Docetism was an error with several variations concerning the nature of Christ. Generally, it taught that Jesus only appeared to have a body--that he was not really incarnate (Greek, "dokeo" = "to seem"). This error developed out of the dualistic philosophy which viewed matter as inherently evil--that God could not be associated with matter; and that God, being perfect and infinite, could not suffer. Therefore, God as the word, could not have become flesh per John 1:1, 14, "In the beginning was the Word, and the Word was with God; and the Word was God . . . And the Word became flesh, and dwelt among us . . . " This denial of a true incarnation meant that Jesus did not truly suffer on the cross, and that He did not rise from the dead.
The basic principle of Docetism was refuted by the Apostle John in 1 John 4:2-3. "By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; 3 and every spirit that does not confess Jesus is not from God; and this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world." Also, 2 John 7, "For many deceivers have gone out into the world, those who do not acknowledge Jesus Christ as coming in the flesh. This is the deceiver and the antichrist."
Ignatius of Antioch (died 98/117) and Irenaeus (115-190), and Hippolatus (170-235) wrote against the error in the early part of the second century.
Docetism was condemned at the Council of Chalcedon in 451.