Docetism is derived from the Greek dokeo, “to seem.” It is the erroneous belief that Christ only seemed to be human, that He did not really have a body of human flesh. It denied the Incarnation; Christ did not really have Mary as His mother according to the flesh. It was one of the first theological errors to appear in church history, for it is probably the target of the warning of 1 John 4:2,3: “Every spirit that confesseth that Jesus Christ is come in the flesh is of God: and every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come: and even now already it is in the world.”
The first known advocate of docetism was Cerinthus (circa 85 AD). He held that Jesus differed from other men only in that He was better and wiser than they, and that the divine Christ descended upon Him at the baptism and left Him at the cross.
The effect of this reasoning was to make the incarnation an illusion. Either there was no human Jesus at all, but only an apparition, or else the real Son of God was simply using the human Jesus as a vehicle of expression, but was not in real union with him.
Marcion (late second century) was willing to concede the reality of the suffering of Christ, but not the reality of His birth. In his version of Luke’s Gospel he made Christ to simply appear in the reign of Tiberius, by which we understand that He descended from heaven.
Docetism stemmed from the view that flesh and physical matter were evil, or the source of evil. Therefore the Saviour could have had no contact with it. All Gnostics, which plagued the early Church, were Docetic.
The essence of this heresy influenced Mohammed, who denied the deity of Christ. It survived in some of the doctrines of Islam concerning Jesus, and in the modern cults which regard matter as evil.