The article analyses theological teaching about human nature expressed in the book “Took the Form of a Servant. To the Matter of Distortions of the Orthodox Understanding of Incarnation in Catholic and Orthodox Theological Thought of the end of XIX – beginning of XXI c.” published in 2020. The authors of the book question the ideas on Incarnation put forward by a number of XX c. theologians, combined under a concept of “incarnation theology”. They believe that teaching about ontological unity of human nature leads to a misinterpretation of Incarnation as the union of Christ with all humanity and implies a doctrine of apocatastasis, the salvation of everyone.The article puts the views of the authors of the book in the context of XX c. Russian theology. It argues that Metropolitan Anthony (Khrapovitskiy) adopted the idea of unity of humankind in the image of Divine Tri-unity from XIX c. “lay theology” and based his “moral theory of Atonement” on it. While not agreeing with the “moral theory”, a number of subsequent theologians, including St. Hilarion (Troitsky), Fr. George Florovsky and Vladimir Lossky, further developed his ideas of the unity of human nature. His teaching on human nature was criticised by St. Seraphim (Sobolev) and Archbishop Theophan (Bystrov). The article analyses their arguments, including, on one hand, the clear distinction between Divine and human natures, and on the other, especially for Archbishop Theophan, questioning the very unity of human nature.The article points out that the arguments in the book continue the arguments of Archbishop Theophan. It analyses the triple definition of human nature put forward by the authors of the book, partly based on the teaching of Thomas Aquinas and neoplatonics on universals. Whereas the article rejects some parts of this definition as poorly compatible with the Orthodox doctrine, the definition of human nature as composition common to all humans is acceptable; however, the authors’ objection against ontological unity of human nature means that they only mean unity as sameness and come close to John Philoponus’ doctrine of “partial substances”. The article demonstrates that misinterpretation of the notions of nature and hypostasis underlies the book authors’ criticism of St. Hilarion (Troitsky) and Vladimir Lossky on the issue of common nature.