The English verb "incarnate" is translated from the Latin term "incarno" (the prefix "in" and the Latin noun "caro" for flesh). While the verb "incarno" or its noun "incarnatio" (incarnation) is not used, the concept of incarnation is developed from John 1:14.

"And the Word became flesh…" (John 1:14)

Latin: "et Verbum caro factum est"

Incarnation describes the condition where deity and humanity exist in the person of Jesus Christ.

For the first few centuries, there have been considerable questions about what this meant. As God's Son (Matt 3:17; 17:5; Mark 9:7; Luke 3:22; 9:35; 2 Pet 1:17; 1 John 5:10), Jesus had a divine nature. As Mary's son (Matt 1:18-25; Luke 2:6-7), Jesus also had a human nature. The mystery was how these two natures could be fully united in one person.

Until this concept was fully understood, there were three evolving views which can be seen and compared in the following chart:

With the establishment of the New Testament, theologians of the early church questioned and debated about the nature of God and His Son Jesus Christ.

Council of Nicaea (325 A.D.)

One of the first controversies confronting the early church was the question of triune nature of God and the relationship of Jesus Christ with His Father. Arius, a Bishop of Alexandria, taught that both Jesus and the Holy Spirit were created by God; because God preexisted, Jesus and the Holy Spirit were not equal in nature to God the Father. This view became known as Arianism.

To end a potential division of the church and society, the Emperor Constantine summoned a Council of Bishops in Nicaea, in which over 300 attended, to settle this question. The theological discussions resulted in the Nicene Creed which formally recognized the unity of Jesus Christ and His Father as One eternal Being. Politically, Arius would be excommunicated, and the creed was used as a basis of authority to validate churches; those who disagreed with the Nicene Creed were excommunicated and the banishment was enforced by Constantine.

Council of Constantinople (381 A.D.)

How Jesus could be both human and divine was difficult to grasp, and the controversy of Arianism did not end with the Nicene Creed. Theodosius I, the last emperor to rule both western and eastern halves of the Roman Empire, called for a meeting of bishops to confront the divisions caused by the persistence of Arianism. Also against Arianism, Apollinaris of Laodicea taught that Jesus Christ had a human body with a divine mind and spirit. Attended by 186 bishops, the Council of Constantinople affirmed the Nicene Creed and made additions to it; the expanded Nicene Creed become known as the Nicene Creed of 381 or the Niceno-Constantinopolitan Creed.

Strongly condemning both Arianism and Apollinarianism, the added statements recognized the deity of the Holy Spirit and in unity with God the Father and Jesus Christ His Son. The doctrine of the Trinity became a part of the official state religion established across both western and eastern halves of the Roman Empire.

Council of Chalcedon (451 A.D.)

Failing to stand up to biblical scrutiny, Arianism finally no longer threatened orthodoxy. But Apollinarianism still lingered, because it was still difficult to understand how Jesus could be both divine and human in nature. New thoughts emerged. Nestorius believed that Jesus had two separate natures and was in essence two persons in one; this teaching became known as Nestorianism. Later, Eutyches taught that Jesus' divine nature absorbed His human nature and morphed into a third sort of divine nature; this became known as Monophysitism / Eutychianism.

To resolve these controversies regarding the nature of Jesus Christ, the Council of Chalcedon was convened. Affirming the theological work of the Council of Nicaea and Constantinople, the Chalcedon Definition taught that Jesus Christ had: a) a human nature and a divine nature, b) His divine nature was exactly the same as God the Father and retaining Its distinctive characteristics, c) His human nature was exactly the same as our human nature and retaining its distinctive characteristics but without sin, and d) these two natures are united together in One Person.

The Chalcedon Definition was also a response to Apollinarianism, Nestorianism, and Monophysitism / Eutychianism by stating what Jesus Christ's nature was not. The principal portion of the definition follows:

Therefore, following the holy fathers, we all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ, at once complete in Godhead and complete in manhood, truly God and truly man, consisting also of a reasonable soul and body; of one substance with the Father as regards his Godhead, and at the same time of one substance with us as regards his manhood; like us in all respects, apart from sin; as regards his Godhead, begotten of the Father before the ages, but yet as regards his manhood begotten, for us men and for our salvation, of Mary the Virgin, the God-bearer; one and the same Christ, Son, Lord, Only-begotten, recognized in two natures, without confusion, without change, without division, without separation; the distinction of natures being in no way annulled by the union, but rather the characteristics of each nature being preserved and coming together to form one person and subsistence, not as parted or separated into two persons, but one and the same Son and Only-begotten God the Word, Lord Jesus Christ; even as the prophets from earliest times spoke of him, and our Lord Jesus Christ himself taught us, and the creed of the fathers has handed down to us.

From the Chalcedon Definition, the phrase "coming together to form one person and subsistence (Greek: hypostasis)" is where the English phrase "hypostatic union" comes from. While it is still beyond human comprehension, the phrase "hypostatic union" describes the union of Jesus Christ's human and divine natures in One Being.