This thesis contains a newly prepared text and translation of portions of the book De incarnatione contra Nestorium / On the Incarnation: Against Nestorius, by John Cassian. Following an introductory chapter which describes the origin of Cassian’s book as well as the story of how it has been largely neglected in comparison to his other works, the thesis continues with the Latin text and English translation of the prologue and the first two chapters on alternating pages. The Latin text is newly prepared directly from the manuscripts, a number of which have never been included by previous editions, the most recent of which was published in 1888. A description of the stemmatic method used in the preparation of the Latin text is described in the closing section of chapter one, and explanatory footnotes are provided to the English translation.