour of death and at other times when he terrifies us with sin and hell. For if he were to succeed in persuading me to regard Christ as only a man who was crucified and died for me, I would be lost. But if my pride and joy is the fact that Christ, both true God and true man, died for me, I find that this outweighs and eclipses all sin, death, hell, and all misery and woe. For if I know that He who is true God suffered and died for me, and also that this same true man rose from the dead, ascended into heaven, etc., then I can conclude with certainty that my sin was erased and death was conquered by Him, and that God no longer views me with anger and disfavor; for I see and hear nothing but tokens and works of mercy in this Person.
Make sure that you comprehend this doctrine in such a way that you leave the Person of Christ intact and assign the functions of each nature to Him despite the difference in these natures. For according to the divine nature, He was not born of a human being, nor did He inherit anything from the Virgin. It is true that God is the Creator and that man is a creature. But here the two have come together in one Person, and now God and man are one Christ. Mary bore a Son, and the Jews crucified a Person who is God and man. Otherwise � if He were only man, as other saints are � He would be unable to deliver us from even one sin or to extinguish one little drop of hell’s fire with all His holiness, His blood, and His death.