(Greek: dokesis, semblance, appearance)
A group of heretics who date back to Apostolic times.
Their teaching, that Christ only "seemed" to be a man, to have been born, to have lived and suffered, sought to destroy the meaning and purpose of the doctrine of the Incarnation.
Docetism, so far as is known, was always an accompaniment of Gnosticism, and later of Manichaeism.
Strictly, it is not a Christian heresy at all, since it did not arise from a misconception of dogma by the faithful but from the principle of antagonism between matter and spirit, which also formed the basis for Gnosticism.
Among its most famous opponents were Saint Ignatius of Antioch, Saint Polycarp, Saint Hippolytus of Rome, and Tertullian.
New Catholic Dictionary