Our Lord was fully God and fully man in an indissoluble union whereby the second person of the Trinity
assumed a human nature that cannot be separated, divided, mixed, or confused.
One can best understand this
mystical union (together united in one subsistence and in one single person)
by examining what it is not, thus from the process of elimination determine what it must be.
The union of the divine and the human natures is not:
1. a denial that our Lord was truly God (
Ebionites, Elkasites, Arians);
2. a dissimilar or different substance (
anomoios) with the Father (
semi-Arianism);
3. a denial that our Lord had a genuine human soul (
Apollinarians);
4. a denial of a distinct person in the Trinity (
Dynamic Monarchianism);
5. God acting merely in the forms of the Son and Spirit (
Modalistic Monarchianism/Sabellianism/United Pentecostal Church);
6. a mixture or change when the two natures were united (
Eutychianism/Monophysitism);
7. two distinct persons (
Nestorianism);
8. a denial of the true humanity of Christ (
docetism);
9. a view that God the Son laid aside all or some of His divine attributes (
kenoticism);
10. a view that there was a communication of the attributes between the divine and human natures (
Lutheranism, with respect to the Lord's Supper); and
11. a view that our Lord existed independently as a human before God entered His body (
Adoptionism).
The
Chalcedonian Definition is one of the few statements that all of orthodox Christendom recognizes as a most faithful summary of the teachings of the Scriptures on the matter of the Incarnate Christ. The Chalcedonian Definition was the answer to the many heterodoxies identified above during the third century.
Two other useful discussions on the matter of the humanity assumed by Our Lord:
Anhypostasis: What Kind of Flesh Did Jesus Take?
http://www.desiringgod.org/articles/anhypostasis-what-kind-of-flesh-did-jesus-take
Enhypostasis: What Kind of Flesh Did the Word Become?
http://www.desiringgod.org/articles/enhypostasis-what-kind-of-flesh-did-the-word-become
AMR