“The First Genus of Communication of Attributes (Genus Idiomaticum)” (Loc. 2675)
Nestorianism separates the human from the divine in Christ, denying suffering and death to the divine nature. In Loc. 2691 Pieper draws a strong connection between Nestorius and Zwingli, who attributed Jesus, suffering only to his human nature. Pieper goes on to illustrate this separation in Zwingli and others.
Counter to this, “it is Scripture itself, and not human speculation, that predicates of the Son of God human birth and suffering and death” (Loc. 2728). He goes on to bring a number of arguments that the Scripture must rule in our understanding. Luther’s argument (Loc. 2764) is that the Scripture discusses God in the person of Christ suffering and dying. We are not at liberty to separate Christ.
Pieper defines this doctrine “as follows: Because the divine and human nature of Christ constitute one Person, the attributes, belonging essentially to only one nature, are always ascribed to the whole person, but the divine attributes according to the divine nature, and the human attributes according to the human nature. In dogmatics this genus is known as the genus idiomaticum” (Loc. 2826). In short, the attributes of each nature belong to the entire person. In general, when pushed, Pieper observes that most theologians will affirm the genus idiomaticum though they will often deny it in their writings. Denying it separates Christ into two persons, neither effectual for salvation.