In Christian terminology, docetism (from the Greekδοκεῖν/δόκησιςdokein (to seem) /dókēsis (apparition, phantom), according to Norbert Brox, is defined narrowly as "the doctrine according to which the phenomenon of Christ, his historical and bodily existence, and thus above all the human form of Jesus, was altogether mere semblance without any true reality."  Broadly it is taken as the belief that Jesus only seemed to be human, and that his human form was an illusion. The word docetai (illusionists) referring to early groups who denied Jesus' humanity, first occurred in a letter by Bishop Serapion of Antioch (197-203), who discovered the doctrine in the Gospel of Peter, during a pastoral visit to a Christian community using it in Rhosus, and later condemned it as a forgery. It appears to have arisen over theological contentions concerning the meaning, figurative or literal, of a sentence from the Gospel of John: "the Word was made Flesh".
Docetism is broadly defined as any teaching that claims that Jesus' body was either absent or illusory. The term ‘docetic’ should be used with caution, since its use is rather nebulous. For Robert Price "docetism", together with "encratism", "Gnosticism", and "adoptionism" has been employed "far beyond what historically descriptive usage would allow". Two varieties were widely known. In one version as in Marcionism, Christ was so divine he could not have been human, since God lacked a material body, which therefore could not physically suffer. Jesus only appeared to be a flesh-and-blood man, his body was a phantasm. Other groups who were accused of docetism held that Jesus was a man in the flesh, but Christ was a separate entity, who entered Jesus’s body in the form of a dove at his baptism, empowered him to perform miracles, and abandoned him on his death on the cross.
Christology and theological implications
Docetism's origin within Christianity is obscure. Ernst Käsemann controversially defined the Christology of St John’s Gospel as “naïve docetism” in 1968. The ensuing debate reached an impasse as awareness grew that the very term ‘docetism’ like ‘gnosticism’ was difficult to define within the religio-historical framework of the debate. It has occasionally been argued that its origins were in heterodox Judaism or Oriental and Grecian philosophies. The alleged connection with Jewish Christianity would have reflected Jewish Christian concerns with the inviolability of (Jewish) monotheism. Docetic opinions seem to have circulated from very early times, 1 John 4:2 appearing explicitly to reject them. Some 1st century Christian groups developed docetic interpretations partly as a way to make Christian teachings more acceptable to pagan ways of thinking of divinity.
In his critique of the theology of Clement of Alexandria, Photius in his Myriobiblon held that Clement’s views reflected a quasi-docetic view of the nature of Christ, writing that Clement "He hallucinates that the Word was not incarnate but only seems to be." (ὀνειροπολεῖ καὶ μὴ σαρκωθῆναι τὸν λόγον ἀλλὰ δόξαι.) In Clement’s time some disputes contended over whether Christ assumed the ‘psychic’ flesh of mankind as heirs to Adam, or the ‘spiritual’ flesh of the resurrection. Docetism largely died out during the first millennium AD.
They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again. They who deny the gift of God are perishing in their disputes".
While these characteristics fit a Monophysite framework, a slight majority of scholars consider that Ignatius was waging a polemic on two distinct fronts, one Jewish, the other docetic, while a distinct minority holds that he is concerned with a group that commingled Judaism and docetism. Other possibilities are that he was merely opposed to Christians who lived Jewishly, or deny that docetism threatened the church, or that his critical remarks were directed at an Ebionite or Cerinthian possessionist Christology, where God descended and took possession of Jesus' body. 
Islam and docetism
The Qur'an has a docetic or gnostic Christology, viewing Jesus as a divine illuminator rather than the redeemer (as he is viewed in Christianity). However, the Islamic docetism is not focused on the general life and person of Jesus or the Christ. In Islam "the Christ" (al-masiH) is not generally viewed as distinct from humanity nor a special spirit being as in docetism or some gnosticisms. Islamic docetism focuses on a denial of the crucifixion of Jesus. Sura4:157–158 reads:
And because of their saying: We slew the Messiah, Jesus son of Mary, Allah's messenger — they slew him not nor crucified him, but it appeared so unto them; and lo! those who disagree concerning it are in doubt thereof; they have no knowledge thereof save pursuit of a conjecture; they slew him not for certain. But Allah took him up unto Himself. Allah was ever Mighty, Wise.
The Qur'an was compiled in the mid-seventh century AD (around 650 CE), corresponding to the period when docetism was still commonly accepted and taught among some Christian sects.
Docetism and Christ myth theory
Since Arthur Drews published his The Christ Myth (Die Christusmythe) in 1909, occasional connections have been drawn between the modern idea that Christ was a myth and docetist theories. Shailer Mathews called Drews' theory a "modern docetism".Frederick Cornwallis Conybeare thought any connection to be based on a misunderstanding of docetism. The idea recurred in ClassicistMichael Grant's 1977 review of the evidence for Jesus, who compared modern scepticism about an historical Jesus to the ancient docetic idea that Jesus only seemed to come into the world "in the flesh". Modern theories did away with "seeming".
^Foster 2009, p. 79.Serapion first approved its use, and only reversed his opinion on returning to his bishopric in Antioch, after being informed of its contents. He wrote a "Concerning the So-Called Gospel of St Peter" which is alluded to in Eusebius of Caesarea's Historia Ecclesiastica 6.12-3-6.
^Gonzalez, Justo (2005). Essential Theologial Terms. Louisville: Westminster John Knox Press. pp. 46–47. ISBN0-664-22810-0. "Docetism is the claim that Jesus did no thave a physical human body, but only the appearance of such."
^Schneemelcher Maurer, p. 220:"N Brox has expressed himself emphatically against a widespread nebulous use of the term, and has sought an exact definition which links up with the original usage (e.g. in Clement of Alexandria). Docetism is ‘the doctrine according to which the phenomenon of Christ, his historical and bodily existence, and thus above all the human form of Jesus, was altogether mere semblance without any true reality.'
^Grant 2004, pp. 199–200:"This skeptical way of thinking reached its culmination in the argument that Jesus as a human being never existed at all and is a myth. In ancient times, this extreme view was named the heresy of docetism (seeming) because it maintained that Jesus never came into the world "in the flesh", but only seemed to; (I John 4:2) and it was given some encouragement by Paul's lack of interest in his fleshly existence. Subsequently, from the eighteenth century onwards, there have been attempts to insist that Jesus did not even "seem" to exist, and that all tales of his appearance upon the earth were pure fiction. In particular, his story was compared to the pagan mythologies inventing fictitious dying and rising gods."
Arendzen, J. (2012) . "Docetae". The Catholic Encyclopedia5. Robert Appleton Company. Retrieved 25 April 2012.
Breidenbaugh, Joel R. (2008). "Docetism". In Hindson, Ed; Caner, Ergun; Verstraete, Edward J. The Popular Encyclopedia of Apologetics: Surveying the Evidence for the Truth of Christianity. Harvest House Publishers. pp. 186–193. ISBN978-0-7369-2084-1. Retrieved 25 April 2012.