From Gnostic Cosmology:
Not all Gnostics considered themselves Christians and Gnosticism seems to have originated apart from Christianity, but those who did synthesize the two movements considered Jesus the divine being who delivered the gnosis that redeems humanity. As the messenger from the divine realm, though, Jesus had to be completely spirit; the divine could inhabit material flesh. So Gnostics had to explain why Jesus appeared human. One solution was to claim just that-he only appeared human. Referred to as docetism , this view held that Jesus put on act of seeming to hunger, to eat, to thirst, to bleed, to die, as he taught his disciples the saving gnosis. The insight was only for the elect who carried the divine spark and so Jesus feigned humanity-as Paul says, Christ came ?in the likeness of sinful flesh? (Romans 8:3)-to conceal the revelation from the non-Gnostics (Ehrman 167). For the same reason Jesus taught in parables, concluded them by saying ?Let he who has ears to hear, hear,? referring to the hidden truth that the elect could find in the parable; and later explained the meaning of the parables to his disciples in private. The parables themselves often depicted the ?kingdom of heaven? as something, such as yeast or a treasure, which one hides . As the anti-Gnostic writings of ecclesiastics including Ireneaus and Tertullian reveal, proto-orthodox Christianity, which would eventually win-in terms of numbers of followers-the intense and polemical theological battles of the second and third centuries, defined much of its theology through conflict with other Christian groups (Filoramo 4). And it declared docetism a heresy at the Council of Chalcedon in 423, deciding that Christ was ?fully human, fully divine.?