The Catechism speaks the plain historical truth of the Church’s message when it declares without compromise:
Belief in the true Incarnation of the Son of God is the distinctive sign of Christian faith: “By this you know the Spirit of God: every spirit which confesses that Jesus Christ has come in the flesh is of God” (1 John 4:2) Such is the joyous conviction of the Church from her beginning whenever she sings “the mystery of our religion”: “He was manifested in the flesh” (1 Timothy 3:16). (CCC 463)
That Jesus Christ is “true God and true man”—fully God and fully human, not half man and half God–lies at the very heart of what the Christian tradition has said about Jesus from the beginning of the Church. And yet, this seemingly simple proposition has met with a host of challenges over the centuries, each one of which has required a response from the Church, and each response requiring the Church to think more and more deeply about who and what Jesus is and what he has accomplished for us.
As we saw in Chapter 4, this was emphatically true concerning the question of the relationship of God the Son with God the Father, but it is even more true regarding the question of how God the Son can be man.
Some difficulties surrounding this question seem obvious given the materialist temper of our age. So post-moderns have little trouble empathizing with Jesus’ contemporaries who could easily accept his humanity but not his deity as they said, “Is not this Jesus, the son of Joseph, whose father and mother we know?” (John 6:42).
Much less easy for post-moderns to grasp are those objections of ancients who had no problem accepting Jesus’ deity but found his humanity to be the incredible thing. For the Docetist of the late first century, the difficulty was with the proposition that God the Son would take on human flesh. And so, the apostle John must warn the Church that denial of Jesus’ humanity, not his deity, comes of the spirit of antichrist:
Beloved, do not believe every spirit, but test the spirits to see whether they are of God; for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit which confesses that Jesus Christ has come in the flesh is of God, and every spirit which does not confess Jesus is not of God. This is the spirit of antichrist. (1 John 4:1-3)
As the history of the Church rolls on, the immense fertility of the human mind in coming up with ways to complicate the very simple words “true God and true man” are on full display. Just as Arianism had threatened to destroy our understanding of the relationship of the Father to the Son, so a series of heresies proposed “solutions” to biblical paradoxes which, if taken seriously, would have been absolutely fatal to the central claim of the entire Christian faith that the Word became flesh.
So after the threat of Docetism faded in the first century, the same second century Gnostic sects who pitted the Son against the Father also pitted the physical world against the spiritual. They claimed that Jesus had come to liberate us from the prison of a physical body and the physical world. The same St. Irenaeus who argued that the Son was not the enemy of the Father also re-asserted the Christian belief that creation was good and that Jesus came to liberate us from sin, not from the physical world. That is why he was raised bodily.
Adoptionists, meanwhile, held that Jesus was a mere man who was supremely virtuous and therefore adopted by the Father as Son of God when the Spirit descended on him at his baptism in the river Jordan. They ignored the witness of Scripture that Jesus was conceived by the Holy Spirit and was therefore fully God and fully human from the first moment of his conception (Luke 1:26-38).
Other sects arose which (as we shall see) attacked the union of humanity and deity in Jesus. But the archetypal assault on the proposition that God became man came in the early fifth century, when a theologian and archbishop of Constantinople named Nestorius advanced an idea that forced the Church to think as deeply about the Incarnation as Arius had forced her to think about the Trinity.