I’m no stranger to questioning tradition and conclusions of councils. I have said that I reject Nicene Orthodoxy and related issues in the past. Recently, one of my friends has been flirting with Eutychianism. That is the idea that Christ has only one nature that possesses both human and Divine properties. I’ll comment on his usage of Dr. Paul Copan(Copan isn’t teaching Eutychianism):

1. The distinction between nature and person. A thing’s nature or essence makes it what it is; it wouldn’t exist if it lacked these features. We all have human-making features—the capacity to choose or act, to be conscious, to communicate, to feel, to hold beliefs—even if we aren’t presently using those capacities, say, when we’re asleep or comatose. God has characteristics that make him God. By person, we mean a center of (self-)consciousness, will, activity, and responsibility. Those who are persons include human and angelic individuals as well as the maximally great divine Persons within the Trinity—Father, Son, and Spirit. What then is the relation between person and nature? A person has a nature; you and I possess something that makes us what we are, the same human nature. Jesus of Nazareth, though one Person, is fully God and fully human. He uniquely possesses two natures—one identical to our human nature and the other nature divine.
2. The distinction between what is “fully” (essentially) human and what is “merely” (commonly) human. Humans commonly have arms, legs, hair, and eyes; but even without these we can still be fully human. Also, humans commonly, even universally, commit moral wrongs; but despite Alexander Pope’s statement, “To err is human,” sinning isn’t essential to being human. After all, Adam and Eve were created sinless; Jesus was sinless; and in the afterlife believers will be sinless as well. Death, too, though common, doesn’t define or partly define human beings. Enoch (Gen 5:24) and Elijah (2 Kgs 2:11) didn’t die even though death has touched all other human beings. You get the idea. This essentially/fully human and commonly/merely human distinction reminds us that certain human features we assume to be essential (part of our nature) often aren’t essential after all; they may just be common. This distinction can help us see that a divine-human incarnation is possible: what is essential to human nature doesn’t exclude the possibility of being fully divine. The image of God figures into our discussion at this point: human beings were made “a little lower than God ” (Ps 8:5) to co-rule creation and to commune with God (Gen 1:26–27). Christ Himself, the new Adam, is the image of God (2 Cor 4:4; Col 1:15). As our representative, He both fulfills the human vocation in reflecting God’s image and also graciously empowers us to live as humans were intended to live. In limited form we share in certain attributes or properties with God—personality, relationality, rationality, morality, freedom, creativity—that enable us to fulfill our vocation. Certain essential human characteristics are derived from divine characteristics: human nature is thus a subcategory of the divine

http://www.paulcopan.com/articles/pdf/did-God-become-a-Jew_A-defense-of-the-incarnation.pdf

The traditional understanding is that Christ has two natures and they are distinct. So, we can predicate creaturely things of Christ because he is truly human and Divine things of Christ because he is truly Divine. The categories of communicable and incommunicable attributes can be looked at two ways. They can imply that God and man share properties in common or they can be looked at as analogical to God’s properties.

1. The Biblical reasons for rejecting this idea is the fact that the Bible teaches that God is unlike and ontologically unique to every created thing. The most exalted being in heaven can’t compare and are completely unlike the one that sits on the throne. I document the defense of this idea in prior articles:

http://spirited-tech.com/COG/2018/12/21/creator-over-all/

http://spirited-tech.com/COG/2018/09/01/mormons-and-ot-polytheism/

2. The philosophical reasons are because it creates a new nature that is a mix between divine and human and not completely the other. Christ prior to the incarnation would have a fully divine nature but on the advent of becoming a man takes to his nature human properties. That implies a Change in Christ divinity. Furthermore, Divine properties are incompatible for a human to possess. God is timeless, spaceless, abstract, eternal, unchanging, omnipotent, omniscient, and a se. Humans are limited, temporal, physical/spiritual, contingent, mutable, ignorant, and dependent. How could a nature contain so many contradictory attributes without being a contradiction? Third, it is not as if divine attributes are properties on a scale with human properties where God’s properties are just quantitatively better attributes and yet not qualitatively different attributes.