Docetism is a Christian heresy. Docetism is the belief that the divine, Godly nature of Christ is irreconcilable with his human birth and existence.
The reason that docetism is a heresy is that it rejects the idea that God would truly send his son in human form. Docetism also rejects the suffering of Christ, especially during his crucifixion, as due to his divine nature suffering would not be possible. The resurrection is also rejected because Christ could not truly have died, he merely appeared to die.
Docetism was a problem for the early church, and the problems seem to have come from the gospel of John. In John's passion narrative Jesus seems to be in control of the situation, willingly giving himself up to the capturing party. His last word upon the cross: "tetellestai" is the greek for "it has been accomplished". This emphasises the glory and the puropose of Jesus's death. An interesting point to add is that in John's gospel there is no mention of Simon of Cyrene, who carries the cross for Jesus in the Markan account. The usual literary explanation for this (John is believed to have been written last) is that John wanted to avoid the docetist heresy that Simon of Cyrene died on the cross instead of the inhuman Jesus.
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sources:
http://cedar.evansville.edu/~ecoleweb/articles/docetism.html
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