Watch the above video starting at the 1:00 mark to see what happens to docetists in hell!
Docetism comes from the Greek word dokeō for ‘seems to be’ or ‘supposedly’ as docetists claimed that Jesus’s body only seemed to be real and was, at some point or other, incorporeal, spiritual, or morphed into a different state. It is hard for us to imagine the attraction of docetism, but in the ancient world it was a compelling option for those with a particular view of God, who cherished the concept of impassibility, and avoidance of shame.
To begin with, some might deny that God or the Christ could take on a crude physical form, so that Jesus was merely a phantasm in his appearance, not truly physical. A certain Saturninus is attributed the view that Jesus was ‘without birth, without body, and without figure, but was, by supposition, a visible man.’ Similarly, in a Gnostic document called Trimorphic Protennoia, Jesus says: ‘I revealed myself to them in their tents as Word, and I revealed myself in the likeness of their shape.’ So Jesus was a bit of a shape-shifter who morphed into human-like appearance but without being actually human. While we might raise eyebrows at this, remember that Paul said that God sent Jesus ‘in the likeness of sinful flesh’ (Gk. en homoiōmati sarkos hamartias), probably meaning that Jesus had a human body without a sinful nature, but such wording gave inspiration and impetus to docetic associations.
Alternatively, others accepted that Jesus did have a physical body, but the ‘Christ’ departed from Jesus prior to the crucifixion, or else Jesus changed form and some other person was crucified in his place, so that Jesus’s divinity was safeguarded from suffering. The logic is that if divinity is impassible, then the divine Christ did not suffer, so obviously the Father must have rescued the heavenly Christ before he was crucified. Such a belief is not without precedent. One Roman author known to us as Ovid wrote a poem about the assassination of Julius Caesar. According to Ovid, just before Caesar was murdered, the goddess Vesta ‘snatched him away and left a naked semblance; what died by the steel was Caesar’s shadow’ so that the real Julius Caesar ‘was raised to the heavens and he found Jupiter’s halls.’ The gods were so impressed with Caesar that they took his soul up to heaven and left only the vestiges of his shadowy self to be assassinated. Pause for a minute and you can see how some people might want to apply that some sort of thing to Jesus in order to protect his divine nature (keep him impassible) and endure his divine honor (keep him from the shame of crucifixion).
This docetic perspective plays out in different ways among ‘heretical’ accounts of the crucifixion. A notorious figure known as Cerinthus, an Egyptian who came to Ephesus and was regarded as the Gnostic opponent of John the apostle, is said to have taught that Jesus was not born of a virgin conception, but was born of Mary and Joseph’s union, that the Christ descended upon Jesus in the form of a dove at his baptism, but later the ‘Christ departed from Jesus, and that then Jesus suffered and rose again, while Christ remained impassible, inasmuch as he was a spiritual being.’ Similarly, a certain Basilides had a ‘trading places’ theory of the atonement and he is alleged to have taught that Simon of Cyrene was transformed into Jesus while Jesus transformed into Simon. According to Irenaeus, Basilides believed that:
He appeared, then, on earth as a man, to the nations of these powers, and wrought miracles. Wherefore he did not himself suffer death, but Simon, a certain man of Cyrene, being compelled, bore the cross in his stead; so that this latter being transfigured by him, that he might be thought to be Jesus, was crucified, through ignorance and error, while Jesus himself received the form of Simon, and, standing by, laughed at them. For since he was an incorporeal power, and the Nous (mind) of the unborn father, he transfigured himself as he pleased, and thus ascended to him who had sent him, deriding them, inasmuch as he could not be laid hold of, and was invisible to all. Those, then, who know these things have been freed from the principalities who formed the world; so that it is not incumbent on us to confess him who was crucified, but him who came in the form of a man, and was thought to be crucified, and was called Jesus, and was sent by the father, that by this dispensation he might destroy the works of the makers of the world.
The same perspective is given a fuller description in the Second Treatise of the Great Seth, a Gnostic document originating in the second century:
For my death, which they think happened, happened to them in their error and blindness, since they nailed their man unto their death. For their Ennoias did not see me, for they were deaf and blind. But in doing these things, they condemn themselves. Yes, they saw me; they punished me. It was another, their father, who drank the gall and the vinegar; it was not I. They struck me with the reed; it was another, Simon, who bore the cross on his shoulder. It was another upon whom they placed the crown of thorns. But I was rejoicing in the height over all the wealth of the archons and the offspring of their error, of their empty glory. And I was laughing at their ignorance. And I subjected all their powers. For as I came downward, no one saw me. For I was altering my shapes, changing from form to form.
When he had said those things, I saw him seemingly being seized by them. And I said ‘What do I see, O Lord? That it is you yourself whom they take, and that you are grasping me? Or who is this one, glad and laughing on the tree? And is it another one whose feet and hands they are striking?’
The Savior said to me, ‘He whom you saw on the tree, glad and laughing, this is the living Jesus. But this one into whose hands and feet they drive the nails is his fleshly part, which is the substitute being put to shame, the one who came into being in his likeness. But look at him and me.’
But I, when I had looked, said ‘Lord, no one is looking at you. Let us flee this place.’
But he said to me, ‘I have told you, “Leave the blind alone!” And you, see how they do not know what they are saying. For the son of their glory instead of my servant, they have put to shame.’
And I saw someone about to approach us resembling him, even him who was laughing on the tree. And he was filled with a Holy Spirit, and he is the Savior. And there was a great, ineffable light around them, and the multitude of ineffable and invisible angels blessing them. And when I looked at him, the one who gives praise was revealed.
And he said to me, ‘Be strong, for you are the one to whom these mysteries have been given, to know them through revelation, that he whom they crucified is the first-born, and the home of demons, and the stony vessel in which they dwell, of Elohim, of the cross, which is under the Law. But he who stands near him is the living Savior, the first in him, whom they seized and released, who stands joyfully looking at those who did him violence, while they are divided among themselves. Therefore he laughs at their lack of perception, knowing that they are born blind. So then the one susceptible to suffering shall come, since the body is the substitute. But what they released was my incorporeal body. But I am the intellectual Spirit filled with radiant light.’ 
Even the Islamic tradition is docetic in its description of the cross, denying that Allah would allow Jesus to be killed or crucified, it only appeared so, since Allah took Jesus up into heaven.
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.
Varieties of docetism were prevalent enough that it was denounced by John the Elder (Asia), Ignatius of Antioch (Syria), and Tertullian (North Africa). The later wrote: ‘Surely he is anti-Christ who denies that Christ has come in the flesh. By declaring that his flesh is simply and absolutely true, and taken in the plain sense of its own nature, the scripture aims a blow at all who make distinctions in it.’
 For a good introduction, see Wilhite 2015, 61-85.
 Irenaus, Adv. Haer. 1.24.2.
 Trim. Prot. 47.14-16 (trans. J.D. Turner).
 Rom 8.3.
 Ovid, Fasti, 3.701-2.
 See Irenaeus, Adv. Haer. 3.3.4; 3.11.1.
 Irenaeus, Adv. Haer. 1.26.1.
 Irenaeus, Adv. Haer. 1.24.4.
 Second Treat. Great Seth 55.30-56.25 (trans. R.A. Bullard and J.A. Gibbons).
 Apoc. Pet. 81.3-83.10 (trans. J. Brashler & R.A. Bullard).
 Sura 4.157-158.
 Tertullian, Carn. Chr. 24.