Final answer:
The ancient form of Christianity that claimed Jesus' body was an illusion and was later declared heretical is known as Docetism. This belief contradicted the eventual orthodox Christian position that Jesus is both fully human and divine, as established by councils like the Council of Chalcedon.
Step-by-step explanation:
The form of Christianity in the ancient world that claimed Jesus' body was an illusion and was eventually condemned as heretical is known as Docetism. Docetism is a belief that asserted that Jesus only seemed to have a physical body and his physical suffering on the cross was a mere illusion. This viewpoint was at odds with orthodox Christian doctrine, which affirmed the full humanity and full divinity of Jesus Christ.
The early Christian Church faced numerous theological debates regarding the nature of Jesus. The affirmation of Jesus as both true God and true man was solidified over time through ecumenical councils, such as the Council of Chalcedon in 451, which formally rejected notions like Docetism and other heretical views.
Christian orthodoxy, as established by these early councils, states that Jesus Christ has two natures, divine and human, united in one person without mixture or confusion, which differ from heretical views that were propounded by groups like the Gnostics and Arians. The establishment of orthodox doctrine was crucial for the identity and unity of the emerging Christian Church, shaping its theology, rituals, and ecclesiastical structure.