The Greek term hypostasis 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, 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; thus the paradox of the hypostatic union was crucial to an abiding faith in the Christian 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".