Hypostatic union is a special term used in Christian beliefs to explain how Jesus Christ has both a human nature and a divine nature in one person. This means Jesus is completely God and completely human at the same time. He has two separate natures, but they exist together in one individual.

The Athanasian Creed agreed with this idea and said it was very important.

Hypostasis

The Greek word hypostasis (ὑπόστασις) was used as a technical term before debates about Jesus' nature in the late 4th and 5th centuries. Before Christianity, Greek philosophy, especially Stoic philosophy, used this term. Some uses of hypostasis in the New Testament showed early ideas about its later technical meaning. While it can be translated as "substance," this has caused confusion. The New American Standard Bible translates it as "subsistence." Hypostasis refers to real, physical existence, not abstract ideas like Platonic ideals.

In Kierkegaard's Philosophical Fragments, the two parts of Jesus' nature are described as a paradox, called "the ultimate paradox." This is because God, seen as a being that is completely good, wise, and powerful, fully became human, as Christians believe—limited by sin and with limits in goodness, knowledge, and understanding. Kierkegaard believed this paradox can only be understood through a "leap of faith," moving away from human reason toward belief in God.

Because the exact way these two parts of Jesus' nature join is too complex for people to fully understand, the term "mystical union" is sometimes used instead of "hypostatic union."

Through history

Apollinaris of Laodicea was the first person to use the word "hypostasis" when trying to explain how Jesus could be both God and human. He believed that the divine and human parts of Jesus were united into one nature and one essence, which he called a single hypostasis.

In the 5th century, a disagreement happened between Cyril of Alexandria and Nestorius. Nestorius argued that the term "theotokos," which means "God-bearer," could not be used to describe Mary, the mother of Jesus. He believed that Jesus had two separate parts: one divine and one human. Nestorius claimed that divinity could not come from a human because the divine nature is unoriginate. In 431, the Council of Ephesus, led by Cyril and Bishop Memnon, called Nestorius a neo-adoptionist, meaning they believed that Jesus was divine only by grace, not by nature. The council removed Nestorius from his position as a heretic. In a letter to Nestorius, Cyril used the word "hypostatic" to describe how Jesus' divine and human natures were united. He wrote, "We must follow these teachings, keeping in mind what 'having been made flesh' means… We say that the Word united to himself hypostatically flesh animated by a rational soul became man." Cyril also emphasized the phrase "one physis of the Word of God made flesh," meaning one nature of God the Word in human form.

Theologian Theodore of Mopsuestia, a leading figure from Antioch, disagreed with the monophysite belief of Apollinarism. He taught that Jesus had two natures—human and divine—and two corresponding hypostases (which meant "essence" rather than "person"). At the time, the word "hypostasis" was often used similarly to "ousia," which clearly meant "essence." However, later interpretations of Theodore's ideas have been studied carefully after his Catechetical Orations were discovered in the Syriac language.

In 451, the Ecumenical Council of Chalcedon stated the Chalcedonian Definition. It agreed with Theodore that the Incarnation involved two natures. However, the council also clarified that the word "hypostasis" should be used as it was in the Trinitarian definition, meaning "person" rather than "nature," as Apollinaris had suggested.

The Oriental Orthodox Churches rejected the Chalcedonian Definition and were called Monophysites because they followed the Eutychian belief that Jesus had one nature. The Orthodox "in two natures" formula, based on early church teachings and Colossians 2:9, was criticized as being similar to Nestorian ideas. However, the Orthodox saw the non-Chalcedonians as leaning toward Eutychian Monophysitism. The non-Chalcedonians claimed they never supported Eutyches' teachings, even though the heresy lasted into the sixth and seventh centuries. They emphasized that Jesus' humanity was the same as human nature and preferred the term "Miaphysite," meaning one united nature, rather than "Monophysite," which implies one singular nature. The Monophysite view holds that Jesus' nature, though originally two, is described as one in its human form because the two natures work together in unity, even though they form a third thing, which contradicts the idea of divine simplicity.

In 1989 and 1990, leaders from the Eastern Orthodox and Oriental Orthodox churches signed joint statements to work toward unity. In 1994, leaders of the Assyrian Church of the East, which honors Nestorius and Theodore, signed an agreement with Catholic Church leaders, stating that their historical differences were about words, not meaning.