A term, not found in the Bible, meaning ‘becoming flesh’ or ‘human’, defining and engaging with the theological issue of the relation of the eternal God with the finite and created world. It is claimed that in the NT there are imprecise and sporadic expressions of this doctrine concerning Jesus but there was no precise definition until there had been five centuries of strenuous debate. Councils of the Church eventually agreed that the orthodox faith in Jesus was that united in his one person were the two natures, full divinity and genuine humanity. That Jesus was truly human carries the corollary that he could not have actually believed himself to be God, an incompatibility with authentic humanity.

There are few indications in the synoptic gospels that Jesus was regarded, or regarded himself, as other than a man with unique gifts of teaching with authority and healing; but soon after the resurrection there are accounts of apostolic preaching in which Jesus is proclaimed as Lord and Messiah (Acts 2: 32–6) and Son (Rom. 1: 3–4). The gospels even mention him as ‘beloved Son’ of God at his baptism (Mark 1: 11) and transfiguration (Mark 9: 7). The two narratives of the virginal conception, in which Joseph is relegated to a powerless background role, push the status of Jesus as Son of God back beyond his birth. There is thus a hint of pre-existence which is confirmed by Paul in Gal. 4: 4. Moreover, Paul identifies Christ as ‘the wisdom of God’ (1 Cor. 1: 24), and this echoes the personification of Wisdom (Wisd. 7: 25; 8: 3) in the intertestamental period: for wisdom is said to have pre-existed with God before particular manifestations. In the hymn extolling Christ's voluntary humiliation in becoming man, Paul appears to accept that he was in the divine arena previously (Phil. 2: 6–11); similarly Col. 1: 15. An incipient doctrine of incarnation in Heb. 1: 1–3 becomes clearer in the gospel of John (1: 14) where Jesus' prior existence with God (John 6: 62) and association with Abraham (John 8: 58) imply a strong notion of pre-existence. But this is not allowed to diminish the clarity of Jesus' humanity; the Word became flesh (John 1: 14); Jesus had human frailties (John 4: 6), he wept (John 11: 35), he died (John 19: 30), and yet without contradicting that humanity he was a revelation of God (John 10: 38; 20: 28). The epistles of John combat the belief that Jesus' humanity was a mere appearance (the belief called docetism; 1 John 4: 2–3).

The traditional doctrine of incarnation is regarded as unsatisfactory by some modern theologians. For example, an alternative way of interpreting the action of God in Christ has been in terms of the Spirit. The Spirit works through the whole evolutionary process of creation and sustaining, and in this continuous divine activity the decisive and focal point is the person of Christ. In him divine self-sacrificing love achieves its full expression in a human personality. Defenders of the doctrine reject any interpretation in which truths about God could be regarded as independent of the life and career of an individual and insist that Jesus Christ is indispensable for the believer's relation, here and now, to God.

Historically, Jesus was a circumcised Jewish man of the first century. In Christian doctrine the Incarnation affirms the identification of the Word (Logos, the New Adam) with humankind, both male and female, in whom is revealed God's will and God's goodness. In him is revealed the invisible Creator. The unity of God is not impaired.