In contrast to De Inst. Coen. and Conlat., Cassian's De Incar. Dom. contains virtually no reference to monasticism, to the human struggle against vice, or to the pursuit of perfection. For the first time, Cassian shifts away from a narrowly monastic audience and writes for the Church as a whole, and he focuses not on the human task of attaining virtue but on God's role in salvation and thus on the person of Christ. In this chapter, I will consider the consistency of Cassian's thought in De Incar. Dom. with the understanding of grace and salvation that I have claimed is present in his monastic writings, and I will examine the ways in which Cassian develops this understanding of grace more fully in the later work. Then I will turn to his technical christology in order to demonstrate that what Cassian writes about Christ's person is directly linked to his understanding of grace as the gift of fellowship with God. Finally, I will consider several troublesome elements of Cassian's christology in the light of the ideas I have examined.
At the inception of his work against Nestorius, Cassian discusses the Latin monk Leporius, who had been guilty of a christological error and had been corrected by Augustine and others. Cassian connects Leporius' christological mistake to Pelagianism by writing that both make Christ into a mere man who was sinless and thus imply that all people can be sinless with God's help. He continues: