1. God the Word subsists in two natures (divine and human) and the two natures do not become one nature, but a compound hypostasis resulted when “the one common nature viewed as a whole in the subsistence of the Word” became incarnate [An Exact Exposition of the Orthodox Faith 3:11 in PG 94:1025B], i.e., the divine nature of the Word united flesh to itself in Person [Summa Theologica III, q. 2, art. 1, ad 1].
Greek: μία φύσις Θeoυ Λόγου σεσαρκωμένη
2. St. Cyril the Great (1/28) used this formula in Epistle 44 to Priest Eulogios of Constantinople [PG 77:225B], Epistle 45 to Bishop Succensus [PG 77:233A], and Epistle 46 to Bishop Succensus [PG 77:240C]. The Doctor of the Incarnation unwittingly derived the formula from the works of the heresiarch Apollinaris of Laodicea (d. 392), but expounded it in an orthodox manner. The Apollinarists circulated fraudulent books [PG 86:1947–1976] under the names of Pope St. Felix I of Rome (5/30), Bishop St. Gregory the Wonderworker of Neocaesarea (11/17), Pope St. Julius I of Rome [PL 8:930BC] (4/12), and the Doctor Patriarch St. Athanasios I the Great of Alexandria (5/2).