The word hypostasis is Latin for “person” or “substance.” It refers to the essential part of a person or what makes a person what he truly is. The term is used in referring to Christ and His nature.
The Council of Nicaea affirmed Scripture’s teaching that Jesus shares His essential nature with the Father—He is “of one substance with the Father.” However, Christ is more, in that He took on human nature as well. The term hypostatic union refers to the uniting of human and divine nature in Christ. The words enhypostasis and anhypostasis are used to describe Jesus’ relationship to human nature.
Jesus has been divine for all eternity, but His humanity became a reality about 2,000 years ago. So how did the Son of God become human? If He took a human hypostasis, it would mean that He took on or inhabited a pre-existing human being—He entered an embryo that had already been conceived in Mary’s womb and that would have, had God not overruled, been born as someone else. To ensure that no one thinks this is Christian teaching, the word anhypostasis is used, an- being a negator. Jesus did not take over a human hypostasis—Jesus did not seize another human and appropriate control of that human’s nature. We could say that the humanity that He put on was impersonal.
We must also be clear that Jesus’ human nature was real. His humanity was not taken from another person, nor was it a “mask” that He wore. Jesus did not pretend to be human—He possessed real human nature. The word enhypostasis is used to denote this fact. En- means the same as the English word in—Jesus was really “in” human nature and was a real human person. So, by using the word enhypostasis, theologians are saying that Jesus did possess real human personhood, but that it could not stand alone (as His divine nature could and did). Jesus added to His divine nature and person, and what was added was a real human nature, not a human person. In the end, Jesus has two natures, but He is only one person—He is the same person that He has been from all eternity, but He has added a human nature to His pre-existing divine nature.
Sometimes words seem to make the issue more complicated than it needs to be. However, technical words are sometimes necessary to make very fine but important theological points. The most important thing is to recognize that, in Christ, God took on genuine human nature. Christ’s genuine human nature is in addition to His genuine divine nature, and He remains one unified person, not two. Because the Second Person of the Trinity entered the human race, He could die for the sins of human beings and also sympathize with our weaknesses (see Hebrews 2:14–18). If the words enhypostasis and anhypostasis help us to understand and explain the Incarnation to others, then they are useful. If they only complicate the issue, we can communicate the same concept this way: “2 natures; 1 person.”
A contrast with the Trinity may help as well. Each Person of the Trinity possesses the one indivisible divine nature. These three Persons make up the One True God. Christ is only one person (as are the Father and the Spirit), but He has two natures—a divine nature (from all eternity) and a human nature (from His conception.)