Gnostics believed that the material world was evil and the only way to salvation was through discovering the "secrets" of the universe. This belief flew in the face of Judaism and Christianity, both of which believed that God created the world (Genesis) and that it was good, not evil. Keeping revelation secret wasn't meant to be; rather, it should be shared openly with others.
Docetism, a spin-off from Gnosticism, comes from the Greek word dokesis, meaning appearance. In the first and second centuries a.d., Docetists asserted that Jesus Christ only appeared to be human. They considered the material world, including the human body, so evil and corrupt that God, who is all good, couldn't have assumed a real human body and human nature. He must have pretended.
The Gnostic antagonism between the spiritual and the material worlds led Docetists to deny that Jesus was true man. They had no problem with His divinity, only with believing in His real humanity. So if that part was an illusion, then the horrible and immense suffering and death of Jesus on the cross meant nothing. If His human nature was a parlor trick, then His Passion also was an illusion.
The core of Christianity, and of Catholic Christianity, is that Jesus died for the sins of all humankind. Only a real human nature can feel pain and actually die. Docetism and Gnosticism were considered hostile to authentic Christianity, or, more accurately, orthodox Christianity. (The word orthodox with a small letter o means correct or right believer. However, if you see the capital letter O, then Orthodox refers to the eastern Orthodox Churches, such as the Greek, Russian, and Serbian Orthodox Churches.)
Even today, remnants of neo-Gnosticism are in some modern ideologies and theories of religion. New Age spirituality and Dianetics, which is the Church of Scientology, propose to reveal secrets and unlock secret powers of human nature. Docetism seems to have pretty much died out, however.