Docetism was a doctrine in the early years of the Christian church that claimed that Jesus didn’t have a physical body. The name comes from the Greek dókēsis which means “to seem.” It refers to the belief that Jesus only seemed to have a physical body.
This seems to be present very early on. This may be what is referred to in 2 John 2:7.
Many deceivers have gone out into the world; they do not confess the coming of Jesus Christ in the flesh. This is the deceiver and the antichrist.
Today it is more likely to deny that Jesus is God but the earliest heresy denied that Jesus was human. It is related to some platonic ideas about body and mind.
It was understood that mind/spirit was good and body was bad. Since Jesus is good, he must be all spirit and not body at all. In modern language, Jesus was almost a hologram. He looked perfectly human but underneath the image, there was no muscle or bone.
Docetism is related to Gnosticism. Gnostics also denied that Jesus had a physical body, just as they denied that God (the Father of Jesus) created the physical world. But Docetism and Gnosticism are not synonymous. Gnosticism is much bigger.
But that is a post for another day…