In Christianity, the doctrine of the Incarnation is the idea that God became human in the person of Jesus Christ, his son. All forms of Christianity believe in some fashion that Jesus is the Son of God. The precise nature of his connection to God has never been universally agreed upon, but the orthodox position held by Catholic, Orthodox, Protestant, and Anglican Christians is that God was incarnated equally human and divine in Jesus. Two church councils can help to shed light on the development of the precise relationship between God and Jesus behind the Incarnation: the Council of Nicaea emphasized the Trinity and the equality between three persons of the Trinity (Father, Son, Spirit), while the Council of Chalcedon emphasized the equality of a human and divine nature united in the single person of Jesus.
The first Council of Nicaea was held in 325 AD. As the first ecumenical council of the church, it was especially important in defining the beliefs held by today’s Catholic, Orthodox, Protestant, Anglican, and Oriental Orthodox churches. The Nicene Creed, a basic outline of Christian belief, professes belief in God, Jesus, and the Holy Spirit. While the original creed does not explicitly define God as a trinity of three persons, one purpose of the Council of Nicaea was to defend Trinitarian belief.
The Council of Chalcedon was held in 451 AD. Among the churches who hold Chalcedonian theology are the Catholic, Orthodox, Protestant, and Anglican churches. The Chalcedonian Definition declared that after the Incarnation, Jesus possessed two natures (human and divine) in one person or “hypostasis.” This has since become the orthodox teaching of Christianity, referred to as dyophysitism, though some churches such as the Oriental Orthodox reject the Chalcedonian Definition. These churches follow miaphysitism, the position that after the incarnation Jesus had only a single nature.
The decisions of the two councils support the dominant position of the Incarnation today: specifically, God exists as a Trinity of three distinct persons who are all of the same essence; the second person of the Trinity, the Son, then became a human called Jesus Christ who in his singular personhood contained two natures, human and divine.
This is significant to Christianity because by affirming the completely human and divine natures of Jesus, as well as his unification with God through the Trinity, he is equipped in the realm of Christian theology to be both savior (from his divinity) and relatable (from his humanity). The Christian God is able to understand humans, because he became one, while also being capable of delivering salvation, thanks to his divinity.