I don't understand why whenever we get into Christology and the Incarnation, people crawl out of the woodwork and start shooting from the hip. This is one area of theology where orthodoxy is very meticulously defined and has been accepted by all major traditions without serious challenge since the fourth century. Why anyone would want to enter the fray with a "Well, I think this: [your novel idea here]" kind of argument is mystifying.
The reason these issues were hashed out so carefully in the early church is that they are absolutely foundational. And it behooves us all to study historical theology and the major creeds on these matters before launching into speculation.
End of lecture. Now here's a simplified synopsis of the historical background that I hope you'll find helpful:
Several early heresies arose in the early centuries of the church. Between them all, they pretty much covered every possible heresy regarding the Person of Christ. You think you have a new way to explain the incarnation? It's no doubt already been done.
For example, the Ebionites insisted that Jesus was a mere man--the holiest of all men, but no more than that. The Apollinarians acknowledged His deity but denied that He had a human soul. The Nestorians made Him both God and man, but in doing so made Him two persons in one body--a man in whom the divine Logos dwelt rather than a single person who was both human and divine. The Eutichians, the Monophysites, and the Monothelites went to the opposite extreme, fusing the divine and human natures of Christ into one new nature. The Arians claimed He was not God, but the highest of all created beings. (Sound familiar? That is the same view held by modern Jehovah's Witnesses.) And the Docetists denied that Christ was really human. Most Docetists taught that Jesus' human body was only an illusion.
Church councils were repeatedly called to examine Scripture and decide between these differing views. As soon as one issue was settled, however, another would surface and need to be dealt with. In 325, the council of Nicea (named for a little town where they met, near Constantinople) condemned Arianism. This council condemned the Arian heresy and proclaimed that Jesus is fully divine.
Nevertheless, in the century that followed, Arianism and several other Christological heresies attempted by subterfuge and intrigue to gain a foothold the church. The doctrine of the incarnation was debated in meticulous detail from every angle, and a series of Church councils subsequently convened to consider virtually every conceivable twist and variation on the nature of the Godhead. What emerged was a solid consensus among the people of God on the doctrine of the Trinity.
During those years of intense debate over the doctrine of the incarnation, the pendulum often swung from one extreme error to another. For example, within 60 years after the Nicene council affirmed the full deity of Christ another church council, the Council of Constantinople, had to deal with Apollinarianism, which went overboard on the side of Christ's deity and was not doing full justice to His humanity. In 381 the council of Constantinople condemned Apollinarianism as heresy.
This war against Christological heresies continued until the council of Chalcedon in 451 issued a statement about the Person of Christ that has stood as the definitive test of orthodoxy from that time until now. The statement is brief. It is all one sentence, so pay careful attention:
We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, perfect in Godhead and also perfect in manhood; very God and very man, of a rational soul and body; coessential [homousios--identical in nature] with the Father according to the Godhead, and consubstantial [homousios--identical in nature] with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the God-bearer [Theotokos], according to the Manhood; one and the same Christ, Son, Lord, Only-begotten, to be acknowledged in two natures without confusion, without change, without division, and without separation; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ, as the prophets from the beginning have spoken of him, and the Lord Jesus Christ himself has taught us, and the Creed of the holy Fathers has handed down to us. [Philip Schaff, Creeds of Christendom, 2:62-63]
The genius of that statement--the element that put an end to incessant heresies on the nature of Christ--is found in the phrase "two natures without confusion, without change, without division, and without separation." Those four negative statements forever defined and delimited how the person of Christ is to be understood. G. C. Berkhouwer called those four negatives "a double row of light-beacons which mark off the navigable water in between and warn against the dangers which threaten to the left and to the right." [Berkhouwer, 85.]
The fact is that every heresy that has ever surfaced with regard to the person of Christ either fuses or separates the deity and the humanity of Christ. Chalcedon declared that the two natures can be neither merged nor disconnected. Christ is both God and man. Truly God and truly man.
There is no terminology outside the Council of Chalcedon's statement that has ever been accepted as orthodox by any major branch of Christianity. So anyone who denies any element of this formula--whether it's the two natures, the union of the two natures, or whatever--is unorthodox on the doctrine of the incarnation. It's as simple as that. And this is not something to treat lightly.
Finally (and this will be on the quiz), the technical term for the union of Christ's two natures is the hypostatic union. It's a doctrine anyone who wants to discuss theology intelligently must be familiar with.