Hypostatic union

Hypostatic union (from the Greek: ὑπόστασις hypóstasis, 'person, subsistence') is a technical term in Christian theology employed in mainstream Christology to describe the union of Christ's human nature and divine nature in one composed hypostasis, or individual personhood.

In the most basic terms, the concept of hypostatic union states that Jesus Christ is both fully God and fully man. He is simultaneously perfectly divine and perfectly human, having two complete and distinct natures at once.

The Athanasian Creed recognized this doctrine and affirmed its importance by stating:

He is God from the essence of the Father, begotten before time; and he is human from the essence of his mother, born in time; completely God, completely human, with a rational soul and human flesh; equal to the Father as regards divinity, less than the Father as regards humanity. Although he is God and human, yet Christ is not two, but one. He is one, however, not by his divinity being turned into flesh, but by God's taking humanity to himself. He is one, certainly not by the blending of his essence, but by the unity of his person. For just as one human is both rational soul and flesh, so too the one Christ is both God and human.

The Greek term hypostasis (ὑπόστασις) had come into use as a technical term prior to the Christological debates of the late fourth and fifth centuries. In pre-Christian times, Greek philosophy (primarily Stoicism) used the word. Some occurrences of the term hypostasis in the New Testament foreshadow the later, technical understanding of the word. Although it can translate literally as "substance", this has been a cause of some confusion; accordingly the New American Standard Bible translates it as "subsistence". Hypostasis denotes an actual, concrete existence, in contrast to abstract categories such as Platonic ideals.

In Kierkegaard's Philosophical Fragments, the dual nature of Christ is explored as a paradox, i.e. as "the ultimate paradox", because God, understood as a perfectly good, perfectly wise, perfectly powerful being, fully became a human, in the Christian understanding of the term: burdened by sin, limited in goodness, knowledge, and understanding. This paradox can only be resolved, Kierkegaard believed, by a leap of faith away from one's understanding and reason towards belief in God.

As the precise nature of this union is held to defy finite human comprehension, the hypostatic union is also referred to by the alternative term "mystical union".

Apollinaris of Laodicea was the first to use the term hypostasis in trying to understand the Incarnation. Apollinaris described the union of the divine and human in Christ as being of a single nature and having a single essence — a single hypostasis.