What is Docetism and why did Christianity feat it?

In Christianity, Docetism (from the Greek [dokeo], "to seem") is the belief that Jesus' physical body was an illusion, as was his crucifixion; that is, Jesus only seemed to have a physical body and to physically die, but in reality he was incorporeal, a pure spirit, and hence could not physically die. This belief has historically been regarded as heretical by most Christian theologians.

This belief is most commonly attributed to the Gnostics, who believed that matter was evil, and hence that God would not take on a material body. This statement is rooted in the idea that a divine spark is imprisoned within the material body, and that the material body is in itself an obstacle, deliberately created by an evil lesser god (the demiurge) to prevent man from seeing his divine origin. Humanity is, in essence, asleep.

Docetism could be further explained as the view that, because the human body is temporary and the spirit is eternal, the body of Jesus therefore must have been an illusion and his crucifixion as well. It could be compared to how a Buddhist speaks about illusion: illusion is everything that is temporary, not everything that is not real. Even so, saying that the human body is temporary has a tendency to undercut the importance of the belief in resurrection of the dead and the goodness of created matter, and is in opposition to this orthodox view.

Docetism was rejected by the ecumenical councils and mainstream Christianity, and largely died out during the first millennium A.D. Catharism, and other surviving gnostic movements, incorporated docetism into their beliefs, but the movement was destroyed by the Albigensian Crusade, though its teachings still exist today. See Catherism

Islam also teaches that Jesus's crucifixion was an illusion. The Qur'an says, "They did not kill him and they did not crucify him, but it was made to seem so to them..."(4:157) Ref. Wikipedia