The Incarnation is the belief in Christianity that the second person in the Christian Godhead, also known as the Son or the Logos (Word), "became flesh" when he was miraculously conceived in the womb of the Virgin Mary. The word Incarnate derives from Latin (in=in or into, caro, carnis=flesh) meaning "to make into flesh" or "to become flesh". The incarnation is a fundamental theological teaching of orthodox (Nicene) Christianity, based on its understanding of the New Testament. The incarnation represents the belief that Jesus, who is the non-created second hypostasis of the triune God, took on a human body and nature and became both man and God. In the Bible its clearest teaching is in the Gospel of John, where in chapter 1 verse 14 (abbreviated as "John 1:14"), it says "And the Word became flesh, and dwelt among us."

In the Incarnation, as traditionally defined, the divine nature of the Son was joined but not mixed with human nature in one divine Person, Jesus Christ, who was both "truly God and truly man". The Incarnation is commemorated and celebrated each year at Christmas, and also reference can be made to the Feast of the Annunciation; "different aspects of the mystery of the Incarnation" are celebrated at Christmas and the Annunciation .

This is central to the traditional faith held by most Christians. Alternative views on the subject (See Ebionites and the Gospel according to the Hebrews) have been proposed throughout the centuries (see below), but all were rejected by mainstream Christian bodies.

In recent decades, an alternative doctrine known as "Oneness" has been espoused among various Pentecostal groups (see below), but has been rejected by the remainder of Christendom.

Description and development of the traditional doctrine

In the early Christian era, there was considerable disagreement amongst Christians regarding the nature of Christ's Incarnation. While all Christians believed that Jesus was indeed the Son of God, the exact nature of his Sonship was contested, together with the precise relationship of the "Father," "Son" and "Holy Ghost" referred to in the New Testament. Though Jesus was clearly the "Son," what exactly did this mean? Debate on this subject raged most especially during the first four centuries of Christianity, involving Gnostics, followers of the Presbyter Arius of Alexandra, and adherents of St. Athanasius the Great, among others.

Eventually, the Christian Church accepted the teaching of St. Athanasius and his allies, that Christ was the incarnation of the eternal second person of the Trinity, who was fully God and fully a man simultaneously. All divergent beliefs were defined as heresies. This included Docetism, which said that Jesus was a divine being that took on human appearance but not flesh; Arianism, which held that Christ was a created being; and Nestorianism, which maintained that the Son of God and the man, Jesus, shared the same body but retained two separate natures. The Oneness belief held by certain modern Pentecostal churches is also seen as heretical by most mainstream Christian bodies.

The most widely-accepted definitions of the Incarnation and the nature of Jesus were made by the early Christian Church at the First Council of Nicaea in 325, the Council of Ephesus in 431, and the Council of Chalcedon in 451. These councils declared that Jesus was both fully God: begotten from, but not created by the Father; and fully man: taking his flesh and human nature from the Virgin Mary. These two natures, human and divine, were hypostatically united into the one personhood of Jesus Christ.

The significance of the Incarnation has been extensively discussed throughout Christian history, and is the subject of countless hymns and prayers. For instance, the Liturgy of St. John Chrysostom, as used by Eastern Orthodox Christians and Byzantine Catholics, includes this "Hymn to the Only Begotten Son":

The Athanasian and Nicene Creeds contain a comprehensive traditional definition of the Incarnation.

Fortuitous and Necessary Incarnation

The link between the Incarnation and the Atonement within systematic theological thought is complex. Within traditional models of the Atonement, such as Substitution, Satisfaction or Christus Victor, Christ must be Divine in order for the Sacrifice of the Cross to be efficacious, for human sins to be "removed" and/or "conquered". In his work The Trinity and the Kingdom of God, Jurgen Moltmann differentiated between what he called a "fortuitous" and a "necessary" Incarnation. The latter gives a soteriological emphasis to the Incarnation: the Son of God became a man so that he could save us from our sins. The former, on the other hand, speaks of the Incarnation as a fulfilment of the love of God, of his desire to be present and living amidst humanity, to "walk in the garden" with us.

Moltmann favours "fortuitous" incarnation primarily because he feels that to speak of an incarnation of "necessity" is to do an injustice to the life of Christ. Moltmann's work, alongside other systematic theologians, opens up avenues of liberation Christology.

Alternative views of the Incarnation

Michael Servetus

During the Reformation, Michael Servetus taught a theology of the Incarnation that denied Trinitarianism, insisting that Trinitarians were Tritheists who had rejected Biblical monotheism in favor of Greek philosophy. The Son of God, said Servetus, was not an eternal being, but rather the Logos (a manifestation of the One True God, not a separate person) incarnate. For this reason, Servetus refused to call Christ the "eternal Son of God" preferring "the Son of the eternal God" instead.

In describing Servetus' theology of the Logos, Andrew Dibb explained: "In Genesis God reveals himself as the creator. In John he reveals that he created by means of the Word, or Logos, Finally, also in John, he shows that this Logos became flesh and 'dwelt among us'. Creation took place by the spoken word, for God said 'Let there be…' The spoken word of Genesis, the Logos of John, and the Christ, are all one and the same."

For defending his belief, Servetus was burnt at the stake in 1553 by Protestants in Geneva at the instigation of John Calvin.

The Oneness view of the Incarnation

In contrast to the traditional view of the Incarnation cited above, adherents of Oneness Pentecostalism believe in the doctrine of Oneness. Although both Oneness and Christianity teach that God is a singular Spirit, Oneness deny that God is a Trinity of persons as in the traditional understanding. Jesus is indeed seen as both fully divine and fully human, but his divine nature is believed to be the Father himself (who is also the Holy Ghost in their theology; "Father", "Son" and "Holy Ghost" being merely titles reflecting the different manifestations of the One True God in the universe) united to Christ's human nature to form one Person: the Son. Thus the Father is not the Son--and this distinction is crucial--but is in the Son as the fullness of his divine nature (Colossians 2:9). Whereas traditional Trinitarians believe that the Son always existed as the eternal second person of the Trinity, Oneness adherents believe that the Son did not come into being until the Incarnation, when the one and only true God took on human flesh for the first, last and only time in history. Oneness doctrine is explained in detail in UPCI minister Dr. David K. Bernard's .

Michael Servetus is held in high regard by Oneness adherents, since his theology definitely reflects a Oneness perspective. In Chapter Ten of The Oneness of God, Bernard refers to Servetus as "a true Oneness believer."