For Apollinaris, Christ was “one nature [mia physis] since He is a simple, undivided Person [prosopon; ibid., Kelly’s Greek text transliterated here]. In fact, Apollinaris originated the phrase which was to become most famous for its incessant use by Cyril of Alexandria (and the later Monophysites who radicalized his principles), “One incarnate nature of the divine Word.” This above all else is the key to Apollinaris’s theology, for, as he himself put it, “The body is not of itself a nature, because it is neither vivifying in itself nor capable of being singled out from that which vivifies it. Nor is the Word, on the other hand, to be distinguished as a separate nature apart from His incarnate state, since it was in the flesh, and not apart from the flesh, that the Lord dwelt on earth.” This concept makes use, as did all Alexandrian theology, of the communicatio idiomatum (communication of properties). Said Apollinaris: “the flesh of the Lord, while remaining flesh even in the union (its nature being neither changed nor lost) shares in the names and properties of the Word; and the Word, while remaining Word and God, in the incarnation shares in the names and properties of the flesh.”
Soteriologically, Apollinaris’s doctrine of the one nature leads to the eucharistic assumption that “The holy flesh is one nature…with the Godhead, and infuses divinity into those who partake of it”–which means that “we are saved by partaking of it as food.” The impact of this eucharistic trajectory would be felt for many centuries to come, one striking example being the charge leveled by Calvinists against Lutherans that the latter’s doctrine of the Supper is “Monophysite.” (Of course, the Lutherans typically reverse the charge, claiming that the Calvinst doctrine is “Nestorian”).
In the year of grace 377, the bishop of Rome, Damasus, held a synod which condemned Apollinaris’s views. This synod was followed up by condemnations in Alexandria (378) and Antioch (379), and in 383, 384, and 385, thanks in part to the untiring efforts of the Cappadocian Fathers, the Emperor Theodosius made Apollinarianism an outlaw doctrine. Why? Kelly summarizes five reasons: (1) Apollinaris’s doctrine was virtually docetic, implying that Christ was not a real human being (but merely “appeared” to be one), (2) Apollinaris’s assumption that two complete entities, divine and human, could not form one real union did not seem defensible, (3) If Christ lacked a real rational human soul, he was not a man at all, (4) Apollinaris’s rejection of a normal human psychology in Christ is contrary to the picture of the very human Savior found in the Gospels (e.g., Christ suffered as a man, exhibited ignorance, and so forth), and (5) most importantly, Apollinarianism undercut redemption at its root by denying that Christ was a real man and therefore making it impossible for God to save real men.