While it is true the Docetists had many things written against them, very little of what they wrote themselves – assuming they commited their teachings to writing – survives to our day. Exegesis of the Soul and the Apocalypse of Peter both have an identifiable docetic slant.
Another, The Second Treatise of the Great Seth, was discovered at Nag Hammadi (Codex VII) and dates from the third century. In this Gnostic work, the reader is treated to an account from the Christ himself! Christ tells us plainly how he jumped into the body of Jesus but then made a hasty exit right before the crucifixion.
Hence, Christ only seemed to die and those who do not get that are “small and ignorant” and needlesly in “fear and slavery” - all because they cherish “a doctrine of a dead man”. The truth of the matter is that poor Simon of Cyrene was accidentaly crucified instead. Jesus reaction was … to laugh. This picture of Jesus even inspired two modern Christ-Mythicist proponents to write a book: Timothy Freke and Peter Gandy, The Laughing Jesus: Religious Lies and Gnostic Wisdom (New York: Harmony Books, 2005), 160.
The author of this "great treatise" is unknown but according to the second-century church father Frenacus, the Gnostic teacher Basilides of Alexandria (circa 132 AD) taught the same thing in regards to this mixup and the resultant laughing Jesus. Also see Epiphanius, Panarion 24.
Irenaeus also mentions this doctrine in connection with Basilides as well as Saturninus of Antioch in Against Heresies 1.24.4. Here is the pertinent section from Seth:
“But I [Christ] was not afflicted at all. Those who were there punished me. But I did not die in reality but in appearance … 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 weralth of the archons and the offspring of their error, of their empty glory. And I was laughing at their ignorance”.
This last sentence describes the way I sometimes feel about any one who believes any of this asinine tripe.
*Also see Bart D. Ehrman, Lost Scriptures: Books that Did Not Make It into the New Testament (New York: Oxford University Press, 2003).