Docetism, is a heretical sect that developed within the early church, its name is derived from the Greek word dokesis which means "semblance" they are particularly dangerous for the Christian faith because of their Christological understanding that Christ only "appeared" or "seemed to be a man”, which in a way denies Christ’s humanity. The danger as exhibited in history is that some Docetists even went as far as claiming that it was Simon of Cyrene who was crucified, while Jesus looked on from a place of safety. They also emphasized on the temporality of the human body and of the material world which puts forward a tendency to undercut the importance of the belief in resurrection of the dead and the goodness of created matter, which opposes the very nature of orthodoxy that stresses on the importance of the salvific work of God in Christ that transpired in the material world that we live in, which could be seen with their disengagement with the affairs of the world at large and their abstinence in partaking the sacrament of the Eucharist.
The Church responded with the assertion of the Scripture’s account of Christ as God Incarnate or God ‘in flesh’ (John 1:14; 1 John 4:13; 2 John 7). Early Christian apologists like Ignatius of Antioch, Irenaeus and Hippolatus who all wrote against it in the early part of the second century.
Learn more about it from the perspective of:
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Further reading: Docetism entry on the Early Church History in the World Wide Web