Historical, Bible-based theology has argued that God is omniscient (all knowing), omnipotent (all powerful), sinless, and incorporeal (without a body) and that these attributes are essential and necessary to deity. Characteristically, human beings do not exhibit these attributes. So how can Jesus simultaneously be fully divine and fully human? Along these lines, people have attacked the doctrine of the incarnation, claiming that it is illogical and contradictory.
This alleged logical contradiction is based on a fundamental misunderstanding of how human nature is defined, according to Thomas V. Morris in his book The Logic of God Incarnate. Morris has argued that the way out of this apparent impasse is to have a clearer understanding of three important concepts: (1) essential versus nonessential properties, (2) essential versus common properties, and (3) the difference between being fully and being merely human.
On the first issue Morris argues that an essential property is a property that, if removed, fundamentally changes the thing in question. So, if God's attributes of omnipotence, omniscience, etc., were removed, then he would no longer be deity. These are essential attributes. While it is a common attribute for a human being to have two hands, this is not an essential property to humanness. The heart of the attack on the incarnation comes from critics on the basis that lack of omniscience, omnipotence, etc., is essential to humanness, since human beings do not have these qualities.
This brings us to Morris's second distinction: essential versus common properties. It is a common property that everyone living on planet earth was born on planet earth, but this is simply a common property; it is not essential to their humanness. Morris then asks the question, on what basis does one know that the absence of the attributes of omniscience and so forth are essential human properties and not just common properties?
Last, Morris argues, "an individual is fully human [in any case where] that individual has all essential human properties, all the properties composing basic human nature. An individual is merely human if he or she has all those properties plus some additional limiting properties as well, properties such as that of lacking omnipotence, that of lacking omniscience, and so on." So orthodox Christians, in affirming the incarnation, are claiming that Jesus was fully human without being merely human.
Ronald Nash summarizes the implications of the argument as follows:
This means two things: Jesus possesses all the properties that are essential to being a human being, and Jesus possesses all the properties that are essential to deity. The historic understanding of the Incarnation expresses the beliefs that Jesus Christ is fully God-that is, He possesses all the essential properties of God: Jesus Christ is also fully human-that is, He possesses all the essential properties of a human being, none of which turn out to be limiting properties: and Jesus Christ was not merely human-that is, he did not possess any of the limiting properties that are complements of the divine attributes. In the face of these distinctions, the alleged contradiction in the Incarnation disappears.
Extracted from the Apologetics Study Bible.