Apollinarianism, named for Apollinarius of Laodecia (4th century) is the belief that Jesus Christ was born in a normal common human body descended from Adam, but had no finite human soul and mind. His immaterial nature was wholly divine and his physical nature wholly human. Sort of like God in a human container.
This is in contradistinction to the scriptural teaching of the “hypostatic union.” The great mystery of the incarnation wherein God the Son was born as the “son of man.” He was and is fully all that God is and all that sinless man is.
We don’t understand that. God simply tells us it’s true.
Apollinarianism was condemned as heresy at the first council of Constantinople in the year 361 and has not seen very much representation since then
Key tenets of Apollinariansm include:
The denial of the human nature of Jesus beyond his physical body.
The belief that a created human nature is by definition incapable of sinlessness. (many problems with this)
Modern-Day Proponents of Apollinarianism:
While mitigating some of the extremes of Apollinarius himself, the Christian apologist William Lane Craig holds to a christology which borrows heavily from Apollinarius. In fairness he claims to reject apollinarianism.