Scholars in the early Christian Church had to wrestle with passages of Scripture such as this:
"In the beginning was the Word, and the Word was with God, and the Word was God." John 1:1.
Later in this passage, the "Word" (Greek logos) was identified with Jesus Christ, the "Word made flesh". This presented them with a trilemma:
One possibility was that the two natures did not coexist in Christ; one or the other was an illusion. Docetism taught that Christ was divine and not actually human. One problem with this idea is that it made Christ's sacrifice for sins also an illusion. In order for sins to be atoned for, a real, human sacrifice was necessary. That is why Christ was called the "Lamb of God, Who takes away the sin of the world" by John the Baptist. As the Reformers taught, "That which is not assumed is not healed". Therefore Christ had to be truly human, as well as truly divine. So the teaching of Docetism was ruled heretical by early church leaders such as Ignatius of Antioch, Irenaeus and Tertullian.