Its story
Apollinaris, bishop of Laodicea in Syria, was a staunch defender of Nicene faith and a friend of Athanasius. Precisely to stress that Christ is truly God and a single subject, he proposed that the divine Word took a body and an animating soul but not a human mind: the Logos itself served as the rational principle. In his view this avoided two subjects in Christ and the possibility that Christ's human will might sin.
His formula 'one incarnate nature of God the Word' (mia physis tou Theou Logou sesarkōmenē) came to circulate under Athanasius' name, and Cyril of Alexandria later used it believing it Athanasian. This fact matters for understanding the 5th-century controversies.
The answer came mainly from the Cappadocians. Gregory of Nazianzus, in his first letter to Cledonius, coined what became a classic principle: 'what has not been assumed has not been healed; what is united to God is saved'. If the human mind is the seat of sin, Christ had to assume it to redeem it. Gregory of Nyssa wrote a treatise against Apollinaris. Roman synods under Pope Damasus (c. 377) and the Council of Constantinople (381, canon 1) condemned the teaching.
Apollinaris' followers formed small communities that dissolved in the early decades of the 5th century. His legacy is the affirmation, shared since then by the major traditions, that Christ is fully human—with human body, soul and mind—as well as fully divine, and Gregory's soteriological principle, which remains central to Christology.