The quintessential book on the two natures of the person Jesus Christ. It was first published in 1570 and expanded by 1578. Chemnitz is addressing Jesus Christ as being both God and man while addressing the Communication of the Attributes of the two natures. The subject is of the highest importance in Christian theology because this determines how we are saved. Basically, Jesus must be man to be the sacrifice for humanity, and he must be God to offer a sacrifice of such value to redeem all humanity. Should the natures be separated, then there is no salvation. The ancient Church, which Chemnitz surveys in extensive detail, affirms the need for clarity on this doctrine and held several Ecumenical Councils on the subject.
The main reason for Chemnitz's writing is confusion within the Lutheran Church on the two natures given the recent opposition from the Reformed (Calvinist and Zwinglian) churches which deny the presence of Christ's body in the Lord's Supper and the continuity of Lutheran teaching with the ancient Church, which Roman Catholics would deny to the Lutherans. Therefore, Chemnitz articulates the scriptural and patristic sources on Christ's natures to more greatly explain three genera within the Communication of Attributes: the Genus Idiomaticum (each nature has attributes peculiar to it that define it), the Genus Apotelesmaticum (each nature is needed in unison for salvation), and the Genus Maiestaticum (the divine nature communicates attributes to the human nature by the will of God for certain actions while the divine nature is not lowered by receiving human attributes).