Q. 1. Can you summarize what was the heresy of Docetism?
A. 1. The heresy of Docetism was the belief that Jesus' physical body was an illusion, as was His crucifixion; meaning that Jesus only seemed to have a physical body and to physically die. But in reality, He was incorporeal, a pure spirit, and hence He could not physically die.
Broadly speaking, it is the belief that Jesus only seemed to be human, and that His human form was a complete illusion.
Docetism was rejected by the Ecumenical Councils and mainstream Christianity. It mostly died out during the first millennium AD.
Docetism was unequivocally rejected by the First Council of Nicaea in 325 AD. It is viewed as heretical by the Catholic Church, Orthodox Church, and many others.
Gnostic movements that continued to promote Docetism, such as Catharism, incorporated it into their beliefs. Those groups were destroyed by the Albigensian Crusade (1209–1229 AD).