If the equal reality of the two natures of Christ is rejected, there are two options:
a) The divine nature is real and the human nature is an illusion. This is the
error of Docetism. This view ignores the many evidences of Christ's humanness as
witnessed by the disciples: he got exhausted and slept, he wept, he bled real blood
and, according to his Centurion executioner, he died.
b) The other option is that the human nature is real but that Christ was not divine. This notion was taught by many early sects such as the Montanists, Nestorians, Arians and others up to the present day. Enlightenment humanists like Thomas Jefferson edited the miracles out of the New Testament but admired Christ as a wise though human teacher.
C.S. Lewis gave an incisive answer to the latter view:
I am trying here to prevent anyone saying the really foolish thing that people often say about Him: 'I'm ready to accept Jesus as a great moral teacher, but I don't accept His claim to be God.' That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic -- on the level with the man who says he is a poached egg -- or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to.
- Mere Christianity, pp. 40-41.