The Homily On the mystical body of our Lord Jesus Christ by George Gennadios II — Scholarios (ca. 1400 — paulo post 1472) was the first original Orthodox theological text to use the word μετουσίωσις (transubstantiatio) as an ex professo Eucharistic term and to adopt the doctrine associated with it. In this paper I propose a new reading of the fragment, in which Scholarios writes that God communicates with the faithful in the Eucharist by substance (κατ’ οὐσίαν). I argue that this fragment was a paraphrase of the third paragraph of chapter 61, book four of Thomas Aquinas’ Summa contra gentiles and should not be interpreted in the context of Palamite theology as has been proposed hitherto. I find support for my case in the manuscript Taurinensis XXIII (C-II-16), a compilation encouraged by Scholarios in 1432 and which contained the translation of the Summa contra gentiles by Demetrios Kydones. In addition, I outline the post Scholarium history of the expression κατ’οὐσίαν (secundum substantiam), which played a key role for the later development of the Eucharistic doctrine of the Orthodox Church in the post-Byzantine period.