Eutyches' heresy involves many other heresies
Leo, bishop of the city of Rome to his well-beloved brother, Julian the bishop.
Although by the hands of our brothers, whom we have dispatched from the city on behalf of the Faith, we hare sent a most full refutation of Eutyches' excessive heresy to our brother Flavian, yet because we have received, through our son Basil, your letter, beloved, which has given us much pleasure from the fervour of its Catholic spirit, we have added this page also which agrees with the other document, that you may offer a united and strenuous resistance to those who seek to corrupt the gospel of Christ, since the wisdom and the teaching of the Holy Spirit is one and the same in you as in us: and whosoever does not receive it, is not a member of Christ's body and cannot glory in that Head in which he denies the presence of his own nature. What advantage is it to that most unwise old man under the name of the Nestorian heresy to mangle the belief of those, whose most devout faith he cannot tear to pieces: when in declaring the only-begotten Son of God to have been so born of the blessed Virgin's womb that He wore the appearance of a human body without the reality of human flesh being united to the Word, he departs as far from the right path as did Nestorius in separating the Godhead of the Word from the substance of His assumed Manhood? From which prodigious falsehood who does not see what monstrous opinions spring? For he who denies the true Manhood of Jesus Christ, must needs be filled with many blasphemies, being claimed by Apollinaris as his own, seized upon by Valentinus, or held fast by Manichæus: none of whom believed that there was true human flesh in Christ. But, surely, if that is not accepted, not only is it denied that He, who was in the form of God, but yet abode in the form of a slave, was born Man according to the flesh and reasonable soul: but also that He was crucified, dead, and buried, and that on the third day He rose again, and that, sitting at the right hand of the Father, he will come to judge the quick and the dead in that body in which He Himself was judged: because these pledges of our redemption are rendered void if Christ is not believed to have the true and whole nature of true Manhood.
Source: Letters (New Advent)