Our Lord was fully God and fully man in an indissoluble union whereby the second subsistence 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 distinguishable subsistence)
by examining what it is not, thus from the process of elimination determine what it must be.
The mystical union of the divine and human natures of Our Lord
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 subsistence 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 subsistences (often called
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 the 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.
Please review the following to better understand what was taking place at the Incarnation:
http://www.desiringgod.org/blog/posts/anhypostasis-what-kind-of-flesh-did-jesus-take
http://www.desiringgod.org/blog/posts/enhypostasis-what-kind-of-flesh-did-the-word-become
AMR