Docetism

Docetism is an early Christian teaching that says that Jesus only appeared to have a human body. The term comes from "dokein": "to seem". 
Early Gnostics, whose teachings began to threaten Christian doctrine and theology, believed all matter was unclean. For this reason, Christ could not have a physical body, as he was seen as the eternal Logos. 
Today, it is unclear where this teaching came from. It might have been influenced by the teachings of Plato. Plato taught that only ideas mattered and that matter itself was inferior. 
Many Gnostic teachings are docetic. For a long time, people believed that docetism and Gnosticism were developed together. This is however not the case, there are also some Gnostic systems that are not docetic, as well as some docetic systems that are not gnostic. 
Today, most Christian theologians see docetism as a heresy.