In the Incarnation, as traditionally defined, the divine nature of the Son was united 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 the Feast of the Incarnation, which is better known as the Annunciation.
This teaching is central to the traditional Christian faith held by the Roman Catholic and Eastern Catholic Churches, the Eastern Orthodox Church, the Oriental Orthodox Church, the Anglican Communion, and most Protestants. Alternative views on the subject have been proposed throughout the centuries (see below), but all were rejected by mainstream Christian bodies.
In the early Christian era, there was considerable disagreement 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 (who ultimately triumphed), among others.
Eventually, the Catholic 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 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 Catholic 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 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 efficious, 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 "fortutious" 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.
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 stubbornly defending this belief, Servetus was burnt at the stake in 1553 by order of John Calvin.
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.