Docetism is an ancient Christian heresy—one of the earliest, in fact. Docetism taught that Jesus' physical body was an illusion. That meant that Jesus' crucifixion was illusory as well. Jesus only seemed to have a physical body and only seemed to die on the cross. The docetists taught that Jesus was pure spirit, incorporeal, so he could not have done the kinds of earthly things real people do, the most earthly of all being to die.