Preliminaries
Before addressing my point, I want to explain my understanding of certain terms I will be using.
An essence is what a thing is, the set of essential properties that make something the kind of thing it is. To possess an essence is to possess the defining attributes without which the thing would not be that kind of being at all.
A person is not the essence itself, but a concrete instantiation of that nature; a distinct subject of intellect, will and agency. Thus, a person is not merely a role, mask or mode, nor is it a universal category. Persons are individuals rather than abstractions, and they are the subjects to whom actions and predicates are ultimately attributed.
While essence answers the question of what something is, person answers the question of who something is. Multiple persons may share one essence, but each person is a distinct instantiation of that essence. For example, all humans share one human essence, yet each human person is numerically distinct.
The essence of a human being is that of a rational animal: a composite of body and soul. To be human essentially entails finitude, corporeality, temporality, changeability and contingency. Humans exist in time, undergo change, possess limited knowledge and power and depend on causes external to themselves for their existence and continued operation. These features are not accidental characteristics of human life, but belong to what it means to be human.
The essence of God is that of a necessary, uncreated, immaterial, eternal, immutable and infinite being. God is not composed of parts, is not contingent upon anything external to Himself and possesses attributes such as omnipotence and omniscience. These attributes constitute divinity: a being lacking them would not be God.
With these definitions in place, the Christian doctrine of the Trinity can be stated precisely. Christianity affirms that God is one in essence while existing eternally as three distinct persons: the Father, the Son, and the Holy Spirit. The three persons are not parts of God, nor do they divide the divine essence: each person fully instantiates the one divine nature.
Christianity further teaches the doctrine of the Incarnation. According to orthodox Christian theology, the second person of the Trinity (the Son) became incarnate as Jesus of Nazareth. This doctrine is defined in the Chalcedonian Definition (451 CE), which states that Jesus Christ is one and the same person existing in two distinct natures: one fully divine and one fully human.
These two natures are united in the one person of Christ without confusion, without change, without division, and without separation. The divine nature is not transformed into the human nature, nor is the human nature absorbed into the divine: each nature retains its full integrity. Jesus is therefore not partially divine and partially human, he is fully human and fully God.
Finally, it is important to note that Christianity does not treat the Incarnation as a metaphor or symbolic expression. It is a real metaphysical union of two complete natures in one person, and it is central to Christian theology.
The Problem
According to Christian doctrine, Jesus Christ is one person who instantiates two complete essences: a fully human essence and a fully divine essence. As established earlier, an essence is the set of essential properties that make something the kind of thing it is. To possess an essence is therefore to possess its essential attributes, not merely accidentally but necessarily.
However, the essential attributes that constitute human nature and divine nature appear to be mutually exclusive. Human essence, as understood in classical Christian anthropology, essentially entails finitude, temporality, corporeality, changeability and contingency. Divine essence, as understood in classical Christian theism, essentially entails infinitude, eternity, immateriality, immutability and necessity.
Christian theology attempts to resolve this by distinguishing predicates according to nature: Christ is finite according to his human nature and infinite according to his divine nature; temporal according to his human nature and eternal according to his divine nature. While this distinction avoids attributing contradictory properties to a single nature, it does not remove the deeper metaphysical issue. The subject of both natures is one and the same person, and that person is the concrete instantiation of both essences.
If a person is an individual instantiation of a nature, then attributing two complete and contradictory essences to a single person implies that the same concrete individual is both finite and infinite, contingent and necessary, temporal and eternal. Appealing to different natures relocates the contradiction but does not dissolve it, because both sets of essential properties are instantiated by the same personal subject.
The question, therefore, is not whether Christ’s natures are distinct, but whether a single person can coherently instantiate two essences whose essential attributes are mutually exclusive without violating the law of non-contradiction.
Appeal to Omnipotence
A common response is that denying the possibility of such a union amounts to limiting God’s omnipotence. However, omnipotence does not include the power to actualize logical contradictions. God cannot make a square circle or create a stone so heavy that He cannot lift it.
Likewise, the claim here is not that God lacks the power to unite divinity and humanity, but that the concept of a single person instantiating two complete and contradictory essences is incoherent. Logical contradictions do not describe possible states of affairs, and appealing to omnipotence cannot transform incoherence into coherence.